DS1921 ,High-Resolution Thermochron iButtonblock diagram in Figure 1 shows the relationships between the major control and memory sections of ..
DS1921 ,High-Resolution Thermochron iButtonFEATURES Digital identification and information by momentary contact ORDERING INFORMATION ®Uni ..
DS1921G ,Thermochron iButtonElectrical Characteristics for Accuracy Specification)form of a histogram. Up to 2048 temperature v ..
DS1921G ,Thermochron iButtonElectrical Characteristics(V = +2.8V to +5.25V, T = -40°C to +85°C.)PUP APARAMETER SYMBOL CONDITION ..
DS1921G-F5 ,2.8 to 5.25 V, thermochron buttonFEATURES Digital identification and information by All dimensions are shown in millimeters. mom ..
DS1921Z ,High Resolution Thermochron iButton Range H: +15°C to +46°C; Z: -5°C to +26°CFEATURES #Denotes a RoHS-compliant device that may include lead(Pb) that is Digital identificatio ..
DS1921
High-Resolution Thermochron iButton
DS1921H/Z
High-Resolution Thermochron iButton
Range H: +15°C to +46°C; Z: -5°C to +26°C SPECIAL FEATURES ��Digital thermometer measures temperature in
1/8°C increments with ±1°C accuracy
��Built-in real-time clock (RTC) and timer has
accuracy of ±2 minutes per month from 0°C
to 45°C
��Automatically wakes up and measures
temperature at user-programmable intervals
from 1 to 255 minutes
��Logs up to 2048 consecutive temperature
measurements in protected nonvolatile (NV)
random access memory
��Records a long-term temperature histogram
with 1/2°C resolution
��Programmable temperature-high and
temperature-low alarm trip points
��Records up to 24 time stamps and durations
when temperature leaves the range specified
by the trip points
��512 bytes of general-purpose read/write NV
random access memory
��Communicates to host with a single digital
signal at 15.4kbits or 125kbits per second
using 1-Wire® protocol
��Fixed range: H: +15°C to +46°C;
Z: -5°C to +26°C
COMMON iButton FEATURES ��Digital identification and information by momentary contact
��Unique, factory-lasered and tested 64-bit reg-
istration number (8-bit family code + 48-bit
serial number + 8-bit CRC tester) assures ab-
solute traceability because no two parts are alike
��Multidrop controller for 1-Wire net
��Chip-based data carrier compactly stores
information
��Data can be accessed while affixed to object
��Durable stainless steel case engraved with
registration number withstands harsh
environments
��Easily affixed with self-stick adhesive
backing, latched by its flange, or locked with
a ring pressed onto its rim
��Presence detector acknowledges when reader
first applies voltage
��Meets UL#913 (4th Edit.). Intrinsically Safe
Apparatus: approved under Entity Concept
for use in Class I, Division 1, Group A, B, C,
and D Locations (application pending)
F5 MICROCAN GND
5.89D6213B2000FBC52B
1-WireThermochron®All dimensions are shown in millimeters.
ORDERING INFORMATION DS1921H-F5 +15°C to +46°C F5 iButton®
DS1921Z-F5 -5°C to +26°C F5 iButton
EXAMPLES OF ACCESSORIES DS9096P Self-Stick Adhesive Pad
DS9101 Multi-Purpose Clip
DS9093RA Mounting Lock Ring
DS9093A Snap-In Fob
DS1921H/Z
iButton DESCRIPTION The DS1921H/Z Thermochron™ iButtons are rugged, self-sufficient systems that measure temperature
and record the result in a protected memory section. The recording is done at a user-defined rate, both as a direct storage of temperature values as well as in the form of a histogram. Up to 2048 temperature
values taken at equidistant intervals ranging from 1 to 255 minutes can be stored. The histogram provides
64 data bins with a resolution of 0.5°C. If the temperature leaves a user-programmable range, the
DS1921H/Z will also record when this happened, for how long the temperature stayed outside the
permitted range, and if the temperature was too high or too low. Additional 512 bytes of read/write NV memory allow storing information pertaining to the object to which the DS1921H/Z is associated. Data is
transferred serially via the 1-Wire protocol, which requires only a single data lead and a ground return.
Every DS1921H/Z is factory-lasered with a guaranteed unique electrically readable 64-bit registration
number that allows for absolute traceability. The durable stainless steel package is highly resistant to
environmental hazards such as dirt, moisture, and shock. Accessories permit the DS1921H/Z to be mounted on almost any object, including containers, pallets, and bags.
APPLICATION The DS1921Z is an ideal device to monitor the temperature of any object it is attached to or shipped with,
such as fresh produce, medical drugs and supplies. It is also ideal for use in refrigerators. The DS1921H
is intended for monitoring the body temperature of humans and animals and for monitoring temperature
critical processes such as curing, powder coating, and painting. Alternatively, the DS1921H can be used for monitoring the temperature of clean rooms, and computer and equipment rooms. It can also aid in
calculating the proportional share of heating cost of each party in buildings with central heating. The
DS1921H has a fixed range of +15°C to +46°C. The DS1921Z has a fixed range of -5°C to +26°C. The
high resolution makes the DS1921H and DS1921Z suitable for scientific research and development. The read/write NV memory can store information such as shipping manifests, dates of manufacture, or other
relevant data written as ASCII or encrypted files.
OVERVIEW The block diagram in Figure 1 shows the relationships between the major control and memory sections of
the DS1921H/Z. The device has seven main data components: 1) 64-bit lasered ROM; 2) 256-bit scratch-
pad; 3) 4096-bit general-purpose SRAM; 4) 256-bit register page of timekeeping, control, and counter registers; 5) 96 bytes of alarm time stamp and duration logging memory; 6) 128 bytes of histogram mem-
ory; and 7) 2048 bytes of data-logging memory. Except for the ROM and the scratchpad, all other mem-
ory is arranged in a single linear address space. All memory reserved for logging purposes, counter reg-
isters and several other registers are read-only for the user. The timekeeping and control registers are
write-protected while the device is programmed for a mission.
The hierarchical structure of the 1-Wire protocol is shown in Figure 2. The bus master must first provide
one of the seven ROM function commands: 1) Read ROM; 2) Match ROM; 3) Search ROM; 4) Condi-
tional Search ROM; 5) Skip ROM; 6) Overdrive-Skip ROM; or 7) Overdrive-Match ROM. Upon
completion of an Overdrive ROM command byte executed at standard speed, the device will enter Overdrive mode, where all subsequent communication occurs at a higher speed. The protocol required for
these ROM function commands is described in Figure 13. After a ROM function command is
successfully executed, the memory functions become accessible and the master may provide any one of
the seven available commands. The protocol for these memory function commands is described in Figure
10. All data is read and written least significant bit first.
DS1921H/Z
DS1921H/Z BLOCK DIAGRAM Figure 1 3V Lithium
PARASITE POWER The block diagram (Figure 1) shows the parasite-powered circuitry. This circuitry “steals” power when-
ever the IO input is high. IO will provide sufficient power as long as the specified timing and voltage re-
quirements are met. The advantages of parasite power are two-fold: 1) by parasiting off this input, lithium is conserved, and 2) if the lithium is exhausted for any reason, the ROM may still be read normally.
64-BIT LASERED ROM Each DS1921 contains a unique ROM code that is 64 bits long. The first eight bits are a 1-Wire family
code. The next 36 bits are a unique serial number. The next 12 bits, called temperature range code, allow
distinguishing the DS1921H and DS1921Z from each other and from other DS1921 versions. The last eight bits are a CRC of the first 56 bits. See Figure 3 for details. The 1-Wire CRC is generated using a
polynomial generator consisting of a shift register and XOR gates as shown in Figure 4. The polynomial
is X8 + X5 + X4 + 1. Additional information about the Dallas 1-Wire Cyclic Redundancy Check is avail-
able in Application Note 27 and in the Book of DS19xx iButton Standards. The shift register bits are initialized to 0. Then starting with the least significant bit of the family code,
one bit at a time is shifted in. After the eighth bit of the family code has been entered, then the serial
DS1921H/Z
HIERARCHICAL STRUCTURE FOR 1-Wire PROTOCOL Figure 2
64-BIT LASERED ROM Figure 3 MSB LSB
MSB LSB MSB LSB MSB LSB MSB LSB
DS1921H/Z
1-Wire CRC GENERATOR Figure 4
MEMORY The memory map of the DS1921H/Z is shown in Figure 5. The 4096-bit general-purpose SRAM make up
pages 0 through 15. The timekeeping, control, and counter registers fill page 16, called Register Page (see
Figure 6). Pages 17 to 19 are assigned to storing the alarm time stamps and durations. The temperature
histogram bins begin at page 64 and use up to four pages. The temperature logging memory covers pages
128 to 191. Memory pages 20 to 63, 68 to 127, and 192 to 255 are reserved for future extensions. The scratchpad is an additional page that acts as a buffer when writing to the SRAM memory or the register
page. The memory pages 17 and higher are read-only for the user. They are written to or erased solely
under supervision of the on-chip control logic.
DS1921H/Z MEMORY MAP Figure 5 0000h to
01FFh Pages 0 to 15
0200h to
021Fh Page 16
0220h to
027Fh Pages 17 to 19
0280h to
07FFh Pages 20 to 63
0800h to
087Fh Pages 64 to 67
0880h to
0FFFh Pages 68 to 127
1000h to
17FFh Pages 128 to 191
1800h to
1FFFh Pages 192 to 255
DS1921H/Z
DS1921H/Z REGISTER PAGE MAP Figure 6 *The first entry in column ACCESS is valid between missions. The second entry shows the applicable access mode while a mission is in progress.
**While a mission is in progress, these addresses can be read. The first attempt to write to these registers
(even read-only ones), however, will end the mission and overwrite selected writeable registers.
TIMEKEEPING The RTC/alarm and calendar information is accessed by reading/writing the appropriate bytes in the register page, address 200h to 206h. Note that some bits are set to 0. These bits will always read 0
regardless of how they are written. The contents of the time, calendar, and alarm registers are in the
DS1921H/Z
RTC and RTC Alarm Register Bitmap
RTC/Calendar The RTC of the DS1921H/Z can run in either 12-hour or 24-hour mode. Bit 6 of the Hours Register (address 202h) is defined as the 12- or 24-hour mode select bit. When high, the 12-hour mode is selected.
In the 12-hour mode, bit 5 is the AM/PM bit with logic 1 being PM. In the 24-hour mode, bit 5 is the 20-
hour bit (20 to 23 hours).
To distinguish between the days of the week the DS1921H/Z includes a counter with a range from 1 to 7. The assignment of counter value to the day of week is arbitrary. Typically, the number 1 is assigned to a
Sunday (U.S. standard) or to a Monday (European standard).
The calendar logic is designed to automatically compensate for leap years. For every year value that is
either 00 or a multiple of 4 the device will add a 29th of February. This will work correctly up to (but not including) the year 2100.
The DS1921H/Z is Y2K-compliant. Bit 7 (CENT) of the Months Register at address 205h serves as a
century flag. When the Year Register rolls over from 99 to 00 the century flag will toggle. It is recom-
mended to write the century bit to a 1 when setting the RTC to a time/date between the years 2000 and 2099.
RTC Alarms The DS1921H/Z also contains a RTC alarm function. The alarm registers are located in registers 207h to
20Ah. The most significant bit of each of the alarm registers is a mask bit. When all of the mask bits are
logic 0, an alarm will occur once per week when the values stored in timekeeping registers 200h to 203h
match the values stored in the time of day alarm registers. Any alarm will set the Timer Alarm Flag (TAF) in the device's Status Register (address 214h). The bus master may set the Search Conditions in the
Control Register (address 20Eh) to identify devices with timer alarms by means of the Conditional Search
function (see ROM Function Commands).
DS1921H/Z
RTC Alarm Control
TEMPERATURE CONVERSION The DS1921H and DS1921Z measure temperatures with a resolution of 1/8th of a degree Celsius.
Temperature values are represented in a single byte as an unsigned binary number, which translates into a range of 32°C. The possible values are 0000 0000 (00h) through 1111 1111 (FFh). The codes 01h to FEh
are considered valid temperature readings. Since the DS1921H and DS1921Z have different starting
temperatures, the meaning of a binary temperature code depends on the device.
If a temperature conversion yields a temperature that is out-of-range, it will be recorded as 00h (if too low) or FFh (if too high). Since out-of-range results are accumulated in histogram bins 0 and 63 the data
in these bins is of limited value (see the Temperature Logging and Histogram section). For this reason the
specified temperature range of the DS1921H and DS1921Z is considered to begin at code 04h and end at
code FBh, which corresponds to histogram bins 1 to 62. With T[7..0] representing the decimal equivalent of a temperature reading, the temperature value is
calculated as
ϑ (°C) = T[7…0] / 8 + 14.500 (DS1921H)
ϑ (°C) = T[7…0] / 8 - 5.500 (DS1921Z)
This equation is valid for converting temperature readings stored in the datalog memory as well as for
data read from the forced temperature conversion readout Register (address 211h). To specify the high or low temperature alarm thresholds, this equation needs to be resolved to
T[7…0] = 8 * ϑ (°C) -116 (DS1921H)
T[7…0] = 8 * ϑ (°C) + 44 (DS1921Z)
A value of 23°C, for example, thus translates into 68 decimal or 44h for the DS1921H, and 228 decimal
or E4h for the DS1921Z. This corresponds to the binary patterns 0100 0100 and 1110 0100 respectively,
which could be written to a Temperature Alarm Register (address 020Bh and 020Ch, respectively).
Temperature Alarm Register Map
DS1921H/Z
SAMPLE RATE
The content of the Sample Rate Register (address 020Dh) determines how many minutes the temperature
conversions are apart from each other during a mission. The sample rate may be any value from 1 to 255,
coded as an unsigned 8-bit binary number. If the memory has been cleared (Status Register bit MEMCLR
= 1) and a mission is enabled (Status Register bit EM = 0), writing a non-zero value to the Sample Rate
Register will start a mission. For a full description of the correct sequence of steps to start a temperature-logging mission see sections Missioning or Missioning Example.
Sample Rate Register Map
CONTROL REGISTER The DS1921H/Z is set up for its operation by writing appropriate data to its special function registers that
are located in the register page. Several functions that are controlled by a single bit only are combined
into a single byte called Control Register (address 20Eh). This register can be read and written. If the
device is programmed for a mission, writing to the Control Register will end the mission and change the
register contents.
Control Register Bitmap
The functional assignments of the individual bits are explained in the table below. Bit 5 has no function.
It always reads 0 and cannot be written to 1.
Control Register Details
DS1921H/Z
Mission Start Delay Counter
The content of the Mission Start Delay Counter determines how many minutes the device will wait before
starting the logging process. The mission start delay value is stored as unsigned 16-bit integer number at addresses 212h (low byte) and 213h (high byte). The maximum delay is 65535 minutes, equivalent to 45
days, 12 hours, and 15 minutes.
For a typical mission, the Mission Start Delay is 0. If a mission is too long for a single DS1921H/Z to store all temperature readings at the selected sample rate, one can use several devices, staggering the
Mission Start Delay to record the full period. In this case, the RO bit in the control register (address
020Eh) must be set to 0 to prevent overwriting of the recorded temperature log after the datalog memory
is full. See Mission Start and Logging Process description and flow chart for details.
Status Register
The Status Register holds device status information and alarm flags. The register is located at address
214h. Writing to this register will not necessarily end a mission.
Status Register Bitmap
The functional assignments of the individual bits are explained in the table below. The bits MIP, TLF,
THF and TAF can only be written to 0. All other bits are read-only. Bit 3 has no function.
Status Register Details
DS1921H/Z
MISSION TIME STAMP
The Mission Time Stamp indicates the time and date of the first temperature conversion of a mission.
Subsequent temperature conversions will take place as many minutes apart from each other as specified
by the value in the Sample Rate Register. Mission samples occur on minute boundaries. Mission Time Stamp Register Bitmap 10 Years
MISSION SAMPLES COUNTER
The Mission Samples Counter indicates how many temperature measurements have taken place during the current mission in progress (if MIP = 1) or during the latest mission (if MIP = 0). The value is stored
DS1921H/Z
Mission Samples Counter Register Map
DEVICE SAMPLES COUNTER
The Device Samples Counter indicates how many temperature measurements have taken place since the
device was assembled at the factory. The value is stored as an unsigned 24-bit integer number. The maximum number that can be represented in this format is 16777215, which is higher than the expected
lifetime of the DS1921H/Z iButton. This counter cannot be reset under software control.
Device Samples Counter Register Map
TEMPERATURE LOGGING AND HISTOGRAM
Once setup for a mission, the DS1921H/Z logs the temperature measurements simultaneously byte after
byte in the datalog memory as well as in histogram form in the histogram memory. The datalog memory
is able to store 2048 temperature values measured at equidistant time points. The first temperature value of a mission is written to address location 1000h of the datalog memory, the second value to address
location 1001h and so on. Knowing the starting time point (Mission Time Stamp), the interval between
temperature measurements, the Mission Samples Counter, and the rollover setting, one can reconstruct
the time and date of each measurement stored in the datalog. There are two alternatives to the way the DS1921H/Z will behave after the 2048 bytes of datalog memory
is filled with data. With rollover disabled (RO = 0), the device will fill the datalog memory with the first
2048 mission samples. Additional mission samples are not logged in the datalog, but the histogram, and
temperature alarm memory continue to update. With rollover enabled (RO = 1), the datalog will wrap
around, and overwrite previous data starting at 1000h for the every 2049th mission sample. In this mode the device stores the last 2048 mission samples.
For the temperature histogram, the DS1921H/Z provides 64 bins that begin at memory address 0800h.
Each bin consists of a 16-bit, non-rolling-over binary counter that is incremented each time a temperature
value acquired during a mission falls into the range of the bin. The least significant byte of each bin is stored at the lower address. Bin 0 begins at memory address 0800h, bin 1 at 0802h, and so on up to
087Eh for bin 63, as shown in Figure 7. The number of the bin to be updated after a temperature
conversion is determined by cutting off the two least significant bits of the binary temperature value. Out
of range values are range locked and counted as 00h or FFh.
DS1921H/Z
HISTOGRAM BIN AND TEMPERATURE CROSS-REFERENCE Figure 7
Since each data bin is 2 bytes it can increment up to 65535 times. Additional measurements for a bin that
has already reached its maximum value will not be counted; the bin counter will remain at its maximum
value. With the fastest sample rate of one sample every minute, a 2-byte bin is sufficient for up to 45 days
if all temperature readings fall into the same bin.
TEMPERATURE ALARM LOGGING
For some applications it may be essential to not only record temperature over time and the temperature
histogram, but also record when exactly the temperature has exceeded a predefined tolerance band and
for how long the temperature stayed outside the desirable range. The DS1921H/Z can log high and low
durations. The tolerance band is specified by means of the Temperature Alarm Threshold Registers, addresses 20Bh and 20Ch in the register page. One can set a high and a low temperature threshold. See
section Temperature Conversion for the data format the temperature has to be written in. As long as the
temperature values stay within the tolerance band (i.e., are higher than the low threshold and lower than
the high threshold), the DS1921H/Z will not record any temperature alarm. If the temperature during a
mission reaches or exceeds either threshold, the DS1921H/Z will generate an alarm and set either the Temperature High Flag (THF) or the Temperature Low Flag (TLF) in the Status Register (address 214h).
This way, if the search conditions (address 20Eh) are set accordingly, the master can quickly identify
devices with temperature alarms by means of the Conditional Search function (see ROM Function
Commands). The device also generates a time stamp of when the alarm occurred and begins recording the
duration of the alarming temperature.
DS1921H/Z
and periods (12 periods for too hot and 12 for too cold). The date and time of each of these periods can be
determined from the Mission Time Stamp and the time distance between each temperature reading.
ALARM TIME STAMPS AND DURATIONS ADDRESS MAP Figure 8
The alarm time stamp is a copy of the Mission Samples Counter when the alarm first occurred. The least
significant byte is stored at the lower address. One address higher than the time stamp the DS1921H/Z
maintains a 1-byte duration counter that stores the number of samples the temperature was found to be beyond the threshold. If this counter has reached its limit after 255 consecutive temperature readings and
the temperature has not yet returned to within the tolerance band, the device will issue another time stamp
at the next higher alarm location and open another counter to record the duration. If the temperature
returns to normal before the counter has reached its limit, the duration counter of the particular time
stamp will not increment any further. Should the temperature again cross this threshold, it will be recorded at the next available alarm location. This algorithm is implemented for the low as well as for the
high temperature threshold.
MISSIONING
The typical task of the DS1921H/Z is recording the temperature of a temperature-sensitive object. Before
the device can perform this function, it needs to be configured. This procedure is called missioning.
First of all, DS1921H/Z needs to have its RTC set to valid time and date. This reference time may be
UTC (also called GMT, Greenwich Mean Time) or any other time standard that was chosen for the
application. The clock must be running (EOSC = 0) for at least one second. Setting a RTC alarm is
optional. The memory assigned to storing alarm time stamps and durations, temperature histogram, as well as the Mission Time Stamp, Mission Samples Counter, Mission Start Delay, and Sample Rate must
be cleared using the Memory Clear command. In case there were temperature alarms in the previous
mission, the TLF and THF flags need to be cleared manually. To enable the device for a mission, the EM
flag must be set to 0. These are general settings that have to be made regardless of the type of object to be
monitored and the duration of the mission.
Next, the low temperature and high temperature thresholds to specify the temperature tolerance band
DS1921H/Z
The state of the Search Condition bits in the Control Register does not affect the mission. If multiple
devices are connected to form a 1-Wire net, the setting of the search condition will enable devices to
participate in the conditional search if certain events such as timer or temperature alarm have occurred. Details on the search conditions are found in the section ROM Function Commands later in this document
and in the Control Register description.
The setting of the RO bit (rollover enable) and sample rate depends on the duration of the mission and the
monitoring requirements. If the most recent temperature history is important, the rollover should be enabled (RO = 1). Otherwise, one should estimate the duration of the mission in minutes and divide the
number by 2048 to calculate the value of the sample rate (number of minutes between temperature
conversions). If the estimated duration of a mission is 10 days (= 14400 minutes) for example, then the
2048-byte capacity of the datalog memory would be sufficient to store a new value every 7 minutes. If the
datalog memory of the DS1921H/Z is not large enough to store all temperature readings, one can use several devices and set the Mission Start Delay to values that make the second device start recording as
soon as the memory of the first device is full, and so on. The RO-bit needs to be set to 0 to disable
rollover that would otherwise overwrite the recorded temperature log.
After the RO bit and the Mission Start Delay are set, the Sample Rate Register is the last element of data that is written. The sample rate may be any value from 1 to 255, coded as an unsigned 8-bit binary
number. As soon as the sample rate is written, the DS1921H/Z will set the MIP flag and clear the
MEMCLR flag. After as many minutes as specified by the Mission Start Delay are over, the device will
wait for the next minute boundary, then wake up, copy the current time and date to the Mission Time
Stamp Register, and make the first temperature conversion of the mission. This increments both the Mission Samples Counter and Device Samples Counter. All subsequent temperature measurements are
taken on minute boundaries specified by the value in the Sample Rate Register. One may read the
memory of the DS1921H/Z to watch the mission as it progresses. Care should be taken to avoid memory
access conflicts. See section Memory Access Conflicts for details.
MEMORY/CONTROL FUNCTION COMMANDS
The Memory/Control Function Flow Chart (Figure 10) describes the protocols necessary for accessing
the memory and the special function registers of the DS1921H/Z. An example on how to use these and
other functions to set up the DS1921H/Z for a mission is included at the end of this document, preceding
the Electrical Characteristics section. The communication between master and DS1921H/Z takes place
either at regular speed (default, OD = 0) or at Overdrive Speed (OD = 1). If not explicitly set into the Overdrive mode, the DS1921H/Z assumes regular speed. Internal memory access during a mission has
priority over external access through the 1-Wire interface. This can affect the Read Memory commands
described below. See section Memory Access Conflicts for details.
ADDRESS REGISTERS AND TRANSFER STATUS
Because of the serial data transfer, the DS1921H/Z employs three address registers, called TA1, TA2, and E/S (Figure 9). Registers TA1 and TA2 must be loaded with the target address to which the data will be
written or from which data will be sent to the master upon a Read command. Register E/S acts like a byte
counter and transfer status register. It is used to verify data integrity with Write commands. Therefore, the
master only has read access to this register. The lower 5 bits of the E/S Register indicate the address of
the last byte that has been written to the scratchpad. This address is called Ending Offset. Bit 5 of the E/S
DS1921H/Z
byte offset. If the target address for a Write command is 13Ch, for example, then the scratchpad will store
incoming data beginning at the byte offset 1Ch and will be full after only 4 bytes. The corresponding
ending offset in this example is 1Fh. For best economy of speed and efficiency, the target address for
writing should point to the beginning of a new page, (i.e., the byte offset will be 0). Thus, the full 32-byte capacity of the scratchpad is available, resulting also in the ending offset of 1Fh. However, it is possible
to write 1 or several contiguous bytes somewhere within a page. The ending offset together with the
Partial and Overflow Flag is mainly a means to support the master checking the data integrity after a
Write command. The highest valued bit of the E/S Register, called AA or Authorization Accepted,
indicates that a valid copy command for the scratchpad has been received and executed. Writing data to the scratchpad clears this flag.
ADDRESS REGISTERS Figure 9
Bit # 7 6 5 4 3 2 1 0
Target Address (TA1) T1 T0
Target Address (TA2)
Ending Address with
Data Status (E/S) (Read Only)
WRITING WITH VERIFICATION
To write data to the DS1921H/Z, the scratchpad has to be used as intermediate storage. First, the master issues the Write Scratchpad command to specify the desired target address, followed by the data to be
written to the scratchpad. In the next step, the master sends the Read Scratchpad command to read the
scratchpad and to verify data integrity. As preamble to the scratchpad data, the DS1921H/Z sends the
requested target address TA1 and TA2 and the contents of the E/S Register. If the PF flag is set, data did
not arrive correctly in the scratchpad. The master does not need to continue reading; it can start a new trial to write data to the scratchpad. Similarly, a set AA flag indicates that the Write command was not
recognized by the device. If everything went correctly, both flags are cleared and the ending offset
indicates the address of the last byte written to the scratchpad. Now the master can continue verifying
every data bit. After the master has verified the data, it has to send the Copy Scratchpad command. This
command must be followed exactly by the data of the three address registers TA1, TA2 and E/S as the master has read them verifying the scratchpad. As soon as the DS1921H/Z has received these bytes, it
will copy the data to the requested location beginning at the target address.
Write Scratchpad Command [0Fh]
After issuing the Write Scratchpad command, the master must first provide the 2-byte target address,
followed by the data to be written to the scratchpad. The data will be written to the scratchpad starting at the byte offset (T4:T0). The ending offset (E4:E0) will be the byte offset at which the master stops writ-
ing data. Only full data bytes are accepted. If the last data byte is incomplete, its content will be ignored
and the partial byte flag (PF) will be set.
DS1921H/Z
byte sent by the master. This CRC is generated using the CRC16 polynomial by first clearing the CRC
generator and then shifting in the command code (0Fh) of the Write Scratchpad command, the Target
Addresses TA1 and TA2 as supplied by the master and all the data bytes. The master may end the Write
Scratchpad command at any time. However, if the ending offset is 11111b, the master may send 16 read time slots and will receive an inverted CRC16 generated by the DS1921H/Z.
The range 200h to 213h of the register page is protected during a mission. See Figure 6, Register
Page Map, for the access type of the individual registers between and during missions.
Read Scratchpad Command [AAh]
This command is used to verify scratchpad data and target address. After issuing the Read Scratchpad command, the master begins reading. The first 2 bytes will be the target address. The next byte will be the
ending offset/data status byte (E/S) followed by the scratchpad data beginning at the byte offset (T4:T0),
as shown in Figure 9. Regardless of the actual ending offset, the master may read data until the end of the
scratchpad after which it will receive an inverted CRC16 of the command code, Target Addresses TA1
and TA2, the E/S byte, and the scratchpad data starting at the target address. After the CRC is read, the bus master will read logical 1s from the DS1921H/Z until a reset pulse is issued.
Copy Scratchpad [55h]
This command is used to copy data from the scratchpad to the writable memory sections. Applying Copy
Scratchpad to the Sample Rate Register can start a mission provided that several preconditions are met.
See Mission Start and Logging Process description and the flow chart in Figure 11 for details. After issuing the Copy Scratchpad command, the master must provide a 3-byte authorization pattern, which can
be obtained by reading the scratchpad for verification. This pattern must exactly match the data contained
in the three address registers (TA1, TA2, E/S, in that order). If the pattern matches, the AA
(Authorization Accepted) flag will be set and the copy will begin. A pattern of alternating 1s and 0s will
be transmitted after the data has been copied until the master issues a reset pulse. While the copy is in progress any attempt to reset the part will be ignored. Copy typically takes 2µs per byte.
The data to be copied is determined by the three address registers. The scratchpad data from the begin-
ning offset through the ending offset will be copied, starting at the target address. Anywhere from 1 to 32
bytes may be copied to memory with this command. The AA flag will remain at logic 1 until it is cleared by the next Write Scratchpad command. Note that Copy Scratchpad when applied to the address range
200h to 213h during a mission will end the mission.
Read Memory [F0h]
The Read Memory command may be used to read the entire memory. After issuing the command, the
master must provide the 2-byte target address. After the 2 bytes, the master reads data beginning from the target address and may continue until the end of memory, at which point logic 0s will be read. It is im-
portant to realize that the target address registers will contain the address provided. The ending offset/data
status byte is unaffected.
The hardware of the DS1921H/Z provides a means to accomplish error-free writing to the memory sec-tion. To safeguard data in the 1-Wire environment when reading and to simultaneously speed up data
transfers, it is recommended to packetize data into data packets of the size of one memory page each.
Such a packet would typically store a 16-bit CRC with each page of data to ensure rapid, error-free data
transfers that eliminate having to read a page multiple times to verify whether if the received data is cor-
DS1921H/Z
MEMORY/CONTROL FUNCTION FLOW CHART Figure 10-1 nd Part
From Figure 10nd Part
DS1921H/Z
MEMORY/CONTROL FUNCTION FLOW CHART Figure 10-2 rd Part
From Figure 10st Part
To Figure 10st Part
From Figure 10rd Part
DS1921H/Z
MEMORY/CONTROL FUNCTION FLOW CHART Figure 10-3
From Figure 10nd Part
To Figure 10nd Part
To Figure 10th Part
From Figure 10th Part
DS1921H/Z
MEMORY/CONTROL FUNCTION FLOW CHART Figure 10-4
From Figure 10rd Part
To Figure 10rd Part
DS1921H/Z
Read Memory with CRC [A5h]
The Read Memory with CRC command is used to read memory data that cannot be packetized, such as
the register page and the data recorded by the device during a mission. The command works essentially
the same way as the simple Read Memory, except for the 16-bit CRC that the DS1921H/Z generates and
transmits following the last data byte of a memory page. After having sent the command code of the Read Memory with CRC command, the bus master sends a 2-
byte address (TA1 = T7:T0, TA2 = T15:T8) that indicates a starting byte location. With the subsequent
read data time slots the master receives data from the DS1921H/Z starting at the initial address and
continuing until the end of a 32-byte page is reached. At that point the bus master will send 16 additional
read data time slots and receive an inverted 16-bit CRC. With subsequent read data time slots the master will receive data starting at the beginning of the next page followed again by the inverted CRC for that
page. This sequence will continue until the bus master resets the device.
With the initial pass through the Read Memory with CRC flow, the 16-bit CRC value is the result of
shifting the command byte into the cleared CRC generator followed by the two address bytes and the contents of the data memory. Subsequent passes through the Read Memory with CRC flow will generate
a 16-bit CRC that is the result of clearing the CRC generator and then shifting in the contents of the data
memory page. After the 16-bit CRC of the last page is read, the bus master will receive logical 0s from
the DS1921H/Z and inverted CRC16s at page boundaries until a reset pulse is issued. The Read Memory
with CRC command sequence can be ended at any point by issuing a reset pulse.
Clear Memory [3Ch]
The Clear Memory command is used to clear the Sample Rate, Mission Start Delay, Mission Time
Stamp, and Mission Samples Counter in the register page and the Temperature Alarm Memory and the
Temperature Histogram Memory. These memory areas must be cleared for the device to be set up for
another mission. The Clear Memory command does not clear the datalog memory or the temperature and timer alarm flags in the Status Register. The RTC oscillator must be on and have counted at least 1
second, before issuing the command. For the Clear Memory command to function the EMCLR bit in
Control Register must be set to 1, and the Clear Memory command must be issued with the very next
access to the device’s memory functions. Issuing any other memory function command will reset the
EMCLR bit. The Clear Memory process takes 500µs. When the command is completed the MEMCLR bit in the Status Register will read 1 and the EMCLR bit will be 0.
Convert Temperature [44h]
If a mission is not in progress (MIP = 0) the Convert Temperature command can be issued to measure the
current temperature of the device. The result of the temperature conversion will be found at memory
address 211h in the register page. This command takes maximum 360ms to complete. During this time the device remains fully accessible for memory/control and ROM function commands.
