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MAX6648MUA+-MAX6692MUA+-MAX6692MUA+T-MAX6692YMUA+
Precision SMBus-Compatible Remote/Local Temperature Sensors with Overtemperature Alarms
General DescriptionThe MAX6648/MAX6692 are precise, two-channel digi-
tal temperature sensors. They accurately measure the
temperature of their own die and a remote PN junction,
and report the temperature in digital form using a 2-wire
serial interface. The remote PN junction is typically the
emitter-base junction of a common-collector PNP on a
CPU, FPGA, or ASIC.
The 2-wire serial interface accepts standard System
Management Bus (SMBus)™ write byte, read byte,
send byte, and receive byte commands to read the
temperature data and to program the alarm thresholds.
To enhance system reliability, the MAX6648/MAX6692
include an SMBus timeout. A fault queue prevents the
ALERTand OVERToutputs from setting until a fault has
been detected one, two, or three consecutive times
(programmable).
The MAX6648/MAX6692 provide two system alarms:
ALERTand OVERT. ALERTasserts when any of four tem-
perature conditions are violated: local overtemperature,
remote overtemperature, local undertemperature, or
remote undertemperature. OVERTasserts when the tem-
perature rises above the value in either of the two OVERT
limit registers. The OVERToutput can be used to activate
a cooling fan, or to trigger a system shutdown.
Measurements can be done autonomously, with the
conversion rate programmed by the user, or in a single-
shot mode. The adjustable conversion rate allows the
user to optimize supply current and temperature
update rate to match system needs.
Remote accuracy is ±0.8°C maximum error between
+25°C and +125°C with no calibration needed. The
MAX6648/MAX6692 operate from -55°C to +125°C, and
measure temperatures between 0°C and +125°C. The
MAX6648 is available in an 8-pin µMAX®package, and the
MAX6692 is available in 8-pin µMAX and SO packages.
ApplicationsDesktop Computers
Notebook Computers
Servers
Thin Clients
Workstations
Test and Measurement
Multichip Modules
FeaturesDual Channel Measures Remote and Local
Temperature+0.125°C ResolutionHigh Accuracy ±0.8°C (max) from +25°C to +125°C
(Remote), and ±2°C (max) from +60°C to +100°C
(Local)Two Alarm Outputs: ALERTand OVERTTwo Default OVERTThresholds Available
MAX6648: +110°C
MAX6692: +85°CProgrammable Conversion Rate SMBus-Compatible InterfaceSMBus TimeoutProgrammable Under/Overtemperature Alarm
ThresholdsCompatible with 90nm, 65nm, and 45nm Process
Technology
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Localemperature Sensors with Overtemperature Alarms
Ordering InformationVCC
DXP
DXN
10kΩ EACH
CLOCK
TO FAN DRIVER OR
SYSTEM SHUTDOWN
3.3V
DATA
INTERRUPTED TO μP
200Ω0.1μF
SDA
SCLK
ALERT
GND
2200pF
OVERT
MAX6648
MAX6692
Typical Operating Circuit19-2545; Rev 4; 6/08
PARTPIN-
PACKAGE
MEASURED
TEMP RANGE
MAX6648MUA8 µMAX0°C to +125°C
MAX6648YMUA8 µMAX0°C to +125°C
MAX6692MUA8 µMAX0°C to +125°C
MAX6692MSA8 SO0°C to +125°C
MAX6692YMUA8 µMAX0°C to +125°C
MAX6692YMSA8 SO0°C to +125°C
SMBus is a trademark of Intel Corp.
Pin Configuration and Functional Diagram appear at end of
Note:All devices operate over the -55°C to +125°C temperature
range.
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Localemperature Sensors with Overtemperature Alarms
ABSOLUTE MAXIMUM RATINGSStresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
(All voltages referenced to GND.)
VCC...........................................................................-0.3V to +6V
DXP.............................................................-0.3V to (VCC+ 0.3V)
DXN.......................................................................-0.3V to +0.8V
SCLK, SDA, ALERT, OVERT.....................................-0.3V to +6V
SDA, ALERT, OVERTCurrent.............................-1mA to +50mA
DXN Current.......................................................................±1mA
Continuous Power Dissipation (TA= +70°C)
8-Pin µMAX (derate 5.9mW/°C above +70°C).............471mW
8-Pin SO (derate 5.9mW/°C above +70°C)..................471mW
ESD Protection (all pins, Human Body Model)................±2000V
Junction Temperature......................................................+150°C
Operating Temperature Range.........................-55°C to +125°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
ELECTRICAL CHARACTERISTICS(VCC= 3.0V to 5.5V, TA= -55°C to +125°C, unless otherwise specified. Typical values are at VCC= 3.3V and TA= +85°C.) (Note 1)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSSupply VoltageVCC3.05.5V
0.125°CTemperature Resolution10Bits
VCC = 3.3V,
TA = +85°CTRJ = +25°C to +125°C-0.8+0.8
TRJ = +60°C to +100°C-1.0+1.0VCC = 3.3V,
+60°C ≤ TA ≤
+100°CTRJ = 0°C to +125°C-1.6+1.6
Remote Temperature Error
n = 1.008
VCC = 3.3V, +0°C
≤ TA ≤ +100°CTRJ = 0°C to +125°C-3.0+3.0
TA = +60°C to +100°C-2.0+2.0Local Temperature ErrorVCC = 3.3VTA = 0°C to +125°C-3.0+3.0°C
TA = + 60°C to + 100°C - 4.0Local Temperature Error
(MAX6648Y/MAX6692Y)V C C = 3.3V TA = 0°C to +125°C-4.4°C
Supply Sensitivity of Temperature
Error±0.2°C/V
Undervoltage Lockout (UVLO)
ThresholdUVLOFalling edge of VCC disables ADC2.42.72.95V
UVLO Hysteresis90mV
Power-On-Reset (POR) ThresholdVCC falling edge2.0V
POR Threshold Hysteresis90mV
Standby Supply CurrentSMBus static3.512µA
Operating CurrentDuring conversion0.450.8mA
0.25 conversions per second4080Average Operating Current2 conversions per second250400µA
Conversion TimetCONVFrom stop bit to conversion completion95125156ms
Conversion Time Error-25+25%
DXP and DXN Leakage CurrentStandby mode100nA
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Localemperature Sensors with Overtemperature Alarms
ELECTRICAL CHARACTERISTICS (continued)(VCC= 3.0V to 5.5V, TA= -55°C to +125°C, unless otherwise specified. Typical values are at VCC= 3.3V and TA= +85°C.) (Note 1)
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITSHigh level80100120Remote-Diode Source CurrentIRJLow level81012µA
ALERT, OVERT
ISINK = 1mA0.4Output Low VoltageISINK = 4mA0.6V
Output High Leakage CurrentVOH = 5.5V1µA
SMBus-COMPATIBLE INTERFACE (SCLK AND SDA)Logic Input Low VoltageVIL0.8V
VCC = 3.0V2.2Logic Input High VoltageVIHVCC = 5.5V2.6V
Input Leakage CurrentILEAKVIN = GND or VCC-1+1µA
Output Low-Sink CurrentISINKVOL = 0.6V6mA
Input CapacitanceCIN5pF
SMBus-COMPATIBLE TIMING (Note 2)Serial Clock FrequencyfSCLK(Note 3)100kHz
Bus Free Time Between STOP and
START ConditiontBUF4.7µs
START Condition Setup Time4.7µs
Repeat START Condition Setup
TimetSU:STA90% to 90%50ns
START Condition Hold TimetHD:STA10% of SDA to 90% of SCLK4µs
STOP Condition Setup TimetSU:STO90% of SCLK to 90% of SDA4µs
Clock Low PeriodtLOW10% to 10%4.7µs
Clock High PeriodtHIGH90% to 90%4µs
Data Setup TimetHD:DAT(Note 4)250µs
Receive SCLK/SDA Rise TimetR1µs
Receive SCLK/SDA Fall TimetF300ns
Pulse Width of Spike SuppressedtSP050ns
SMBus TimeouttTIMEOUTSDA low period for interface reset253755ms
Note 1:All parameters tested at a single temperature. Specifications over temperature are guaranteed by design.
Note 2:Timing specifications guaranteed by design.
Note 3:The serial interface resets when SCLK is low for more than tTIMEOUT.
Note 4:A transition must internally provide at least a hold time to bridge the undefined region (300ns max) of SCLK’s falling edge.
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Localemperature Sensors with Overtemperature Alarms
Typical Operating Characteristics(VCC= 3.3V, TA= +25°C, unless otherwise noted.)
STANDBY SUPPLY CURRENT
vs. SUPPLY VOLTAGEMAX6648/92 toc01
SUPPLY VOLTAGE (V)
STANDBY SUPPLY CURRENT (
OPERATING SUPPLY CURRENT
vs. CONVERSION RATE
MAX6648/92 toc02
CONVERSION RATE (Hz)
OPERATING SUPPLY CURRENT (
REMOTE TEMPERATURE ERROR
vs. REMOTE-DIODE TEMPERATURE
MAX6648/92 toc03
TEMPERATURE (°C)
TEMPERATURE ERROR (
TA = +85°C
FAIRCHILD 2N3906
LOCAL TEMPERATURE ERROR
vs. DIE TEMPERATUREMAX6648/92 toc05
TEMPERATURE (°C)
TEMPERATURE ERROR (
REMOTE TEMPERATURE ERROR
vs. 45nm REMOTE DIODE TEMPERATURE
MAX6648/92 toc04
TEMPERATURE (°C)
TEMPERATURE ERROR (
°C)807060100
TEMPERATURE ERROR
vs. POWER-SUPPLY NOISE FREQUENCYMAX6648/92 toc06
FREQUENCY (Hz)
TEMPERATURE ERROR (
°C)
10k1k110100
0.1100k
LOCAL ERROR
REMOTE ERROR
VIN = SQUARE WAVE APPLIED TO VCC
WITH NO 0.1μF VCC CAPACITOR
TEMPERATURE ERROR
vs. COMMON-MODE NOISE FREQUENCYMAX6648/92 toc07
TEMPERATURE ERROR (
100k10k101001k1
REMOTE ERROR
LOCAL ERROR
VIN = AC-COUPLED TO DXN
VIN = 100mVP-P
TEMPERATURE ERROR
vs. DIFFERENTIAL-MODE NOISE FREQUENCY
MAX6648/92 toc08
TEMPERATURE ERROR (
100k10k101001k1
VIN = 20mVP-P SQUARE WAVE
APPLIED TO DXP-DXN
TEMPERATURE ERROR
vs. DXP-DXN CAPACITANCEMAX6648/92 toc09
TEMPERATURE ERROR (
0.100100.000
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Localemperature Sensors with Overtemperature Alarms
Detailed DescriptionThe MAX6648/MAX6692 are temperature sensors
designed to work in conjunction with a microprocessor
or other intelligence in thermostatic, process-control, or
monitoring applications. Communication with the
MAX6648/MAX6692 occurs through the SMBus-com-
patible serial interface and dedicated alert pins. ALERT
asserts if the measured local or remote temperature is
greater than the software-programmed ALERThigh
limit or less than the ALERTlow limit. ALERTalso
asserts if the remote-sensing diode pins are shorted or
unconnected. The overtemperature alarm, OVERT,
asserts if the software-programmed OVERTlimit is
exceeded. OVERTcan be connected to fans, a system
shutdown, a clock throttle control, or other thermal-
management circuitry.
The MAX6648/MAX6692 convert temperatures to digital
data either at a programmed rate or in single conver-
sions. Temperature data is represented as 10 bits plus
sign, with the LSB equal to 0.125°C. The “main” tempera-
ture data registers (at addresses 00h and 01h) are 8-bit
registers that represent the data as 7 bits with the final
MSB indicating the diode fault status (Table 1). The
remaining 3 bits of temperature data are available in the
“extended” registers at addresses 11h and 10h (Table 2).
ADC and MultiplexerThe averaging ADC integrates over a 60ms period
(each channel, typically), with excellent noise rejection.
The multiplexer automatically steers bias currents
through the remote and local diodes. The ADC and
associated circuitry measure each diode’s forward volt-
age and compute the temperature based on this volt-
age. Both channels are automatically converted once
the conversion process has started, either in free-run-
ning or single-shot mode. If one of the two channels is
not used, the device still performs both measurements,
and the user can ignore the results of the unused chan-
Pin Description
PINNAMEFUNCTION1VCCSupply Voltage Input, 3V to 5.5V. Bypass VCC to GND with a 0.1µF capacitor. A 200Ω series
resistor is recommended but not required for additional noise filtering.DXP
Combined Remote-Diode Current Source and A/D Positive Input for Remote-Diode Channel. DO
NOT LEAVE DXP DISCONNECTED; connect DXP to DXN if no remote diode is used. Place a2200pF capacitor between DXP and DXN for noise filtering.
3DXNCombined Remote-Diode Current Sink and A/D Negative Input. DXN is internally biased to one
diode drop above ground.OVERTOvertemperature Alert/Interrupt Output, Open Drain. OVERT is logic low when the temperature is
above the software-programmed threshold.GNDGroundALERT
SMBus Alert (Interrupt) Output, Open Drain. ALERT asserts when temperature exceeds user-set
limits (high or low temperature). ALERT stays asserted until acknowledged by either reading the
status register or by successfully responding to an alert response address, provided that the fault
condition no longer exists. See the ALERT Interrupts section.SDASMBus Serial-Data Input/Output, Open DrainSCLKSMBus Serial-Clock Input
TEMP (°C)DIGITAL OUTPUT1300 111 1111
1270 111 1111
1260 111 11110 001 10010 000 00000 000 00000 000 0000
-250 000 0000
Diode fault
(short or open)1 000 0000
Table 1. Main Temperature Data Register
Format (00h, 01h)
MAX6648/MAX6692nel. If the remote-diode channel is unused, connect
DXP to DXN rather than leaving the pins open.
The DXN input is biased to one VBEabove ground by
an internal diode to prepare the ADC inputs for a differ-
ential measurement. The worst-case DXP-DXN differen-
tial input voltage range is 0.25V to 0.95V. Excess
resistance in series with the remote diode causes
+0.5°C (typ) error per ohm.
A/D Conversion SequenceA conversion sequence consists of a local temperature
measurement and a remote temperature measurement.
Each time a conversion begins, whether initiated auto-
matically in the free-running autonomous mode (RUN= 0)
or by writing a one-shot command, both channels are
converted, and the results of both measurements are
available after the end of a conversion. A BUSY status bit
in the status byte indicates that the device is performing a
new conversion. The results of the previous conversion
are always available, even if the ADC is busy.
Low-Power Standby ModeStandby mode reduces the supply current to less than
10µA by disabling the ADC and timing circuitry. Enter
standby mode by setting the RUNbit to 1 in the configu-
ration byte register (Table 6). All data is retained in mem-
ory, and the SMBus interface is active and listening for
SMBus commands. Standby mode is not a shutdown
mode. With activity on the SMBus, the device draws more
supply current (see Typical Operating Characteristics). In
standby mode, the MAX6648/MAX6692 can be forced to
perform A/D conversions through the one-shot command,
regardless of the RUNbit status.
If a standby command is received while a conversion is
in progress, the conversion cycle is truncated, and the
data from that conversion is not latched into a tempera-
ture register. The previous data is not changed and
remains available.
Supply-current drain during the 125ms conversion peri-
od is 500µA (typ). Slowing down the conversion rate
reduces the average supply current (see Typical
Operating Characteristics). Between conversions, the
conversion rate timer consumes about 25µA of supply
current. In standby mode, supply current drops to
about 3µA.
SMBus Digital Interface From a software perspective, the MAX6648/MAX6692
appear as a set of byte-wide registers that contain tem-
perature data, alarm threshold values, and control bits.
A standard SMBus-compatible 2-wire serial interface is
used to read temperature data and write control bits
and alarm threshold data. These devices respond to the
The MAX6648/MAX6692 employ four standard SMBus
protocols: write byte, read byte, send byte, and receive
byte (Figures 1, 2, and 3). The shorter receive byte proto-
col allows quicker transfers, provided that the correct
data register was previously selected by a read byte
instruction. Use caution when using the shorter protocols
in multimaster systems, as a second master could over-
write the command byte without informing the first master.
Temperature data can be read from the read internal
temperature (00h) and read external temperature (01h)
registers. The temperature data format for these regis-
ters is 7 bits plus 1 bit, indicating the diode fault status
for each channel, with the LSB representing 1°C (Table
1). The MSB is transmitted first.
An additional 3 bits can be read from the read external
extended temperature register (10h), which extends the
data to 10 bits plus sign and the resolution to 0.125°C
per LSB (Table 2). An additional 3 bits can be read
from the read internal extended temperature register
(11h), which extends the data to 10 bits (plus 1 bit indi-
cating the diode fault status) and the resolution to
0.125°C per LSB (Table 2).
When a conversion is complete, the main temperature
register and the extended temperature register are
updated simultaneously. Ensure that no conversions
are completed between reading the main register and
the extended register, so that both registers contain the
result of the same conversion.
To ensure valid extended data, read extended resolu-
tion temperature data using one of the following
approaches:Put the MAX6648/MAX6692 into standby mode by
setting bit 6 of the configuration register to 1. Initiate
a one-shot conversion using command byte 0Fh.
When this conversion is complete, read the contents
of the temperature data registers.
Precision SMBus-Compatible Remote/Localemperature Sensors with Overtemperature Alarms
FRACTIONAL TEMP (°C)DIGITAL OUTPUT0.000000X XXXX
0.125001X XXXX
0.250010X XXXX
0.375011X XXXX
0.500100X XXXX
0.625101X XXXX
0.750110X XXXX
0.875111X XXXX
Table 2. Extended Resolution Temperature
Register Data Format (10h, 11h)
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Localemperature Sensors with Overtemperature AlarmsIf the MAX6648/MAX6692 are in run mode, read the
status byte. If the BUSY bit indicates that a conversion
is in progress, wait until the conversion is complete
(BUSY bit set to zero) before reading the temperature
data. Following a conversion completion, immediately
read the contents of the temperature data registers. If
no conversion is in progress, the data can be read
within a few microseconds, which is a sufficiently short
period of time to ensure that a new conversion cannot
be completed until after the data has been read.
SMBCLKCDEFGHIJK
SMBDATA
tSU:STAtHD:STA
tLOWtHIGH
tSU:DATtHD:DATtSU:STOtBUF
A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVE
E = SLAVE PULLS SMBDATA LINE LOW M
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO MASTER
H = LSB OF DATA CLOCKED INTO MASTER
I = MASTER PULLS DATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO SLAVE
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION
M = NEW START CONDITION
Figure 2. SMBus Write Timing Diagram
Write Byte Format
Read Byte Format
Send Byte FormatReceive Byte FormatSlave Address: equiva-
lent to chip-select line of
a 3-wire interface
Command Byte: selects which
register you are writing to
Data Byte: data goes into the register
set by the command byte (to set
thresholds, configuration masks, and
sampling rate)
Slave Address: equiva-
lent to chip-select line
Command Byte: selects
which register you are
reading from
Slave Address: repeated
due to change in data-
flow direction
Data Byte: reads from
the register set by the
command byte
Command Byte: sends com-
mand with no data, usually
used for one-shot command
Data Byte: reads data from
the register commanded
by the last Read Byte or
Write Byte transmission;
also used for SMBus Alert
Response return addressS = Start conditionShaded = Slave transmission
P = Stop condition/// = Not acknowledged
Figure 1. SMBus Protocols
ADDRESSRDACKDATA///P7 bits8 bits
SACKCOMMANDACKP8 bits
ADDRESS7 bits
ACKDATA8 bits
ACKCOMMAND8 bits
ACKWRADDRESS7 bits
ADDRESSWRACKCOMMANDACKSADDRESS7 bits8 bits7 bits
ACKDATA8 bits
///P
MAX6648/MAX6692
Precision SMBus-Compatible Remote/Localemperature Sensors with Overtemperature AlarmsSMBCLK
A = START CONDITION
B = MSB OF ADDRESS CLOCKED INTO SLAVE
C = LSB OF ADDRESS CLOCKED INTO SLAVE
D = R/W BIT CLOCKED INTO SLAVECDEFGHIJ
SMBDATA
tSU:STAtHD:STA
tLOWtHIGH
tSU:DATtSU:STOtBUFK
E = SLAVE PULLS SMBDATA LINE LOW
F = ACKNOWLEDGE BIT CLOCKED INTO MASTER
G = MSB OF DATA CLOCKED INTO SLAVE
H = LSB OF DATA CLOCKED INTO SLAVE
I = MASTER PULLS DATA LINE LOW
J = ACKNOWLEDGE CLOCKED INTO SLAVE
K = ACKNOWLEDGE CLOCK PULSE
L = STOP CONDITION
M = NEW START CONDITION
Figure 3. SMBus Read Timing Diagram
Alarm Threshold RegistersFour registers store ALERTthreshold values—one high-
temperature (THIGH) and one low-temperature (TLOW)
register each for the local and remote channels. If
either measured temperature equals or exceeds the
corresponding ALERTthreshold value, the ALERTinter-
rupt asserts.
The power-on-reset (POR) state of both ALERTTHIGH
registers is full scale (0101 0101, or +85°C). The POR
state of both TLOWregisters is 0000 0000, or 0°C.
Two additional registers store remote and local alarm
threshold data corresponding to the OVERToutput. The
values stored in these registers are high-temperature
thresholds. If either of the measured temperatures
equals or exceeds the corresponding alarm threshold
value, an OVERToutput asserts. The POR state of the
OVERTthreshold is 0110 1110 or +110°C for the
MAX6648, and 0101 0101 or +85°C for the MAX6692.
Diode Fault AlarmA continuity fault detector at DXP detects an open cir-
cuit between DXP and DXN, or a DXP short to VCC,
GND, or DXN. If an open or short circuit exists, the
external temperature register is loaded with 1000 0000.
If the fault is an open-circuit fault bit 2 (OPEN) of the
status byte, it is set to 1 and the ALERTcondition is
activated at the end of the conversion. Immediately
after POR, the status register indicates that no fault is
present. If a fault is present upon power-up, the fault is
not indicated until the end of the first conversion.
ALERTInterruptsThe ALERTinterrupt occurs when the internal or exter-
nal temperature reading exceeds a high- or low-tem-
perature limit (user programmed) or when the remote
The ALERTinterrupt output signal is latched and can
be cleared only by either reading the status register or
by successfully responding to an alert response
address. In both cases, the alert is cleared only if the
fault condition no longer exists. Asserting ALERTdoes
not halt automatic conversion. The ALERToutput pin is
open drain, allowing multiple devices to share a com-
mon interrupt line.
The MAX6648/MAX6692 respond to the SMBus alert
response address, an interrupt pointer return-address
feature (see the Alert Response Address section). Prior
to taking corrective action, always check to ensure that
an interrupt is valid by reading the current temperature.
Fault Queue Register In some systems, it may be desirable to ignore a single
temperature measurement that falls outside the ALERT
limits. Bits 2 and 3 of the fault queue register (address
22h) determine the number of consecutive temperature
faults necessary to set ALERT(see Tables 3 and 4).
Alert Response AddressThe SMBus alert response interrupt pointer provides
quick fault identification for simple slave devices that
lack the complex, expensive logic needed to be a bus
master. Upon receiving an ALERTinterrupt signal, the
host master can broadcast a receive byte transmission
to the alert response slave address (0001 100).
Following such a broadcast, any slave device that gen-
erated an interrupt attempts to identify itself by putting
its own address on the bus.
The alert response can activate several different slave
devices simultaneously, similar to the I2C general call. If
more than one slave attempts to respond, bus arbitration
rules apply, and the device with the lower address
