MC74HC165ADR2 ,8-Bit Serial or Parallel-Input/Serial Output Shift RegisterMAXIMUM RATINGS*ÎÎÎÎÎÎ Symbol ParameterÎÎÎÎÎ ValueÎÎÎ UnitThis device contains protectioncircuitry ..
MC74HC165ADR2 ,8-Bit Serial or Parallel-Input/Serial Output Shift Register
MC74HC165ADR2 ,8-Bit Serial or Parallel-Input/Serial Output Shift Register
MC74HC165ADT ,8-Bit Serial or Parallel-Input/Serial-Output Shift RegisterThe MC74HC165A is identical in pinout to the LS165. The deviceinputs are compatible with standard C ..
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MC74HC14ADT-MC74HC165ADR2
Hex Schmitt-Trigger Inverter
AND8144/D
CMOS Shift Registers Solve
I/O Issues in Automotive
Applications
Prepared by: Fred ZlotnickON Semiconductor
There are a number of very popular shift registers
available to solve Input Output problems very economically.
The automotive industry, in particular, often needs to send
data from one set of switches to a micro−controller that is
several meters away. Sending many parallel bit is terribly
inefficient. Using a dedicated micro−controller is still an
expensive solution. The available industry−standard shift
registers make a very economical solution to this problem.
This article will try to point out the pitfalls in using these
devices properly.
The choice of standard family is generally the first
decision. Two families are the most widely used for
automotive uses, the Metal Gate CMOS Family also known
as the “4000” family and the High Speed Logic or “HC”
family. ON Semiconductor offers many part types in these
two families, and also in some of the newer, faster lines.
These applications usually demand 16 or more bits of
information, representing the position of a mechanical
switch, and speed is usually of no consequence. Since a high
speed serial bus of say 10 MHz might cause RFI1 , a clock
rate of 250 kHz or 500 kHz is sufficient and even preferable
to high speed data transmission. However, many designers
attempting to keep RFI to a minimum unwittingly introduce
delays in the system that wreak havoc on the timing and
cause unexpected results.
The beauty of the shift register is its simplicity, but there
are a few critical parameters that must be taken into account.
All shift registers have a “set−up and hold” time. It is
assumed that the board is running from 12 V nominal, with
an on−board 5 V regulator. The MC74HC165ADR2 is
selected as the shift register. This device is rated for
automotive temperatures, and is inexpensive and available
in a small 14 pin packages. This device is quite flexible, with
8 bits of parallel inputs, Q and Q−Bar outputs, a clock
inhibit, and the ability to string several registers together, to
create any string of parallel inputs desired. Since the
microcontroller is providing the clock signal, there is no
need for clock recovery. This design is SPI compatible in the
MISO2 mode. The system designer may choose to use a SPI
port or simply use a few I/O pins of the microcontroller,
while “bit−banging”.
Since the MCU is polling the shift register, simple RC
circuit will suffice for the parallel inputs. If the data is in
transition, all that is necessary is for the MCU to come back
and poll this data again, before taking action. A polling rate
is 10 ms should be sufficient for de−bounce. At a 250 kHz
clock rate, it only takes 32 μs, to read 8 bits of data. The
capacitors need to have parallel resistors to bleed off the
charge after a switch is opened. This combination of filtering
and polling is a very lost cost implementation of de−bounce.
For reasons of RFI, the maximum data rate will be set to
250kHz. This is well below the 20 MHz rating of the part,
even at 125°C.
specification that requires that the data is present and valid
(in proper logic level state) for 22 ns (at 5 V). Fig. 1 shows
the use of one “HC165” to expand 8 bits into a 3 wire serial
bus. The MCU must “poll” the shift register at a desired rate,
say every 10 ms. The act of polling consists of setting pin 1
High, and holding it High, and “clocking” 8 pulses. After
each clock pulse the data will be present on pin 9, 45 ns later.
All that is necessary to read this data is for the software
designer to send out a clock pulse, wait sufficient time for the
clock pulse to arrive at the shift register and get transferred
to its output, in this case 400 ns and then read the data. He
does this 8 times and reads all 8 bits. It is highly
recommended that the designer use Schottky diodes and a
resistor to form an RC time constant, to limit the noise on the
lines. This is all very straight−forward. The only critical
point is to wait long enough, to make sure the data read is
valid. A preferred solution is to recognize the data, as valid
after receiving the same data twice, in sequence. RFI − Radio Frequency Interference MISO − Master In Slave Out
