7.7 Shift registers in microcontrollers

7.7 Shift registers

There are two types of shift registers: input and output. Input shift registers receive data in parallel, through 8 lines and then send it serially through two lines to a microcontroller. Output shift registers work in the opposite direction; they receive serial data and on a "latch" line signal, they turn it into parallel data. Shift registers are generally used to expand the number of input-output lines of a microcontroller. They are not so much in use any more though, because most modern microcontrollers have a large number of pins. However, their use with microcontrollers such as PIC16F84 is very important.

7.7.1 Input shift register 74HC597

Input shift registers transform parallel data into serial data and transfers it to a microcontroller. Their working is quite simple. There are four lines for the transfer of data: Clock, Latch, Load and Data. Data is first read from the input pins by an internal register through a 'latch' signal. Then, with a 'load' signal, data is transferred from the input latch register to the shift register, and from there it is serially transferred to a microcontroller via 'data' and 'clock' lines.


An outline of the connection of the shift register 74HC597 to a micro, is shown below.


In order to simplify the main program, a macro can be used for the input shift register. Macro HC597 has two parameters:
HC597 macro Var, Var1
Var variable where data from shift register input pins is transferred
Var1 loop counter
Example: HC597 data, counter
Data from the input pins of the shift register is stored in data variable. Timer/counter variable is used as a loop counter.
Macro listing:


Example of how to use the HC597 macro is given in the following program. Program receives data from a parallel input of the shift register and moves it serially into the RX variable of the microcontroller. LEDs connected to port B will indicate the result of the data input.


7.7.2 Output shift register

Output shift registers transform serial data into parallel data. On every rising edge of the clock, the shift register reads the value from data line, stores it in temporary register, and then repeats this cycle 8 times. On a signal from 'latch' line, data is copied from the shift register to input register, thus data is transformed from serial into parallel data.


An outline of the 74HC595 shift register connections is shown on the diagram below:


Macro used in this example can be found in hc595.inc file, and is called HC595.
Macro HC595 has two parameters:
HC595 macro Var, Var1
Var variable whose contents is transferred to outputs of shift register.
Var1 loop counter
Example: HC595 Data, counter
The data we want to transfer is stored in data variable, and counter variable is used as a loop counter.


An example of how to use the HC595 macro is given in the following program. Data from variable TX is serially transferred to shift register. LEDs connected to the parallel output of the shift register will indicate the state of the lines. In this example value 0xCB (1100 1011) is sent so that the seventh, sixth, third, first, and zero LEDs are illuminated.