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EE Design > In Technical
Title: Expanding Outputs using the 74LS164 Shift Register Author: Vincent Wong Date: 29th March 2005 Sometimes it is necessary to expand the outputs of the PIC Micro. This is often for PIC with low pin count, those with less than 28 pins. This can be done using a shift register type serial in parallel out. The 74164 is an 8-bit serial in - parallel output shift register.  It requires 8 clock cycles at 'CLK' pin to shift the serial data from input 'B' to the 8 parallel outputs. After 8 shifts, the first serial bit will be in output QH, and only now the data at output is valid. Relays & LEDs can be connected to the output. However, connecting a speaker or buzzer is not appropriate. This is because the shift of data may contribute noise to the sound device. If these kind of sensitive devices are to be connected to these outputs, i suggest to connect an Octal D-type latch (74HC373) in between to hold the outputs and disallow the invalid data to appear at the outputs. Only when all 8-valid data shifted to the output, the new data is allowed to exit at the latch's outputs. To cascade more 74164 shift register IC, you will need to connect the last output (QH) to the 'B' input of the second 74164 shift register IC. Now, it is necessary to shift 16 times before you get a valid output. In this case, all the clock pins are tied togather in the same line. |