SCEA117 July   2022 SN74HCS164 , SN74HCS164-Q1 , SN74HCS165 , SN74HCS165-Q1 , SN74HCS595 , SN74HCS595-Q1

 

  1.   Abstract
  2.   Trademarks
  3. 1Overview
    1. 1.1 Types of Shift Registers
    2. 1.2 Default State of a Shift Register
    3. 1.3 164 Function Shift Registers
    4. 1.4 165 Function Shift Registers
    5. 1.5 595 Function Shift Registers
    6. 1.6 Daisy-Chain Two Shift Registers
  4. 2Design Challenges
    1. 2.1 Controller Loading Limits
    2. 2.2 Operating over Large Distances
    3. 2.3 Data Loss Due to Signal Timing
    4. 2.4 Data Rate Limitations
    5. 2.5 Software Overview
  5. 3Example Design - Daisy Chain 72 Shift Registers
    1. 3.1 System Overview
    2. 3.2 System Design
    3. 3.3 Software Examples
  6. 4References

1.6 Daisy-Chain Two Shift Registers

Any shift register can be daisy-chained, so long as it has a serial data input and direct access to the last serial register’s contents. In order to connect two shift registers in series, connect the serial output of one device (usually QH or QH’) to the serial input (usually SER) of the next device. Typically, all of the clock signals will be shorted together, although they can be separated for unique cases.

Figure 1-4 Daisy-Chaining two 595 Function Shift Registers Together

Figure 1-4 provides an example of connecting two 595 function shift registers together to convert 3 output pins into 16 output pins (O0 to O15). The first device connects to the system controller as usual, with the serial data and clocks coming directly from the controller. For the second device in the chain, the only difference is that the data input comes from the QH’ output of the previous device rather than the controller.

Figure 1-5 Block Diagram for Adding 16 Outputs and 16 Inputs to a Controller Using 4 I/Os

Figure 1-5 shows an example of connecting two parallel-in and two parallel-out shift registers to provide 16 new inputs and 16 new outputs. By using this configuration, the number of I/Os required by the controller remains only four, while the number of outputs (or inputs) can be increased almost indefinitely. There are multiple design challenges that need to be addressed to ensure proper operation. See Section 2 for details.