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74HC4021 Shift Register Module - 8-bit Parallel-to-Serial Input

Device driver for CD4021B 8-bit shift register (input expander). Supports daisy-chaining and parallel chains.

Hardware: CD4021B CMOS 8-stage static shift register Interface: Parallel input, serial output Supply: 3V-18V (3.3V compatible) Clock Freq: Up to 3MHz at 5V, 8.5MHz at 15V

Pin Connections:

Pin 10 → CLK (clock)
Pin 9 → P/!S (parallel load / serial shift, active LOW for load)
Pin 11 → Q7 (serial data output)
Pins 7,6,5,4,13,14,15,1 → P0-P7 (parallel data inputs)

Daisy Chaining: Connect Q7 (pin 11) of first device to parallel input of next device.

Parallel Chains: Multiple chains can share CLK and LATCH pins with separate data lines.

Data Layout: When using multiple devices (example: 2 daisy-chained, 2 parallel):

states0-7 = Chain 0, Parallel 0
states8-15 = Chain 1, Parallel 0
states16-23 = Chain 0, Parallel 1
states24-31 = Chain 1, Parallel 1

Example:

import nimphea
import nimphea/dev/sr4021

# Single device
var sr: ShiftRegister4021[1, 1]
var cfg: ShiftRegister4021Config[1, 1]
cfg.clk = newPin(PORTB, 0)
cfg.latch = newPin(PORTB, 1)
cfg.data[0] = newPin(PORTB, 2)
cfg.delay_ticks = 10

sr.init(cfg)
sr.update()  # Read inputs
if sr.state(0):  # Check pin P0
  echo "Button pressed"

Imports

nimphea

Types

ShiftRegister4021[NumDaisy; NumParallel] {. importcpp: "daisy::ShiftRegister4021<#,#>", bycopy.} = object: Source Edit
ShiftRegister4021Config[NumDaisy; NumParallel] {. importcpp: "daisy::ShiftRegister4021<#,#>::Config", bycopy.} = object clk* {.importcpp: "clk".}: Pin latch* {.importcpp: "latch".}: Pin data* {.importcpp: "data".}: UncheckedArray[Pin] delay_ticks* {.importcpp: "delay_ticks".}: uint32: Source Edit

Procs

proc getConfig[ND, NP](sr: ShiftRegister4021[ND, NP]): ShiftRegister4021Config[ ND, NP]: Get the current configuration Source Edit
proc init[ND, NP](sr: var ShiftRegister4021[ND, NP]; cfg: ShiftRegister4021Config[ND, NP]): Initialize shift register(s)

Parameters:

cfg: Configuration with pins and delay settings

Template Parameters:

ND: Number of daisy-chained devices (default 1)

NP: Number of parallel chains (default 1)

Total inputs = 8 × ND × NP
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proc state[ND, NP](sr: ShiftRegister4021[ND, NP]; index: int): bool: Get the state of an input

Parameters:

index: Input index (0 to 8×ND×NP - 1)

Returns: true if input is HIGH, false if LOW

Note: Call update() first to refresh the input states
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proc update[ND, NP](sr: var ShiftRegister4021[ND, NP]): Read all inputs from shift register(s)

Call this periodically (e.g., in your main loop) to sample all inputs. The states are stored internally and can be read with state().
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