681 QEI (Quadrature Encoder Interface)
681 : QEI (Quadrature Encoder Interface)
- Author: Jake Drew
- Description: Decodes A/B quadrature input. Outputs last step direction and 15-bit count.
- GitHub repository
- Open in 3D viewer
- Clock: 0 Hz
How it works
Quadrature x4 decoder with 15-bit up/down count and last-step direction.
- Inputs:
A=ui_in[0],B=ui_in[1](two-stage sync toclk) - Decoding:
00→01→11→10→00= +1 per edge; reverse = −1 - Outputs:
uo_out[7]— DIR (1 = forward, 0 = backward)uo_out[6:0]— COUNT[6:0]uio_out[7:0]— COUNT[14:7]
Notes:
- DIR reports the last step direction; it is not a signed count.
- The exposed count bits give you 15 LSBs across
uio_outanduo_out[6:0].
How to test
On hardware
- Drive encoder A/B into
ui[0]andui[1]. - Read:
uo[7]→ DIR (1 = forward, 0 = backward)uo[6:0]→count[6:0]uio[7:0]→count[14:7]
- Turn the encoder forward: count increases and
uo[7]=1. Reverse: count decreases anduo[7]=0.
Simulation (local)
- Run:
make -C test clean test - View waveform:
gtkwave test/tb.vcd
External hardware
- If your encoder is single-ended, connect A/B directly (3.3V logic).
- If your encoder is differential (A/A′, B/B′), use an RS-422/line-receiver to convert to 3.3V CMOS before
ui[0:1]. - Index (Z) is not used in this design.
IO
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | A | COUNT0 | COUNT7 |
| 1 | B | COUNT1 | COUNT8 |
| 2 | COUNT2 | COUNT9 | |
| 3 | COUNT3 | COUNT10 | |
| 4 | COUNT4 | COUNT11 | |
| 5 | COUNT5 | COUNT12 | |
| 6 | COUNT6 | COUNT13 | |
| 7 | DIR | COUNT14 |