774 UART-Programmable 2-Tap FIR Filter
774 : UART-Programmable 2-Tap FIR Filter
- Author: Mert Erman
- Description: 2-tap direct-form FIR filter with runtime-configurable Q7 coefficients via UART 9600 8N1. 8-bit signed input/output with saturation.
- GitHub repository
- Open in 3D viewer
- Clock: 25000000 Hz
UART-Programmable 2-Tap FIR Filter
How it works
This design implements a 2-tap direct-form FIR filter with runtime-configurable coefficients.
Coefficient Loading (UART)
Send a 5-byte packet at 9600 baud, 8N1 on uio[0]:
[ 0xA5 ] [ c0 ] [ c1 ] [ c2 ] [ c3 ]
(Note: To fit in a 1x1 footprint, c2 and c3 are ignored by the filter, but they must still be sent to complete the UART packet protocol.)
Coefficients are 8-bit signed Q7 fixed-point (divide by 128 to get the real value):
| Float value | Byte to send |
|---|---|
| +1.0 | 0x7F |
| +0.5 | 0x40 |
| +0.25 | 0x20 |
| 0.0 | 0x00 |
| -0.5 | 0xC0 |
| -1.0 | 0x80 |
Default coefficients at reset: all 0x40 (+0.5) — box / moving-average filter.
Sample Processing
- Place 8-bit signed sample on
ui_in[7:0] - Pulse
uio[2](sample_valid) HIGH for ≥1 clock cycle - Wait 1 clock cycle
- Read result from
uo_out[7:0]whenuio[3](out_valid) pulses HIGH
Filter equation
y[n] = (c0*x[n] + c1*x[n-1]) >> 7
Output is saturated to [-128, +127].
Filter presets (Python)
import serial
ser = serial.Serial('COM3', 9600)
def q7(v):
return int(v * 128) & 0xFF
def load(c0, c1, c2, c3):
ser.write(bytes([0xA5, q7(c0), q7(c1), q7(c2), q7(c3)]))
load(0.5, 0.5, 0.0, 0.0) # Low-pass (box)
load(0.5, -0.5, 0.0, 0.0) # High-pass
How to test
- Minimum test: DIP switches on
ui_in, LED bar onuo_out, button onuio[2] - UART test: Python script sends coefficient packets; verify
uio[4](coeff_we) pulses - Signal test: Arduino generates sine wave on
ui_in; observe filtering on oscilloscope - Audio demo: Connect R-2R DAC ladder to
uo_outand speaker — hear LP/HP effect live
External hardware
| Pin | Connection |
|---|---|
ui_in[7:0] |
8-bit signed sample (DIP switches / ADC / Arduino) |
uo_out[7:0] |
8-bit signed filtered output (LEDs / DAC / oscilloscope) |
uio[0] |
UART RX from USB-UART adapter (CP2102 / CH340) |
uio[2] |
sample_valid strobe (button / MCU GPIO) |
uio[3] |
out_valid indicator (LED) |
uio[4] |
coeff_we indicator (LED) |
IO
| # | Input | Output | Bidirectional |
|---|---|---|---|
| 0 | Sample in bit 0 (LSB) | Filtered output bit 0 (LSB) | UART RX - coefficient programming 9600 8N1 (input) |
| 1 | Sample in bit 1 | Filtered output bit 1 | UART TX - reserved, tie HIGH (output) |
| 2 | Sample in bit 2 | Filtered output bit 2 | sample_valid - pulse to process ui_in (input) |
| 3 | Sample in bit 3 | Filtered output bit 3 | out_valid - pulses when uo_out ready (output) |
| 4 | Sample in bit 4 | Filtered output bit 4 | coeff_we - pulses when coefficients updated (output) |
| 5 | Sample in bit 5 | Filtered output bit 5 | |
| 6 | Sample in bit 6 | Filtered output bit 6 | |
| 7 | Sample in bit 7 (MSB, sign) | Filtered output bit 7 (MSB, sign) |