45 UART Positioning PWM Interface :: Quicker, easier and cheaper to make your own chip!

How it works

This project implements a UART-controlled servo positioning interface with a built-in sweep mode and inactivity failsafe.

At the default 25 MHz project clock, the design generates a repeating 20 ms frame on uo[0]. Within each frame, the high pulse width selects the servo position:

minimum: about 1.0 ms
center: about 1.5 ms
maximum: about 2.0 ms

Feature summary:

UART preset control

The command interface is intentionally simple:

ui[0] receives UART data
digits 0 to 9 map to evenly spaced preset positions between minimum and maximum
c or C selects the center preset
m selects the minimum preset
M selects the maximum preset
s or S enables an automatic sweep mode that steps through the preset range once per frame

Direct preset overrides

The dedicated inputs ui[1], ui[2], and ui[3] force center, minimum, and maximum output positions. These inputs take precedence over the UART-controlled target and give a simple hardware override path.

Sweep/self-test mode

The sweep mode repeatedly walks the target position between minimum and maximum. It is intended for bring-up and bench observation, where a moving output is easier to validate than a static preset.

Inactivity failsafe

A watchdog-style failsafe returns the output to center after a programmable number of inactive frames. This keeps the module useful as a practical command interface block rather than only as a waveform generator demo.

The design also exposes internal status on the regular Tiny Tapeout outputs:

uo[1] toggles each time a valid UART command is received
uo[5:2] shows the currently selected preset code
uo[6] goes high while the failsafe mode is active
uo[7] goes high while the sweep mode is active

How to test

Drive the project with a 25 MHz clock and hold rst_n low during reset.

For direct input testing:

drive ui[1] high to force the center servo position
drive ui[2] high to force the minimum servo position
drive ui[3] high to force the maximum servo position

For UART testing on ui[0]:

send m for minimum
send M for maximum
send c or C for center
send any digit from 0 to 9 for an intermediate preset
send s or S to enable sweep mode

Expected outputs:

uo[0] produces the servo control pulse
uo[1] toggles on each accepted command
uo[5:2] reflects the selected preset number
uo[6] goes high when failsafe has forced the output back to center
uo[7] goes high while sweep mode is running

The repository also includes a feature-oriented cocotb regression rooted at test/test.py. The detailed coverage plan is documented in verif.md.

External hardware

Recommended external hardware:

a 25 MHz clock source
a USB-to-UART adapter or microcontroller TX line connected to ui[0]
a hobby servo or an oscilloscope/logic analyzer connected to uo[0]

The servo must be powered externally. The Tiny Tapeout output only provides the control waveform.

#	Input	Output
0	UART RX	Servo PWM
1	Center preset override	Command acknowledge toggle
2	Minimum preset override	Preset code bit 0
3	Maximum preset override	Preset code bit 1
4		Preset code bit 2
5		Preset code bit 3
6		Failsafe active
7		Sweep mode active

45 UART Positioning PWM Interface