gemstonedevteam opened a new pull request, #20138:
URL: https://github.com/apache/nuttx/pull/20138
> Opening as draft since this depends on #20137 will mark ready once that
lands.
### Summary
* Adds a PWM lower-half driver for the AM67 eCAP modules in APWM mode,
where
the module is used as a single-channel PWM generator rather than a
capture
unit.
* Period is programmed in `CAP1` and the compare value in `CAP2`, with the
counter free-running from the eCAP functional clock at 125 MHz.
* `start()` reprograms the time base only when the frequency changes, so a
duty-only update does not clear `TSCNTSTP` or zero `TSCTR` on a running
counter.
* t3-gem-o1 registers eCAP1 as `/dev/ecap1` and eCAP2 as `/dev/ecap2`.
Output
pad selection is left to the board; the candidate pads collide with I2C0
on
this design.
### Impact
* Is new feature added? **YES** — eCAP APWM support for AM67.
* Impact on user? **NO** — new driver, opt-in by Kconfig.
* Impact on build? **NO**.
* Impact on hardware? **YES** — arch/arm/am67 and board t3-gem-o1.
* Impact on documentation? **YES** — provided in this PR (second commit).
* Impact on security? **NO**.
* Impact on compatibility? **NO** — additive.
* Anything else? Depends on PRs 3–5.
### Testing
```
Build Host(s): Pardus GNU/Linux 25 (Debian 13 base), x86_64, host gcc 14
Toolchain: arm-none-eabi-gcc 14.2.1 20241119 (15:14.2.rel1-1)
Target: arm / TI AM67 (J722S) Cortex-R5F, board t3-gem-o1:nsh
nuttx-apps: 5a7ab4200
Loaded on the main-domain R5F by Linux remoteproc; console on UART-MAIN1.
```
Testing logs before change: no eCAP support on this arch.
The version string in the logs is from the tested build; the commits were
GPG-signed afterwards, which rewrites hashes. The code is unchanged.
Testing logs after change:
```
make distclean && ./tools/configure.sh t3-gem-o1:nsh && make -j
-> nuttx ELF 543332 bytes, 0 compiler warnings
Console:
nsh> pwm -p /dev/ecap1 -f 1000 -d 50 -t 1
pwm_main: starting output with frequency: 1000 channel: 1 duty: 00007fff
channel: 2 duty: 00007fff
pwm_main: stopping output
nsh> pwm -p /dev/ecap2 -f 1000 -d 50 -t 1
pwm_main: starting output with frequency: 1000 channel: 1 duty: 00007fff
channel: 2 duty: 00007fff
pwm_main: stopping output
nsh> pwm -p /dev/ecap1 -f 1000 -d 25 -t 2
pwm_main: starting output with frequency: 1000 channel: 1 duty: 00003fff
channel: 2 duty: 00007fff
pwm_main: stopping output
nsh> pwm -p /dev/ecap1 -f 1000 -d 75 -t 2
pwm_main: starting output with frequency: 1000 channel: 1 duty: 0000bfff
channel: 2 duty: 00007fff
pwm_main: stopping output
nsh>
Test plan result:
console_alive PASS prompt responsive
version PASS NuttX version 0.0.0 aaf0c2583d Sep 10 2026
13:49:06 t3-gem-o1:nsh
procfs PASS procfs readable
tasks PASS 2 task lines
dev_nodes PASS all expected nodes present
pwm_ecap1 PASS /dev/ecap1 1 kHz / 50% for 1 s
pwm_ecap2 PASS /dev/ecap2 1 kHz / 50% for 1 s
ecap_duty_only PASS 25% then 75% at 1 kHz accepted
PASS=8
```
Measured time base, captured 2026-09-11, read back from the A53 over
`/dev/mem` while NuttX drives the output.
```
NuttX: nsh> pwm -p /dev/ecap1 -f 1000 -d 50 -t 25 &
A53, while that is running (eCAP registers are 32-bit natively):
/dev/ecap1 @ 0x23110000 APRD = 124999, ACMP = 62498
/dev/ecap2 @ 0x23120000 APRD = 124999, ACMP = 62498
frequency = 125 MHz / (124999 + 1) = 1000.00 Hz
duty = 62498 / 125000 = 49.998 %
TSCTR sampled twice 0.4 s apart: 12959 -> 109982 (ecap1) and
67938 -> 93142 (ecap2), so the counter is free-running.
This is also the direct evidence for the duty-only path above: the eCAP
clock is FICLK/2 of the EPWM's, and the 125000-tick period confirms the
125 MHz figure the driver assumes.
```
### PR verification Self-Check
* [x] This PR introduces only one functional change.
* [x] I have updated all required description fields above.
* [x] My PR adheres to Contributing Guidelines and Documentation.
* [ ] My PR is still work in progress (not ready for review).
* [x] My PR is ready for review and can be safely merged into a codebase.
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