ESP32-S3 MCPWM has a separate prescaler for each timer, buf IDF code doesn't use them. (IDFGH-13989)

[eriksl]

Answers checklist.

• [X] I have read the documentation ESP-IDF Programming Guide and the issue is not addressed there.
• [X] I have updated my IDF branch (master or release) to the latest version and checked that the issue is present there.
• [X] I have searched the issue tracker for a similar issue and not found a similar issue.

IDF version.

5.3.1

Espressif SoC revision.

ESP32-S3 0.1

Operating System used.

Linux

How did you build your project?

Command line with idf.py

If you are using Windows, please specify command line type.

None

Development Kit.

LilyGO S3 T7

Power Supply used.

USB

What is the expected behavior?

I expected to be able to run different timers within one MCPWM group to be able to run at different frequencies. I have selected APB frequency for timer0 and APB / 16 for timer1, so there should be no problem. The technical reference manual says (register overview) there is a common prescaler for all timers and a specific prescaler for each timer, so it should be possible. This is also what the text says, although the diagram of the MCPWM module is indecisive on this matter.

What is the actual behavior?

These reports and abort called:

E (10) mcpwm: mcpwm_set_prescale(242): group prescale conflict, already is 2 but attempt to 1
E (10) mcpwm: mcpwm_new_timer(125): set prescale failed

Besides the first report being very cryptic, it looks like the code simply doesn't take the timer-specific prescaler into account. Or there is none and the documentation is incorrect.

Steps to reproduce.

1. create a timer on mcpwm group 0, frequency is 80 MHz (1x APB prescaler)
2. create a second timer on mcpwm group, frequency is 5 MHz (16x APB prescaler)

Nothing more required than this.

When you create both timers with the same frequencies it works. If the timers are on separate groups, it works as well.

Debug Logs.

E (10) mcpwm: mcpwm_set_prescale(242): group prescale conflict, already is 2 but attempt to 1
E (10) mcpwm: mcpwm_new_timer(125): set prescale failed

More Information.

I am forced to use MCPWM for my lighting ("LED") purposes as the "LEDPWM" has a resolution of only 14 bits, which is really too little. I don't understand why the "fast LEDPWM" channels where dropped (with the high resolution timers). I am now investigating ways to achieve similar PWM resolutions. The 16 bit width of MCPWM is step one, but I'd really like even more. I am thinking of having the cpu updating the PWM "value" on the fly after one or more timer overflow interrupts, but it sounds complex to implement. On the other hand, it may even be done on the LEDPWM module.

Other possible paths:

• use the modulator + duty cycle of the MCPWM to add 3 bits of resolution
• use the modulator of the RMT module, but I can't find maximum parameters
• use the UART, but you can't select 0 stop bits. If that was possible, it could work (although it would require a lot of memory or a lot of data pumping).

I am interested in any interesting hints to workaround this omission.

[suda-morris]

Didn't reproduce the issue with the following init code:

TEST_CASE("mcpwm timer with different resolutions", "[mcpwm]")
{
    mcpwm_timer_config_t timer_config = {
        .group_id = 0,
        .clk_src = MCPWM_TIMER_CLK_SRC_DEFAULT,
        .resolution_hz = 80 * 1000 * 1000,
        .period_ticks = 20 * 1000,
        .count_mode = MCPWM_TIMER_COUNT_MODE_UP,
    };
    mcpwm_timer_handle_t timer0, timer1;
    printf("create mcpwm timers\r\n");
    TEST_ESP_OK(mcpwm_new_timer(&timer_config, &timer0));
    timer_config.resolution_hz = 5 * 1000 * 1000;
    TEST_ESP_OK(mcpwm_new_timer(&timer_config, &timer1));

    printf("delete timers\r\n");
    TEST_ESP_OK(mcpwm_del_timer(timer0));
    TEST_ESP_OK(mcpwm_del_timer(timer1));
}

[eriksl]

That's interesting. I have tested it more than once. I have run the same code on one and on two mcpwm groups, using two groups it works like a charm, on one group it doesn't.

[eriksl]

I think I know where's the difference. I am always using a 16 bit (max) resolution timer.

Please try with:

mcpwm_timer_config_t timer_config =
{
    .group_id = 0,
    .clk_src = MCPWM_TIMER_CLK_SRC_DEFAULT,
    .resolution_hz = FREQUENCY,
    .count_mode = MCPWM_TIMER_COUNT_MODE_UP,
    .period_ticks = TIMER_TICKS,
    .intr_priority = 0,
    .flags =
    {
        .update_period_on_empty = 0,
        .update_period_on_sync = 0,
    },
};

where either

• FREQUENCY = 160000000 / 16 and TIMER_TICKS = 65535
• FREQUENCY = 160000000 / 1 and TIMER_TICKS = 65535

This is where the problem occurs.

[suda-morris]

@eriksl this can be workaround by set the higher resolution first then the smaller resolution.

e.g.

TEST_CASE("mcpwm timer with different resolutions", "[mcpwm]")
{
    mcpwm_timer_config_t timer_config = {
        .group_id = 0,
        .clk_src = MCPWM_TIMER_CLK_SRC_DEFAULT,
        .resolution_hz = 160 * 1000 * 1000,
        .period_ticks = 65535,
        .count_mode = MCPWM_TIMER_COUNT_MODE_UP,
    };
    mcpwm_timer_handle_t timer0, timer1;
    printf("create mcpwm timers\r\n");
    TEST_ESP_OK(mcpwm_new_timer(&timer_config, &timer0));
    timer_config.resolution_hz = 10 * 1000 * 1000;
    TEST_ESP_OK(mcpwm_new_timer(&timer_config, &timer1));

    printf("delete timers\r\n");
    TEST_ESP_OK(mcpwm_del_timer(timer0));
    TEST_ESP_OK(mcpwm_del_timer(timer1));
}