RMT resolution divider rounding error (IDFGH-14301)

[rickou]

Answers checklist.

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

IDF version.

v5.3.2

Espressif SoC revision.

ESP32

Operating System used.

Windows

How did you build your project?

VS Code IDE

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

None

Development Kit.

ESP32-Wroom

Power Supply used.

USB

What is the expected behavior?

I need very precise RMT output frequency, so I would like to configure the RMT with a resolution of 720721Hz (based on 80MHz APB clock)
=> this give a theorical divider of 110.999956
=> expected to choose the nearest divider (111) in order to minimize the resolution loos

What is the actual behavior?

The divider is not round to nearest, but truncated to int.
in my case, i get lot of loss !
727272Hz (+7kHz !) instead of 720720.7207Hz (-0.2793Hz)

Steps to reproduce.

rmt_tx_channel_config_t tLocTxChannelConfig = {
.clk_src = RMT_CLK_SRC_DEFAULT,
.gpio_num = xxx,
.mem_block_symbols = 64,
.resolution_hz =720721,
.trans_queue_depth = 10,
.flags.invert_out = false,
.flags.with_dma = false,
.flags.io_loop_back = true
};
ESP_LOGI(TAG, "Init RMT for %s (res=%i)", psInContext->cpsConfig->cpcName, (int)tLocTxChannelConfig.resolution_hz);
ESP_ERROR_CHECK(rmt_new_tx_channel(&tLocTxChannelConfig, &psInContext->tTxChannelHandle));

Debug Logs.

I (4700) Motors: Init RMT for RA (res=720721)
W (4700) rmt: channel resolution loss, real=727272

More Information.

suggestion to round to nearest integer:

uint32_t real_div = group->resolution_hz;
real_div <<=1;
real_div +=config->resolution_hz;
real_div /=config->resolution_hz;
real_div >>=1;

or something like that: 🤢
uint32_t real_div= (uint32_t)(((float)group->resolution_hz/config->resolution_hz)+0.5)

[wanckl]

@rickou you are right, thanks for your report, we will fix soon