What exacly esp_hal::rmt::Channel.receive() do?

[trinhanhtuan]

Hello,
I am trying to write myself a NEC IR receiver using esp_hal::rm unfortunately its API document is too brief for a development guide. Can someone please help me some follwoing questions?

1- Because ESP32 RMT is a general pupose one, how can I config the RxChannelConfig to recornize 38kHz NEC IR? Making sure its demodulation will work correctly?
2- How to make sure we have configured the RXChannel works on "pulse distance modulation"? i.e. the one that NEC IR relies on but not any other modulations.
3- What does exacly esp_hal::rmt::Channel.receive() do if it receive a NEC IR message? e.g. let say when I press a button on my NEC IR remote control. Will it fill up the data: &mut [u32] with the whole below NEC message data (a to g) or just the address & command (c to f)?
3.a- a 9ms leading pulse burst; (16 times the pulse burst length used for a logical data bit)
3.b- a 4.5ms space;
3.c- the 8-bit address for the receiving device;
3.d- the 8-bit logical inverse of the address;
3.e- the 8-bit command;
3.f- the 8-bit logical inverse of the command;
3.g- a final 562.5µs pulse burst to signify the end of message transmission.

Any NEC IR related examples are appriciated. Thank you very much.

[bjoernQ]

Thanks for pointing out the lack of documentation regarding the RMT driver.

Until now most people are (ab)using the peripheral for other uses than its original purpose (most popular is using it to drive addressable LEDs).

Maybe looking at this ESP-IDF example might help: https://github.com/espressif/esp-idf/tree/master/examples/peripherals/rmt/ir_nec_transceiver

While the API is considerably different both should do and support the same level of functionality. If our driver is lacking something that is by accident and should be added.

For general information about the peripheral the TRM chapter and ESP-IDF's documentation might be helpful:

• https://docs.espressif.com/projects/esp-idf/en/stable/esp32/api-reference/peripherals/rmt.html (the API is different but the basics of the peripheral apply)
• https://www.espressif.com/sites/default/files/documentation/esp32_technical_reference_manual_en.pdf#rmt (you might want to refer to the TRM of a newer chip since the hardware is very similar, but the documentation is more in-depth, e.g. https://www.espressif.com/sites/default/files/documentation/esp32-c6_technical_reference_manual_en.pdf#rmt )

[trinhanhtuan]

Yes, I know the driver is not responsible for NEC protocol, what I mean is if I config the receiver as following :

    let freq: Rate<u32, 1, 1> = 80.MHz();
    let rmt = Rmt::new(peripherals.RMT, freq).unwrap().into_async();
    let rx_config = RxChannelConfig {
        clk_divider: 80,
        carrier_modulation: true,
        filter_threshold: 125,
        idle_threshold: 12000,
        ..RxChannelConfig::default()};

    let mut channel = rmt.channel2.configure(peripherals.GPIO4, rx_config).expect("Err: ");
    let mut data: [u32; 32] = [PulseCode::empty(); 32];

    loop {
        channel.receive(&mut data).await.unwrap();
        ....
   }

then will the &mut data contain data demodulated correctly with carrier 38kHz? It may be important to ensure that before any further processing e.g. NEC protocol decoding.

In the ESP-IDF, there is a configuration for that demodulation (https://docs.espressif.com/projects/esp-idf/en/latest/esp32c6/api-reference/peripherals/rmt.html#_CPPv420rmt_carrier_config_t) while I can't find anywhere in esp_hal::rmt.

[bjoernQ]

With that config - no.

The driver is a bit more low-level by letting the user configure carrier_high, carrier_low and carrier_level - in the end that is what esp-idf configures here: https://github.com/espressif/esp-idf/blob/b5ac4fbdf9e9fb320bb0a98ee4fbaa18f8566f37/components/esp_driver_rmt/src/rmt_rx.c#L470-L507

[trinhanhtuan]

But the issue is I can't find anywhere to do that necessary configuration, the only API is that RxChannelConfig. For the ESP-IDF, yes it can do but we then have to switch to C and ESP-IDF, no more Rust with esp-hal :-(

[bjoernQ]

I don't really understand the part about C and ESP-IDF t.b.h.

You could calculate carrier_high and carrier_low and set it in RxChannelConfig

[trinhanhtuan]

Thank you, I have manualy calculate and set the config to receive NEC IR messages. Now in progress to decode it.