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Throw exception for illegal tx gpio ports in RMT tx channel #92

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Throw exception for illegal tx gpio ports in RMT tx channel #92

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cd90393
fix(#1686): throw an argument out range exception if gpio port is not…
MrCSharp22 Nov 5, 2025
3423a54
chore: remove an unneeded double quote symbol
MrCSharp22 Nov 5, 2025
1ae34d9
chore: replace 'port' with 'pin' as the correct terminology
MrCSharp22 Nov 5, 2025
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4 changes: 4 additions & 0 deletions README.md
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Expand Up @@ -73,6 +73,10 @@ using var rxChannel = new ReceiverChannel(rxChannelSettings);
rxChannel.Start(clearBuffer: true);
```

## GPIO Pin Restrictions

The ESP32 MCU restricts which GPIO pins can be used for the Transmit Channel (TX). Ports 34 to 39 (inclusive) cannot be used for TX. Attempting to use these ports will cause the `TransmitChannelSettings` class to throw an `ArgumentOutOfRangeException`.
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@josesimoes josesimoes Nov 5, 2025

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  1. Shouldn't these be referred as pins, instead of ports?
  2. are these valid for all ESP32 series?

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@MrCSharp22 MrCSharp22 Nov 5, 2025

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  1. Absolutely! Sorry about that.. been doing a lot of web dev and networking-related stuff for a while now 😁
  2. I based this on what I read in the RMT legacy driver code. Essentially rmt_config(..) calls rmt_set_gpio(..) which performs the following validation:
ESP_RETURN_ON_FALSE(((GPIO_IS_VALID_GPIO(gpio_num) && (mode == RMT_MODE_RX)) ||
                         (GPIO_IS_VALID_OUTPUT_GPIO(gpio_num) && (mode == RMT_MODE_TX))), ESP_ERR_INVALID_ARG, TAG, RMT_GPIO_ERROR_STR);

The GPIO_IS_VALID_OUTPUT_GPIO macro is used to make sure the pins are not 34-39:

/// Check whether it can be a valid GPIO number of output mode
#define GPIO_IS_VALID_OUTPUT_GPIO(gpio_num) ((gpio_num >= 0) && \
                                              (((1ULL << (gpio_num)) & SOC_GPIO_VALID_OUTPUT_GPIO_MASK) != 0))

There aren't any compiler directive around this validation code which tells me it is applicable to all ESP32 boards. However, The RMT docs on the Espressif website for IDF version 4.4.6 makes no specific mention of this.

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@josesimoes josesimoes Nov 5, 2025

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Please note that we're currently at IDF v5.4. Not sure if that makes any difference in the API/docs.


## Build status

| Component | Build Status | NuGet Package |
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7 changes: 7 additions & 0 deletions nanoFramework.Hardware.Esp32.Rmt/TransmitChannelSettings.cs
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Expand Up @@ -144,8 +144,15 @@ public TransmitChannelSettings(int pinNumber) : this(channel: -1, pinNumber)
/// <param name="channel">The channel number to use. Valid value range is 0 to 7 (inclusive).</param>
/// <param name="pinNumber">The GPIO Pin number to use with the channel.</param>
/// <exception cref="ArgumentOutOfRangeException"><paramref name="channel"/> must be between 0 and 7.</exception>
/// <exception cref="ArgumentOutOfRangeException"><paramref name="pinNumber"/> must not be between 34 and 39.</exception>
public TransmitChannelSettings(int channel, int pinNumber) : base(channel, pinNumber)
{
// ESP32 RMT TX channels cannot use GPIO pins 34-39
if (pinNumber >= 34 && pinNumber <= 39)
{
throw new ArgumentOutOfRangeException();
}

_enableCarrierWave = true;
_carrierLevel = true;
_carrierWaveFrequency = 38_000;
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