serial telem: initialize as RX

[dakejahl]

By default the serial line is configured in TX mode as push-pull. This prevents sharing the serial line with other ESCs if the ESC has not received a telemetry request before. This fixes the issue by ensuring the serial line is initialized as RX.

This should be added to all of the targets. Opening now as a draft to get review, and then I can add to all of the targets if we agree this is the right solution.

scope shot of one uninitialized ESC pulling the signal

[dakejahl]

@AlkaMotors thoughts?

[AlkaMotors]

This works for me, I can't see any issues it would cause.

[dakejahl]

Want me to update the other targets as well? I don't have the means of testing them

[AlkaMotors]

yes please. if you have time.

[dakejahl]

I'm not confident I can make the changes to the other targets without breaking them, since I can't test them. The reason I am not confident is because of how send_telem_DMA() looks:

void send_telem_DMA(uint8_t bytes)
{ // set data length and enable channel to start transfer
    LL_USART_SetTransferDirection(USART1, LL_USART_DIRECTION_TX);
    LL_DMA_DisableChannel(DMA1, LL_DMA_CHANNEL_2);
    LL_DMA_SetDataLength(DMA1, LL_DMA_CHANNEL_2, bytes);
    LL_DMA_EnableChannel(DMA1, LL_DMA_CHANNEL_2);
    LL_USART_SetTransferDirection(USART1, LL_USART_DIRECTION_RX);
}

It sets the transfer direction to RX immediately after passing the bytes and enabling DMA. According to this logic the transfer shouldn't work at all because the USART transmitter is immediately disabled after enabling DMA.

That being said, not all of the other targets do this. Some of them enable the transmitter in init() and never disable it.