No software support for UARTE various data bits lenghts

[staead]

I am developing a modbus server application for nrf54L15 in nrf sdk v3.2.3. But I ran into an issue where using MODBUS_MODE_ASCII requires UART_CFG_DATA_BITS_7. The modbus library tries to use seven databits for the ascii mode, but when configuring the serial, it fails due to the serial library rejecting any other data bits length than eight.

This is being set in modbus_serial.c:

if (ctx->mode == MODBUS_MODE_ASCII) {
	uart_cfg.data_bits = UART_CFG_DATA_BITS_7;
} else {
	uart_cfg.data_bits = UART_CFG_DATA_BITS_8;
}

and the problem is in uart_nrfx_uarte.c:

if (cfg->data_bits != UART_CFG_DATA_BITS_8) {
	return -ENOTSUP;
}

But to my understanding, nrf45L15 should support various data bit lengths, according to the datasheet (8.25.5 Data frame size):

"UARTE implements a configurable data frame size of 4 bits to 9 bits"

So is there really no software support for the new hardware or am I doing something wrong?

[FedericaMayorga]

Hello staead,

Did you try commenting out the section in uart_nrfx_uarte.c that rejects data bit lengths other than 8, to see if it works?

Best regards,
Federica from OWL Services

[staead]

Hello FedericaMayorga,

I tried to comment it out, not only that, I also change other parameters in the sdk code, because the condition is just a hard stop, but removing it does not change the frame size.

#if NRF_UARTE_HAS_FRAME_SIZE
	uarte_cfg.frame_size = NRF_UARTE_FRAME_SIZE_8_BIT;
	uarte_cfg.endian = NRF_UARTE_ENDIAN_MSB;
#endif

But nothing I tried worked.
Also, I feel like changing the SDK libraries is not really a long-term solution, and I was hoping to get a statement from the Nordic developer team regarding this issue.

[Kenneth]

Hello,

I have reported it internally now so it an be improved in future.

Kenneth

[Kenneth]

Hello,

I am not sure if and when this will be prioritized, but one of the developers have theorized a bit what must be done:

1. remove conditional preventing non-8-bit setting: sdk-zephyr/drivers/serial/uart_nrfx_uarte.c at a09b9a98666f250a84059b7c88e1e01e96316488 · nrfconnect/sdk-zephyr

2. enable CONFIG_UART_USE_RUNTIME_CONFIGURE=y

3. in application context, before using uart for MODBUS, call sdk-zephyr/include/zephyr/drivers/uart.h at a09b9a98666f250a84059b7c88e1e01e96316488 · nrfconnect/sdk-zephyr on MODBUS uart device, providing struct uart_config *cfg as argument, where cfg must contain a valid configuration with .data_bits = UART_CFG_DATA_BITS_7,. Rest of configuration can be taken from this line: sdk-zephyr/drivers/serial/uart_nrfx_uarte.c at a09b9a98666f250a84059b7c88e1e01e96316488 · nrfconnect/sdk-zephyr as starting point.

4. UART_CFG_DATA_BITS_x must be translated into nrfx HAL-specific defines, by replacing this line: sdk-zephyr/drivers/serial/uart_nrfx_uarte.c at main · nrfconnect/sdk-zephyr with following pseudocode:

   switch (cfg->data_bits)
{
case UART_CFG_DATA_BITS_8:
    uarte_cfg.frame_size = NRF_UARTE_FRAME_SIZE_8_BIT; break;
    case UART_CFG_DATA_BITS_7: uarte_cfg.frame_size = NRF_UARTE_FRAME_SIZE_7_BIT; break;
    default: return -ENOTSUP;
}

Hope that helps,
Kenneth