fsl_usart.c: How to properly handle RX FIFO overflows, i.e., kStatus_USART_RxError

danielholala — Fri, 16 Jan 2026 15:46:02 GMT

Hello,

on LPC5528/LPC5526/LPC55xx, I'm using the USART driver from the SDK, see

https://github.com/nxp-mcuxpresso/mcuxsdk-core/blob/main/drivers/flexcomm/usart/fsl_usart.c#L1123

The code works properly when requesting to receive single bytes using

USART_TransferReceiveNonBlocking()

It also recovers on RX FIFO overflows by reinitiating reception.

Situation is different when using

USART_TransferReceiveNonBlocking()

with xfer->dataSize > 1. For example, the receive callback inspects the RX FIFO level and if the RX FIFO level is > 1, it calls USART_TransferReceiveNonBlocking() with the exact number of bytes of in the RX FIFO to retrieve these bytes more efficiently, say 10 bytes.

However, if the system is busy handling other interrupts, the RX FIFO might overflow. Then

void USART_TransferHandleIRQ(USART_Type *base, usart_handle_t *handle)

clears the RX FIFO overflow bit, empties the FIFO and calls the callback with kStatus_USART_RxError. All the bytes from the FIFO are therefore discarded and cannot be received any more.

However, it does not reset rxDataSize. So the callback will not be called again until the number of bytes (in the example ten bytes) have been received. Until this, the receive mechanics hangs.

How do you recover from this situation?

Resetting the dataSize by calling USART_TransferReceiveNonBlocking() with xfer->dataSize = 1does not work as the receive mechanics is still in status kUSART_RxBusy.

So I wonder what is the proper way to handle RX FIFO overflows with the SDK drivers.

Thanks.

Daniel

PS: sorry for the strange text formatting. I tried to markup code so it is easier to read but screwed up.

Re: fsl_usart.c: How to properly handle RX FIFO overflows, i.e., kStatus_USART_RxError

Habib_MS — Tue, 20 Jan 2026 20:40:33 GMT

Hello @danielholala,
I understand that you have an issue with how to clear rxDataSize. From what I can see, the SDK (version 25.09) does not handle this automatically, but you can call the function USART_TransferAbortReceive inside the callback. This function clears the rxDataSize, as shown in the following image:

However, this function also aborts the current transfer, so you will need to resume the transmission manually.

Lef me know if this helps you.
BR
Habib

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Re: fsl_usart.c: How to properly handle RX FIFO overflows, i.e., kStatus_USART_RxError