MIMXRT1050 lpuart bug when changing serial format

[grumpyengineer]

If you are using one of the MIMXRT lpuarts and call set_format to change the serial format the UART can either stop working completely or will transmit but at a very strange baud rate. Example if set to 115200 I was seeing an actual baud of 142900, if anything was transmitted at all.

I was using BufferedSerial in my project.
I did not test UnbufferedSerial as I found the issue in the target's support files.

Target(s) affected by this defect ?

MIMXRT1050_EVK

Toolchain(s) (name and version) displaying this defect ?

ARM Compiler 6.16

What version of Mbed-os are you using (tag or sha) ?

mbed OS 6.15.1

The bug still exists in the github master

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

MBed studio 1.4.4
ARMC 6.16

How is this defect reproduced ?

I was working with lpuart3. The default console is running on lpuart1 with default settings apart from baud rate with no issues.

#define CONTROL_SERIAL_TX	GPIO_AD_B1_06
#define CONTROL_SERIAL_RX	GPIO_AD_B1_07
...
	BufferedSerial *serialPort;
	char buf[64];

	serialPort = new BufferedSerial(CONTROL_SERIAL_TX, CONTROL_SERIAL_RX);

	serialPort->set_blocking(true);
	serialPort->set_baud(115200);
	serialPort->set_format(8, BufferedSerial::None, 1);

	sprintf(buf, "This is a test message");
	serialPort->write(buf, strlen(buf));

The bug is in:
mbed-os\targets\TARGET_NXP\TARGET_MCUXpresso_MCUS\TARGET_MIMXRT1050\TARGET_EVK\serial_api.c

void serial_format(serial_t *obj, int data_bits, SerialParity parity, int stop_bits)
{
    LPUART_Type *base = uart_addrs[obj->index];
    uint8_t temp;
    /* Set bit count and parity mode. */
    temp = base->CTRL & ~(LPUART_CTRL_PE_MASK | LPUART_CTRL_PT_MASK | LPUART_CTRL_M_MASK);
...
    base->CTRL = temp;

All the control registers in the IMXRT1050 are 32bit. base->CTRL has a default value of 0x002c0000.
temp is a uint8_t so when you read in base->CTRL it gets truncated to 0x00.
When it is written back to base->CTRL the lpuart stops working.

Changing temp to a uint32_t resolves the bug

[mbedmain]

@grumpyengineer thank you for raising this issue.Please take a look at the following comments:

Could you add some more detail to the description? A good description should be at least 25 words.

NOTE: If there are fields which are not applicable then please just add 'n/a' or 'None'. This indicates to us that at least all the fields have been considered.
Please update the issue header with the missing information.

[0xc0170]

cc @ARMmbed/team-nxp

[multiplemonomials]

Thanks for finding this issue and finding a patch for it! I have submitted your change as a PR on the mbed-ce fork: mbed-ce#134

[0xc0170]

Changing temp to a uint32_t resolves the bug

@grumpyengineer could you please send a fix via a pull request?

[grumpyengineer]

Pull request #15382 created. I also added the fix to the MIMXRT1170_EVK as it has the same uint8_t temp var to hold a 32 bit register.