Setting BufferedSerial pin to NC throws runtime assertion

[mark-psl]

Description of defect

A runtime assertion happens on FRDM-K64F when assigning a pin name as "NC". Specifically this happens on BufferedSerial() when assigning the Rx pin to be NC. This is in conflict with the mbed-os API document on BufferedSerial().

TX and RX pins - you can specify either pin as Not Connected (NC) for simplex (unidirectional) communication or both as valid pins for full duplex (bidirectional) communication.

The following is the assertion:

++ MbedOS Error Info ++ Error Status: 0x80FF0144 Code: 324 Module: 255 Error Message: Assertion failed: pin != (PinName)NC Location: 0x85F7 File: ./mbed-os/targets/TARGET_Freescale/TARGET_MCUXpresso_MCUS/api/pinmap.c+27 Error Value: 0x0 Current Thread: main Id: 0x1FFF21DC Entry: 0x91CD StackSize: 0x1000 StackMem: 0x1FFF0C30 SP: 0x1FFF1AD0 For more info, visit: https://mbed.com/s/error?error=0x80FF0144&tgt=K64F -- MbedOS Error Info --

It appears the assertion happens in the pin_function() routine called by the initialization code in serial_api.c for this target.
In other targets where this is not an issue (eg. FRDM-KL46Z) the call to pin_function() is first checked to ensure the pin is not NC.
In the target in question (FRDM-K64F) the pin_function() calls are done without the check even through if(!=NC) checks are done just below it to enable the Tx and Rx pins.

Starting at line 66 it currently looks like:

pin_function(pinmap->tx_pin, pinmap->tx_function);
pin_function(pinmap->rx_pin, pinmap->rx_function);

if (pinmap->tx_pin != NC) {
    UART_EnableTx(uart_addrs[obj->serial.index], true);
    pin_mode(pinmap->tx_pin, PullUp);
}
if (pinmap->rx_pin != NC) {
    UART_EnableRx(uart_addrs[obj->serial.index], true);
    pin_mode(pinmap->rx_pin, PullUp);
}

I moved the pin_function() calls into the if(!=NC) sections for Tx and Rx as such:

if (pinmap->tx_pin != NC) {
    pin_function(pinmap->tx_pin, pinmap->tx_function);
    UART_EnableTx(uart_addrs[obj->serial.index], true);
    pin_mode(pinmap->tx_pin, PullUp);
}
if (pinmap->rx_pin != NC) {
    pin_function(pinmap->rx_pin, pinmap->rx_function);
    UART_EnableRx(uart_addrs[obj->serial.index], true);
    pin_mode(pinmap->rx_pin, PullUp);
}

This corrects the issue and complies with the documentation on BufferedSerial() in the mbed-os API documentation.

Target(s) affected by this defect ?

FRDM-K64F

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

Mbed Studio v1.3.1

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

mbed-os v6.8

It also appears to be in v6.7

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

Mbed Studio v1.3.1

How is this defect reproduced ?

Set the Rx pin for BufferedSerial() to NC. This will compile and link cleanly but throw a runtime assertion and restart the board.

[ciarmcom]

Thank you for raising this detailed GitHub issue. I am now notifying our internal issue triagers. Internal Jira reference: https://jira.arm.com/browse/IOTOSM-3515

[0xc0170]

Thanks for the report. @mark-psl can you please send a pull request fixing this?

[mark-psl]

Sure. Not sure how to do that correctly for this issue. Do you have a branch that I can refer to?

[0xc0170]

Create your own branch with a fix and send it via "new pull request"

[mark-psl]

I'm unable to create a branch from master. I can only filter on this repo.

[ladislas]

you need to fork the repo first.

[mark-psl]

Done. PR#14387 submitted. I'm a noob at this so thanks for your patience.