Establishing serial communication over UART instead of USB in MAX25405 EVKIT

I was experimenting with the different options on the MAX32620FTHR board to print serial messages. I created some simple code to test behaviour using the latest MbedOS 6.16.

I made life easier and also used a wrapper library, which you would need to import into your project: os.mbed.com/.../

Here is the example code:

/* mbedOS 6.16
 * Board: MAX32620FTHR
 * Copyright (c) C Gerrish
 * SPDX-License-Identifier: Apache-2.0
 */

#include "mbed.h"
#include "BufferedSerial.h"
#include "SerialStream.h"
#include "USBSerial.h"

// Blinking rate in milliseconds
#define BLINKING_RATE       1000ms

#define UART_BAUD_RATE      115200

BufferedSerial fthrUART(P3_1,P3_0, UART_BAUD_RATE);
// This SerialStream is a wrapper for Stream.h to give you printf
SerialStream<BufferedSerial> RXTX(fthrUART);

// true here blocks until USB serial monitor opens
// in Arduino you would use while(!Serial) for same effect
USBSerial USB(true, 0x0b6a, 0x4360, 0x0001);

int main()
{
    // Initialise the digital pin LED1 as an output
    DigitalOut led(LED1);

    int cntr = 0;

    while (true) {
        led = !led;
        ThisThread::sleep_for(BLINKING_RATE);
        USB.printf("hello USB world %d\r\n", cntr);
        RXTX.printf("hello RXTX world %d\r\n", cntr);
        printf("hello DEBUG world %d\r\n", cntr);
        cntr++;
    }
}

The mbed_app.json file:

{
    "target_overrides": {
        "*": {
            "platform.stdio-baud-rate": 115200,
            "platform.stdio-buffered-serial": 1
        }
    }
}

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RE: Establishing serial communication over UART instead of USB in MAX25405 EVKIT

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[Post EDIT]

Looking at the original MAX25x05 gesture library again to see how the serial part could be adapted to OS6.16 I would say that you could use...

the UnbufferedSerial API for when USE_UART_INTERFACE = 1 as this only provide putc etc.

I've just realised that the above statement is not correct.

In OS6.xx, it is not that simple to use putc anymore. In fact, I think the only way I could find is though the SerialStream wrapper. Not worth the effort, IMHO. If I had read the UnbufferedSerial API documentation more carefully then I would see that it only provides a write function. Sorry about that.

and you could use BufferedSerial API for when USE_UART_INTERFACE = 0 as this provide buffered writes etc.

Hope that helps.

I was thinking about how we could minimise changes in the code, so here is one option that simplifies things.

This code will either print to the debug/console UART pins (as defined in pinNames.h) or to UART pins of choice.

#include "mbed.h"

#define USE_UART_INTERFACE 1

#if USE_UART_INTERFACE
    #define printf(f_, ...) fprintf((f_), __VA_ARGS__);
#else
    #define printf(f_, ...) printf( __VA_ARGS__);
#endif

BufferedSerial otherUART(P3_1,P3_0,MBED_CONF_PLATFORM_STDIO_BAUD_RATE);
FILE* myTX = fdopen(&otherUART, "r+");

// main() runs in its own thread in the OS
int main()
{
    printf(myTX,"Hello New World\r\n");

    while (true) {

    }
}

I measured on Tx 0V and Rx 0.8V

Interesting. RX is OK but TX should be in logical 1 when indle but your pin is in logical 0 state for some reason. Does the 0V remains after reset?

Yes Tx remains 0V after reset as well. I'll try some example firmware for UART to check if it works. 

Ok. Currently I am out of ideas which can I provide without having access to real device.

I flashed the board with the code provided by BigG now I read 1.3V on Rx (as written on the PCB of MCU) and 0V on Tx pin (as written on the MCU PCB). I think the interpretation of Tx and Rx could be different on Maxim boards. 

Btw what pin exactly do you use as TX and RX? MAX32620 supports four UARTs. Do you use pins expossed on shield on JP5 or do you connect directly to some pinheader of FTHR board?

No issues. I'll try some more methods to check the UART. Let me know if you succeed with this over the weekend. 

The code in the framework has defined pins P3_0 and P3_1 as UART pins.  

#if USE_UART_INTERFACE
Serial *serial = new Serial(P3_1,P3_0,UART_BAUD_RATE);

I'm using the same pins connecting wires directly to the pin headers of P3_1 and P3_0

It is the same behaviour. 1.3V is the same as 0.8V. RX pin is input. MAX32620 set this pin as input which means that it do not connect this pin directly to GND (0V) or VDDIO(H) (1.8 or 3.3V depending on fimrware selection). Because pin is virtualy disconnected there theoreticaly is only voltage provided externaly like noise, inducted voltage from your body, signal from aliens and so on. Practicaly this voltage come also from implementation of digital logic.

Yes. Pins 3.0 and 3.1 should be correct. These pins are also routed to shield to JP5.

Yes. Pins 3.0 and 3.1 should be correct. These pins are also routed to shield to JP5.