Trouble settings SODAQ EXPLORER to 300,E,7,1

[GiroSinTornillo]

Hi guys
I am trying to setup a sodaq explorer board to communicate in 300,e,7,1 as described at the section "Parameters" in the Arduino Reference https://www.arduino.cc/reference/en/language/functions/communication/serial/begin/

The board seems to not be responding to such configuration .
The same sketch run fine on a Arduino Mega (changind port names obviously).

Sodaq sketch:

    #include "Arduino.h"`

    #define debugSerial SerialUSB
    #define SerialPort Serial

    //#define AL
    #define AC

    #if defined AC
      int BaudRate = 300;
      int BitsParityStop = SERIAL_7E1; // ---->this setup only work in arduino mega sketch, not here in sodaq explorer
    #elif defined AL
      int BaudRate = 1200; 
      int BitsParityStop = SERIAL_8N1;
    #endif
    void setup() {


      while ((!debugSerial) && (millis() < 10000)){
        // wait 10 seconds for serial monitor
      }
      
      debugSerial.begin(115200);
      SerialPort.begin (BaudRate,BitsParityStop);   // Inicializo el puerto serial
    }

    void loop() {  

        //send and receive data with serial
         if (debugSerial.available()){
          while (debugSerial.available()) {
            uint8_t inChar = debugSerial.read();
            SerialPort.write(inChar);
          }
         }

         if (SerialPort.available()){
          while (SerialPort.available()) {
            uint8_t inChar = SerialPort.read();
            debugSerial.write(inChar);
          }
         }
    }

Arduino Mega Sketch (Working fine)

#include "Arduino.h"

#define debugSerial Serial
#define SerialPort Serial1

//#define AL
#define AC 

#if defined AC
  int BaudRate = 300;
  int BitsParityStop = SERIAL_7E1; ---->this setup only work fine here  in the arduino mega sketch
#elif defined AL
  int BaudRate = 1200; 
  int BitsParityStop = SERIAL_8N1; // 
#endif

void setup() {

  while ((!debugSerial) && (millis() < 10000)){
  }
  
  debugSerial.begin(115200); 
  SerialPort.begin (BaudRate,BitsParityStop);   // Inicializo el puerto serial 
}

void loop() {  

    //send and receive data with serial
     if (debugSerial.available()){
      while (debugSerial.available()) {
        uint8_t inChar = debugSerial.read();
        SerialPort.write(inChar);
      }
     }

     if (SerialPort.available()){
      while (SerialPort.available()) {
        uint8_t inChar = SerialPort.read();
        debugSerial.write(inChar);
      }
     }
}

Can tell me how to see the library that process the parameters to check if is correctly implemented?
Or what suggest to solve this issue?

THanks in advance.

[GiroSinTornillo]

I just discovered the problem as it originates, it is actually something much more serious than I thought. SODAQ EXPLORER does not respect timings for 300bps, it sends 9 bits in 32.6 milliseconds instead of 30 milliseconds.

Can anyone tell me why this speed error is due? Is it a bug?

Thanks in advance.

[Jan]

Hi @GiroSinTornillo,

SODAQ EXPLORER does not respect timings for 300bps

Do you have this issue with both hardware serials?

SerialUSB has a autobaud function, can you try to request this baudrate from your pc?
Serial should respect your requested settings.

Best regards,
Jan

[GiroSinTornillo]

Hi Jan
Thanks by your fast reply. I have tested this fail on Serial that uses D0/D1,
A fast verification is to setup the port to 300,n,8,1 and send 0x00, then there are 9 bits to zero, counting the StartBit.
@300 bps, each bit must have exactly 3,3333 mSec. so, 9 bits must remain low over 30,0mSec. and i verify on scope that SODAQ EXPLORER remains low for 32,6mSeg, it{s 276 bps, and error of 7,9% in the timming.

Best Regards

Hugo

[GiroSinTornillo]

Hi Jan
Do you need some additional info or proof of concept about the issue?
I suggest to send a byte in 300,n,8,1 and check in an oscilloscope the timming. Bit rate is not 300 bps, is less, about 276 bps from my readings.

Thanks in advance

[Jan]

Hi @GiroSinTornillo,

Looking into the datasheet I don't know if this can be improved. Look like there is always some small error rate on the baudrate. Datasheet:

SAMD21-Family-DataSheet-DS40001882D.pdf

Best regards,
Jan

[GiroSinTornillo]

ook like there is always some small error rate on the baudrate. Datasheet:

Hi Jan
Could you specify the page numbers from where you deduce that the error present in 300bps is acceptable according to the data sheet? and quote the text please, I don't see anything that supports this theory.

Can you refer to the library section where baud rate is setted after a .begin methoed execution?
To review them.

Regards

[Jan]

Page 463
25.6.2.3
Here is points out that the baudrate is calculated by the used clock.
There will be always an error rate.

Can you refer to the library section where baud rate is setted after a .begin methoed execution?

.begin is a function from the Arduino SAMD Core.
See their GitHub page: https://github.com/arduino/ArduinoCore-samd

Best regards,
Jan

[GabrielNotman]

Can you try transmitting 0xAA and then graph that on the scope.
The alternating bit pattern should make it easy to see the duration of the start bit, the data bits and the stop bit.

This should help figure out where the issue is.

[GabrielNotman]

Also, our core uses slightly different code than the main Arduino SAMD core for UARTs running < 600 BAUD.

If you want you can try modifying the following file in the Sodaq SAMD core.

In Uart.cpp remove lines 70-74 leaving only:

sercom->initUART(UART_INT_CLOCK, SAMPLE_RATE_FRACT_x16, baudrate);

If you are using Windows you can normally find that file in:
%localappdata%/Arduino15/packages/SODAQ/hardware/samd/1.6.20/cores/arduino

[GiroSinTornillo]

see the duration of the start bit,

Yes, Gabriel, i made it.
Bit rate is not 300 bps, is less, about 276 bps from my readings.

SODAQ EXPLORER does not respect timings for 300bps, it sends 9 bits in 32.6 milliseconds instead of 30 milliseconds.

1 bit have 0,0036222 seconds (3,6222 mSeg), so 1/Tbit=276 bps baudrate. Checked on the osciloscope.

[GiroSinTornillo]

In Uart.cpp remove lines 70-74 leaving only:

sercom->initUART(UART_INT_CLOCK, SAMPLE_RATE_FRACT_x16, baudrate);

That apparent arithmetic rounding, instead of using a fractional factor may be the reason for the error, later I will be testing it, on the sodaq board, after editing that library and recompile.

It is very interesting what you explain to me where the libraries are located, I did not know how to get to that sourcecode.

What was the intended objective for that conditional adjustment for speeds below 600bps?

By the way, the constants SAMPLE_RATE_ARITH_x16 and SAMPLE_RATE_FRACT_x16 where they are defined? To better understand how it works.

I clarify that my knowledge about library management and creation is very basic, especially if we talk about the Arduino core.

I will update the results for the change suggested, later in my lab.

Thanks in advance.

[GabrielNotman]

We forked from the Arduino core quite early on in its development.

We need a fix for one of our projects, which is what you see there.
Later the Arduino core applied a difference fix and we have not merged the two.

Almost all developments within the Arduino core are pulled into our core.
I believe we are mostly level with Arduino core release 1.6.20.

SAMPLE_RATE_FRACT_x16 can be found in an enum SERCOM.h.

I'm curious if the just using the fractional mode resolves this.

[GiroSinTornillo]

In Uart.cpp remove lines 70-74 leaving only:

sercom->initUART(UART_INT_CLOCK, SAMPLE_RATE_FRACT_x16, baudrate);

If you are using Windows you can normally find that file in:
%localappdata%/Arduino15/packages/SODAQ/hardware/samd/1.6.20/cores/arduino

Hi Gabriel
bad news... fractional mode does not resolve the issue, on the contrary, the error worsens, leaving a baudrate of 1666 bps.

Without library modification, transmiting "U" 0x55 character, at 300n81, the waveform is this

and with the suggested change, the waveform is:

I don't know if you want me to do another test, you tell me ..

Regards

Hugo

[GiroSinTornillo]

I was thinking in to try SoftwareSerial, but i see this post http://forum.sodaq.com/t/sodaq-not-finding-softwareserial-h/338 So, I discard the idea of trying it with a serial library by software.

[GabrielNotman]

I believe there are a number of things at play here, and ultimately the combination of the clock setup and low baudrates doesn't seem to mix well.

One thing you could try is changing line 81 of Sercom.cpp to: (Edit: First revert any changes suggested above.)

term1 += (SystemCoreClock / 2);   // Some rounding

This will change the direction of the rounding.

Alternatively, you could try replacing the following files in the Sodaq Core.

1. Sercom.h
2. Sercom.cpp
3. Uart.cpp

You should replace them with the files from the Arduino Core.
This should make the UART behave exactly as in the main Arduino Core (I'm not sure if this will help).

arduino/ArduinoCore-samd