SoftwareSerial Freezes on Certain Pins (Analog?)

[chrissbarr]

Describe the bug
SoftwareSerial seems to cause program execution to freeze at startup when using certain pins.

So far in my testing it looks like the freeze happens if:

• Pins are specified using the Ax analog naming convention, or
• The PYn convention is used with pins that map to an Ax convention in variant.h

The freeze only seems to happen when both pins meet the above - if one maps to an analog pin and the other does not, the freeze does not happen.

Results are consistent on both 1.9.0 and current master branch. I am testing with a RUMBA32 board which is the only STM32 board I have to hand at the moment, but I can try to round up a different dev board shortly.

Is SoftwareSerial expected to work when using the Ax & PYn pin naming conventions?

To Reproduce
I am testing with the following sketch:

#include <SoftwareSerial.h>

#define PIN_SS_RX PA3
#define PIN_SS_TX PA4

SoftwareSerial mySerial(PIN_SS_RX, PIN_SS_TX);

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {  
  digitalWrite(LED_BUILTIN, HIGH);
  delay(500);
  digitalWrite(LED_BUILTIN, LOW);
  delay(500);
}

With the pin pairs that seem to cause a 'freeze', the LED does not blink. I've tested with the following pin pairs.

These work (LED blinks / sketch runs):

// Two analog - one using PYn, the other digital pin number
#define PIN_SS_RX 3
#define PIN_SS_TX PA4

// Two analog - one using PYn, the other digital pin number
#define PIN_SS_RX PA3
#define PIN_SS_TX 4

// Two analog - one using PYn, the other digital pin number
#define PIN_SS_RX 4
#define PIN_SS_TX PA3

// Two analog - one using PYn, the other digital pin number
#define PIN_SS_RX PA4
#define PIN_SS_TX 3

// Two non-analog - both using PYn
#define PIN_SS_RX PA8
#define PIN_SS_TX PA9

// Two analog - one using PYn, the other digital pin number
#define PIN_SS_RX 4
#define PIN_SS_TX PB0

// One analog, one digital, both using PYn
#define PIN_SS_RX PA8
#define PIN_SS_TX PB0

// One analog using Ax, one digital using PYn
#define PIN_SS_RX A0
#define PIN_SS_TX D4

These do not work (no LED blinking):

// Two analog - both using PYn
#define PIN_SS_RX PA3
#define PIN_SS_TX PA4

// Two analog - both using PYn (swapped)
#define PIN_SS_RX PA4
#define PIN_SS_TX PA3

// Two analog - both using PYn (non-adjacent pins)
#define PIN_SS_RX PA4
#define PIN_SS_TX PB0

// Two analog - both using Ax
#define PIN_SS_RX A0
#define PIN_SS_TX A1

// Two analog - both using Ax (different pins just in case)
#define PIN_SS_RX A5
#define PIN_SS_TX A6

These particular pins as described in the RUMBA32 variant.h:

#define PA3   A0  //D3
#define PA4   A1  //D4
#define PA8   8   //D8
#define PA9   9   //D9
#define PB0   A13 //D16
#define PC3   A5  //D35
#define PC4   A6  //D36

Steps to reproduce the behavior:

1. Try the above sketch with two analog pins (try Ax naming convention, and PYn, etc.)
2. Check if the LED blinks?

Expected behavior
Program should not freeze / should execute as expected when using these pins.

Desktop (please complete the following information):

• OS: Windows
• Arduino IDE version: 1.8.12 and 1.9.0-beta
• STM32 core version: 1.9.0 and master branch (f190402)
• Tools menu settings if not the default: USART: Enabled (Generic 'Serial'), USB: CDC (Generic 'Serial')
• Upload method: STM32CubeProgrammer DFU

Board (please complete the following information):

• Name: RUMBA32
• Hardware Revision: Rev 1.0D

Additional context
I noticed this during testing of the SERIAL_TIMER as part of PR #1092

[fpistm]

Hi @chrissbarr
seems strange.

Is SoftwareSerial expected to work when using the Ax & PYn pin naming conventions?

Yes it should anyway after testing with a Nucelo_F411RE I've got the same issue.
We will investigate that.

[chrissbarr]

Hi @fpistm,

Yes it should anyway after testing with a Nucelo_F411RE I've got the same issue.

Well, I'm glad it's not just me - thought I was going crazy...

Thanks for trying this so quickly.