Is it safe to use SerialPIO with FreeRTOS SMP?

[agent106]

The FreeRTOS doc page says that
"While the core now supports FreeRTOS, most (probably all) Arduino libraries were not written to support preemptive multithreading."

How about SerialPIO? I want to create two separate tasks that use a few software serial UART interfaces and if I use mutexes to guard each individual SerialPIO instance - should it be okay? In other words, is it safe to use different software UARTS simultaneously from different threads.

Thank you!!

[earlephilhower]

It should be safe, and if it's not then I'm sure it can be made so. (It's not tested, though, so be prepared to debug a little!)

If you're only using a single thread to talk to each SerialPIO there shouldn't be a need to use a mutex to control access. In fact, there's already one (like in the SerialUSB/UART) to enable both cores to write to it.

The only thing I can see that could be a problem is calling SerialPIO::begin() in multiple threads. The issue is there's shared infrastructure below them (PIOProgram and some static arrays) which are not protected and could get scrambled if run on both cores simultaneously or partially started in 1 task and then restarted in a 2nd swapped in task. So, if you can SerialPIO::begin() from a single task and not the individual tasks I think that would be safe.

[agent106]

Got it, thank you! I will try that, but meanwhile, I'm running into another issue. I want to use 4 SerialPIO connections and I need a separate WiFi connection. 4 soft serial connections seem to work wine, but as soon as I add WiFi connection - the Pico freezes. However, this only happens when I initialize WiFI after setting up the soft serial - if WiFi is initialized first and then I init the 4 serials - everything is okay. Another weirdness is that the problem goes away if I only have 3 soft serials.

Here is the test code that reproduces this:

#include <SoftwareSerial.h>
#include <WiFi.h>

const char *wifi_ssid = "*****";
const char *wifi_pass = "*****";

const int fifo_size = 256;
SerialPIO soft_serial1(15, 14, fifo_size);
SerialPIO soft_serial2(13, 12, fifo_size);
SerialPIO soft_serial3(11, 10, fifo_size);
SerialPIO soft_serial4(7, 6, fifo_size);

void initWifi() {
  Serial.print("Connecting WiFi...");
  WiFi.begin(wifi_ssid, wifi_pass);
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.print("OK IP: ");
  Serial.println(WiFi.localIP());    
}

void setup() {
  Serial.begin(115200);  
  Serial.print("Waiting to start");
  for(int i=0; i<10; i++) {
    delay(1000);
    Serial.print(".");    
  }
  Serial.println("Starting!");


  // The problem
  soft_serial1.begin(115200, SERIAL_8N1);
  soft_serial2.begin(115200, SERIAL_8N1);
  soft_serial3.begin(115200, SERIAL_8N1);
  soft_serial4.begin(115200, SERIAL_8N1); // COMMENT THIS OUT - AND IT WORKS!!!
  
  initWifi();

  // ---------------------------------
}

void loop() {
  Serial.println("I'm okay!");
  delay(1000);
}

To recap, what fails is: init 4 serials before WiFi. To fix, either init the WiFi before serials, or reduce number of serials to 3 or less. The reason I need to init WiFi after I init serials, is that I don't always need WiFI connection - I want to be able to connect and disconnect in loop().

Any ideas? Thanks again!

[earlephilhower]

It's just not possible to use WiFi and 4 soft serials. You're out of PIO state machines. Each serial port needs 2 SMs, and the WiFi core from RPi also needs 2 (I believe...in any case at least one). There are 8 total on-chip, 4 per PIO.

The crash is really a panic in the RPI core when it's not able to allocate a PIO SM to run the soft-SPI that connects up the WiFi chip. If you hook up a Picoprobe you should see the call.

Not really anything you can do about it other than, if possible, only use 1-directional soft-serial port (i.e. if you're only ever sending or only ever receiving) which only needs 1 SM.

[agent106]

Ah! Sorry, I didn't use Picoprobe - my bad! It makes perfect sense - thank you very much!

[agent106]

@earlephilhower Do hardware serials use state machines? I mean, can I have 2 hardware UARTs, 3 software and a WiFi connection?

[earlephilhower]

Serial1 and Serial2 use the hardware UARTs, no PIO SM required. So you can have 3 soft serials, 2 hard serials, and wifi if you're so inclined, like you suggest.