Direct port access in Arduino IDE

Stefan_69 · August 22, 2020, 3:29pm

Hello,

just received my first XIAO board and I’m quite impressed with its performance so far.

As I want to use it to manipulate an incoming 800 Hz PWM signal (connected to an input pin set up for trggering pin change interrupts) I’m looking for a way to access (i.e. read from and write to) the digital IO pins directly without using digitalRead/digitalWrite.

Is there a similar approach as for Arduinos where one can use
for example PORTB = PINB | B00100000 to set a bit?

Thanks and best regards,
Stefan

ansonhe97 · August 24, 2020, 1:28am

Something like this:

EPortType port = g_APinDescription[ulPin].ulPort; 
 uint32_t pin = g_APinDescription[ulPin].ulPin; 
 uint32_t pinMask = (1ul << pin); 
PORT->Group[port].OUTCLR.reg = pinMask;

PORT->Group[port].OUTCLR.reg = pinMask; //LOW

PORT->Group[port].OUTSET.reg = pinMask; //HIGH

Stefan_69 · August 25, 2020, 7:17pm

Hello ansonhe97,

thanks for the feedback, just tested the proposed code and it executes ~ 1 us faster than digitalWrite.
Going to explore the speed up potential some more.

All the best,
Stefan

s.micic · September 2, 2020, 5:44am

Hello Stefan_69,

Can you present here piece of your code where you are using this approach in order to make your code faster?
The proposed code is somehow unclear for me to understand…

regards,
s.micic

Stefan_69 · September 8, 2020, 4:33pm

Hello s.micic,

sorry for the late response, was quite busy at work … Anyhow, I’ll paste my code below, as mentioned in the original post I want to modify the duty cylce of an incoming PWM signal while keeping its frequency constant. You will find both methods (“Arduino style” and “fast style”) for setting the GPIO pin.

If you are using the Arduino IDE you can also have a look at “wiring_digital.c” which can be found under “AppData\Local\Arduino15\packages\Seeeduino\hardware\samd\1.7.9\cores\arduino”

Hope this helps,
Stefan

const byte inPin = 1;
const byte interruptPin = 1;
const byte outPin = 3;
const byte testPin = 5;
uint32_t ulPin = 5;
uint32_t myPin = 1 ;

volatile byte state = LOW;
volatile byte myvar;
//
volatile EPortType port = g_APinDescription[ulPin].ulPort;
volatile uint32_t pin = g_APinDescription[ulPin].ulPin;
volatile uint32_t pinMask = (1ul << pin);

void setup() {
pinMode(inPin, INPUT);
pinMode(outPin, OUTPUT);
pinMode(testPin, OUTPUT);
pinMode(interruptPin, INPUT_PULLUP);
attachInterrupt(digitalPinToInterrupt(interruptPin), blink, CHANGE);
}

void loop() {
}

void blink() {
PORT->Group[port].OUTCLR.reg = pinMask;
if (digitalRead(inPin) == 0) {
// PORT->Group[port].OUTCLR.reg = pinMask; //LOW
digitalWrite(outPin,LOW);
}
else {
delayMicroseconds(10); // modify duty cycle
// PORT->Group[port].OUTSET.reg = pinMask; //HIGH
digitalWrite(outPin,HIGH);
}
}

Stefan_69 · September 8, 2020, 4:37pm

… sorry, forgot to include the option for “fast reading” the GPIO pin (used in the if statement):