Initial quick and dirty PWM implementation using Adafruit's HardwarePWM.h

code/parasite/lib/parasite/pwm.cpp

@@ -0,0 +1,34 @@
+#include "pwm.h"
+
+#include <Arduino.h>
+#include <HardwarePWM.h>
+#include <nrf.h>
+
+namespace parasite {
+
+namespace {
+// nRF52 PWM specs/guide:
+// https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.nrf52832.ps.v1.1%2Fpwm.html
+// Adafruit's HardwarePWM wapper:
+// https://github.com/adafruit/Adafruit_nRF52_Arduino/blob/a86fd9f36c88db96f676cead1e836377a37c7b05/cores/nRF5/HardwarePWM.cpp
+
+// No scaling. The PWM counter will increase with a frequency of 16MHz.
+constexpr unsigned long kPWMFrequencyPrescale = PWM_PRESCALER_PRESCALER_DIV_1;
+// In conjunction with the PWM clock frequency, kPWMMaxCounter defines the
+// frequency of the square wave.
+constexpr int kPWMMaxCounter = 32;
+// Since we want a duty cycle of 0.5, we flip the PVM output when the counter
+// reaches kPWMMaxCounter / 2/;
+constexpr int kPWMFlipCount = kPWMMaxCounter;
+
+}  // namespace
+
+void SetupSquareWave(int pin_number) {
+  HwPWM0.addPin(pin_number);
+  HwPWM0.setClockDiv(PWM_PRESCALER_PRESCALER_DIV_1);
+  HwPWM0.setMaxValue(kPWMMaxCounter);
+  HwPWM0.writePin(pin_number, kPWMFlipCount);
+  HwPWM0.begin();
+}
+
+}  // namespace parasite

code/parasite/lib/parasite/pwm.h

@@ -0,0 +1,13 @@
+#ifndef _PARASITE_PWM_H_
+#define _PARASITE_PWM_H_
+
+namespace parasite {
+
+// This is a simple, single-channel PWM-based square wave generator.
+// This only ever works for a single pin. If you call this function
+// twice with different pin numbers, nothing good will come out of it.
+// I am particularly proud of how well I resisted making this "generic"
+// and "reusable".
+void SetupSquareWave(int pin_number);
+}  // namespace parasite
+#endif  // _PARASITE_PWM_H_

code/parasite/platformio.ini

@@ -10,9 +10,9 @@
 
 [env:e73-tbb]
 platform = nordicnrf52
-; board = adafruit_feather_nrf52832
+board = adafruit_feather_nrf52832
 ; Additionally, I had to force use_adafruit = True in platforms/nordicnrf52/builder/main.py
-board = e73-tbb
+; board = e73-tbb
 framework = arduino
 build_flags = -DNRF52 -DS132 -DNRF51_S132
 board_build.ldscript = ./ldscripts/nrf52832_s132_v6.ld

code/parasite/src/main.cpp

@@ -1,20 +1,24 @@
 #include <Arduino.h>
 
+#include "pwm.h"
+
 constexpr int kLED1Pin = 17;
 constexpr int kLED2Pin = 18;
+constexpr int kPWMPin = 19;
 
 void setup() {
   Serial.begin(9600);
   pinMode(kLED1Pin, OUTPUT);
   pinMode(kLED2Pin, OUTPUT);
+  parasite::SetupSquareWave(kPWMPin);
 }
 
 void loop() {
   Serial.print("Hello, world\n");
   digitalWrite(kLED1Pin, LOW);
   digitalWrite(kLED2Pin, LOW);
-  delay(1000);
+  delay(500);
   digitalWrite(kLED1Pin, HIGH);
   digitalWrite(kLED2Pin, HIGH);
-  delay(1000);
+  delay(500);
 }

resources.md

@@ -94,5 +94,13 @@ if self.board_config(board).get("build.bsp.name",
                     "package"] = "framework-arduinoadafruitnrf52"
 ```
 
+## What does each line in the linker mean?
+* [The most commented linker script in the world](https://github.com/theacodes/Winterbloom_Castor_and_Pollux/blob/main/firmware/scripts/samd21g18a.ld)
+
 ### Memory layout
-The [SoftDevice S132 spec](https://infocenter.nordicsemi.com/index.jsp?topic=%2Fsds_s112%2FSDS%2Fs1xx%2Fmem_usage%2Fmem_resource_map_usage.html&anchor=mem_resource_map_usage) specifies the layout as the soft device begins at addr 0x0 and the application code comes after it, at the address `APP_CODE_BASE`.
+The [SoftDevice S132 spec](https://infocenter.nordicsemi.com/index.jsp?topic=%2Fsds_s112%2FSDS%2Fs1xx%2Fmem_usage%2Fmem_resource_map_usage.html&anchor=mem_resource_map_usage) specifies the layout as the soft device begins at addr 0x0 and the application code comes after it, at the address `APP_CODE_BASE`.
+
+# PWM
+* [HardwarePWM.cpp](https://github.com/adafruit/Adafruit_nRF52_Arduino/blob/a86fd9f36c88db96f676cead1e836377a37c7b05/cores/nRF5/HardwarePWM.cpp#L112) by Adafruit is a good reference. [Servo.c](https://github.com/adafruit/Adafruit_nRF52_Arduino/blob/4d703b6f38262775863a16a603c12aa43d249f04/libraries/Servo/src/nrf52/Servo.cpp#L154) uses it.
+* [nrf52832 pwm spec](https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.nrf52832.ps.v1.1%2Fpwm.html)
+* [nrf5 SDK PWM reference](https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.sdk5.v11.0.0%2Fgroup__nrf__pwm__hal.html) HAL => Hardware Abstraction Layer?