Implement LED PWM brightness control

Makefile

@@ -10,6 +10,9 @@ LDFLAGS += -g -nostdlib -mmcu=am1808.pru1.specs
 
 all: $(BUILD_TARGET).bin
 
+# Manual dependency tracking
+main.o: am1808.h
+
 $(BUILD_TARGET).bin: $(BUILD_TARGET).elf
 
 $(BUILD_TARGET).elf: start.o main.o

am1808.h

@@ -0,0 +1,71 @@
+#ifndef __PRU_AM1808_H__
+#define __PRU_AM1808_H__
+
+// PRU0/1 Local INTC
+#pragma ctable_entry 0  0x00004000
+// Timer64P0
+#pragma ctable_entry 1  0x01C20000
+// I2C0
+#pragma ctable_entry 2  0x01C22000
+// PRU0/1 Local Data (self)
+#pragma ctable_entry 3  0x00000000
+// PRU1/0 Local Data (other)
+#pragma ctable_entry 4  0x00002000
+// MMC/SD
+#pragma ctable_entry 5  0x01C40000
+// SPI0
+#pragma ctable_entry 6  0x01C41000
+// UART0
+#pragma ctable_entry 7  0x01C42000
+// McASP0 DMA
+#pragma ctable_entry 8  0x01D02000
+// Reserved
+#pragma ctable_entry 9  0x01D06000
+// Reserved
+#pragma ctable_entry 10 0x01D0A000
+// UART1
+#pragma ctable_entry 11 0x01D0C000
+// UART2
+#pragma ctable_entry 12 0x01D0D000
+// USB0
+#pragma ctable_entry 13 0x01E00000
+// USB1
+#pragma ctable_entry 14 0x01E25000
+// UHPI Config
+#pragma ctable_entry 15 0x01E10000
+// Reserved
+#pragma ctable_entry 16 0x01E12000
+// I2C1
+#pragma ctable_entry 17 0x01E28000
+// EPWM0
+#pragma ctable_entry 18 0x01F00000
+// EPWM1
+#pragma ctable_entry 19 0x01F02000
+// Reserved
+#pragma ctable_entry 20 0x01F04000
+// ECAP0
+#pragma ctable_entry 21 0x01F06000
+// ECAP1
+#pragma ctable_entry 22 0x01F07000
+// ECAP2
+#pragma ctable_entry 23 0x01F08000
+
+/* 24 and 25 are variable */
+// 24 = PRU0/1 Local Data   0x00000n00
+// 25 = McASP0 Control      0x01D00n00
+
+// Reserved
+#pragma ctable_entry 26 0x01D04000
+// Reserved
+#pragma ctable_entry 27 0x01D08000
+
+/* 28-31 are variable */
+// 28 = DSP Megamodule RAM/ROM      0x11nnnn00
+// 29 = EMIFA SDRAM                 0x40nnnn00
+// 30 = Shared RAM                  0x80nnnn00
+// 31 = mDDR/DDR2 Data              0xC0nnnn00
+
+// Timer0 peripheral (only what we need)
+static volatile uint32_t * const TIMER0_TIM34 = (volatile uint32_t *)0x01C20014;
+
+#endif

main.c

@@ -1,3 +1,52 @@
+#include <stdint.h>
+
+#include "am1808.h"
+
+// Shared RAM (control input)
+typedef struct shared_ram {
+    // Declare this as a u32 to force more-efficient codegen
+    uint32_t pwms;
+} shared_ram;
+// XXX: The real address in use here is 0x80010000
+// There appears to be a compiler bug where ctable entries with the MSB set
+// do not get optimized correctly. We lie to the PRU compiler here, but the
+// hardware indeed contains the correct address we *actually* want.
+static volatile shared_ram * const SHARED = (volatile shared_ram *)0x7f010000;
+#pragma ctable_entry 30 0x7f010000
+
+// LED definitions
+// LEDx corresponds to DIODEx in the EV3 schematics
+// LEDx defined as n ==> pin is PRU1_R30[n]
+#define LED0    12
+#define LED1    10
+#define LED2    13
+#define LED3    11
+
+static inline void update_pwm(uint8_t val, uint8_t time_now, uint32_t gpio_bit) {
+    // We want to force generation of the optimized set/clr opcodes
+    if (time_now < val) {
+        asm volatile("set r30, r30, %0"::"I"(gpio_bit));
+    } else {
+        asm volatile("clr r30, r30, %0"::"I"(gpio_bit));
+    }
+}
+
 void main() {
-    __halt();
+    uint32_t pwms = 0;
+    while (1) {
+        // 24 MHz / 256 ==> 93.75 kHz tick rate for this counter
+        uint8_t time_now = (*TIMER0_TIM34) >> 8;
+
+        if (time_now == 0) {
+            // 24 MHz / 256 / 256 ~= 366 Hz update rate
+            pwms = SHARED->pwms;
+        }
+
+        update_pwm(pwms >> 0, time_now, LED0);
+        update_pwm(pwms >> 8, time_now, LED1);
+        // Intentional swap of 2 and 3
+        // This puts the pins into the order R, G, R, G
+        update_pwm(pwms >> 16, time_now, LED3);
+        update_pwm(pwms >> 24, time_now, LED2);
+    }
 }