platform/technic_hub: fix LED full brightness when small duty cycle

b22ad15 didn't fully fix this problem. There are still certain
duty cycle values that cause the LED to be full brightness. This appears
to be a hardware bug where the CxIF interrupt just doesn't fire at at
all. We work around this by manually comparing the CNT and CCRx
registers just as the timer does internally. Colors might be a bit off
at low brightness values, but at least they will now error on the side
of being off rather than full brightness.

Fixes: pybricks/support#224

lib/pbio/platform/technic_hub/platform.c

@@ -324,13 +324,19 @@ const pbdrv_pwm_stm32_tim_platform_data_t
 // for the LEDs. The pin mux doesn't work out, so we have to manually write
 // the GPIOs in the timer interrupt handler.
 void TIM2_IRQHandler(void) {
+    uint32_t cnt = TIM2->CNT;
     uint32_t sr = TIM2->SR;
     uint32_t handled = TIM_SR_UIF;
 
+    // NB: there seems to be hardware bug where sometimes we don't get the CCxIF
+    // interrupt or only get it 1/2 of the time for certain bands of small duty
+    // cycle values. So we are comparing CNT to CCRx in addition to just looking
+    // at the interrupt. This works best when the red and green values are near
+    // each other since that minimizes the error.
+
     // green LED
-    if (TIM2->CCR1 == 0 || sr & TIM_SR_CC1IF) {
-        // If channel 1 duty cycle is 0 or we have reached the CC1 count,
-        // turn the GPIO off
+    if (cnt >= TIM2->CCR1 || sr & TIM_SR_CC1IF) {
+        // If we have reached the CC1 count, turn the GPIO off
         GPIOA->BRR = GPIO_BRR_BR11;
         handled |= TIM_SR_CC1IF;
     } else if (sr & TIM_SR_UIF) {
@@ -339,9 +345,8 @@ void TIM2_IRQHandler(void) {
     }
 
     // red LED
-    if (TIM2->CCR2 == 0 || sr & TIM_SR_CC2IF) {
-        // If channel 2 duty cycle is 0 or we have reached the CC2 count,
-        // turn the GPIO off
+    if (cnt >= TIM2->CCR2 || sr & TIM_SR_CC2IF) {
+        // If we have reached the CC2 count, turn the GPIO off
         GPIOB->BRR = GPIO_BRR_BR15;
         handled |= TIM_SR_CC2IF;
     } else if (sr & TIM_SR_UIF) {