[stm32] F4: fix ppm input timer frequency for TIM2

- krooz: don't use TIM2 for PWM
- change to 6 ticks per usec to fit all frequencies
- on the F1 we assume to run at 72MHz for HCLK and both timer clocks
  TIM1 -> APB2 = HCLK = 72MHz
  TIM2 -> 2 * APB1 = 2 * 36MHz = 72MHz
- on the F4 we assume 2 * AHB clock:
  TIM1 -> 2 * APB2 = 168MHz
  TIM2 -> 2 * APB1 = 84MHz

closes #470

sw/airborne/arch/stm32/subsystems/radio_control/ppm_arch.c

@@ -52,6 +52,20 @@ uint32_t ppm_last_pulse_time;
 bool_t   ppm_data_valid;
 static uint32_t timer_rollover_cnt;
 
+/*
+ * Timer clock frequency (before prescaling) is twice the frequency
+ * of the APB domain to which the timer is connected.
+ */
+
+#ifdef STM32F1
+/**
+ * HCLK = 72MHz, Timer clock also 72MHz since
+ * TIM1_CLK = APB2 = 72MHz
+ * TIM2_CLK = 2 * APB1 = 2 * 32MHz
+ */
+#define PPM_TIMER_CLK       AHB_CLK
+#endif
+
 #if USE_PPM_TIM2
 
 PRINT_CONFIG_MSG("Using TIM2 for PPM input.")
@@ -60,6 +74,14 @@ PRINT_CONFIG_MSG("Using TIM2 for PPM input.")
 #define PPM_PERIPHERAL      RCC_APB1ENR_TIM2EN
 #define PPM_TIMER           TIM2
 
+#ifdef STM32F4
+/* Since APB prescaler != 1 :
+ * Timer clock frequency (before prescaling) is twice the frequency
+ * of the APB domain to which the timer is connected.
+ */
+#define PPM_TIMER_CLK       (rcc_ppre1_frequency * 2)
+#endif
+
 #elif USE_PPM_TIM1
 
 PRINT_CONFIG_MSG("Using TIM1 for PPM input.")
@@ -68,6 +90,12 @@ PRINT_CONFIG_MSG("Using TIM1 for PPM input.")
 #define PPM_PERIPHERAL      RCC_APB2ENR_TIM1EN
 #define PPM_TIMER           TIM1
 
+#ifdef STM32F4
+#define PPM_TIMER_CLK       (rcc_ppre2_frequency * 2)
+#endif
+
+#else
+#error Unknown PPM input timer configuration.
 #endif
 
 void ppm_arch_init ( void ) {
@@ -86,7 +114,7 @@ void ppm_arch_init ( void ) {
   timer_set_mode(PPM_TIMER, TIM_CR1_CKD_CK_INT,
                  TIM_CR1_CMS_EDGE, TIM_CR1_DIR_UP);
   timer_set_period(PPM_TIMER, 0xFFFF);
-  timer_set_prescaler(PPM_TIMER, (AHB_CLK / (RC_PPM_TICKS_PER_USEC*ONE_MHZ_CLK)) - 1);
+  timer_set_prescaler(PPM_TIMER, (PPM_TIMER_CLK / (RC_PPM_TICKS_PER_USEC*ONE_MHZ_CLK)) - 1);
 
  /* TIM configuration: Input Capture mode ---------------------
      The Rising edge is used as active edge
@@ -96,7 +124,7 @@ void ppm_arch_init ( void ) {
 #elif defined PPM_PULSE_TYPE && PPM_PULSE_TYPE == PPM_PULSE_TYPE_NEGATIVE
   timer_ic_set_polarity(PPM_TIMER, PPM_CHANNEL, TIM_IC_FALLING);
 #else
-#error "Unknown PM_PULSE_TYPE"
+#error "Unknown PPM_PULSE_TYPE"
 #endif
   timer_ic_set_input(PPM_TIMER, PPM_CHANNEL, PPM_TIMER_INPUT);
   timer_ic_set_prescaler(PPM_TIMER, PPM_CHANNEL, TIM_IC_PSC_OFF);

sw/airborne/arch/stm32/subsystems/radio_control/ppm_arch.h

@@ -33,10 +33,17 @@
 #include "mcu_periph/sys_time.h"
 
 /**
- * The ppm counter is running at cpu freq / 72 or 168 / 8
- * so the counter has 1/8 us resolution
+ * The ppm counter is set-up to have 1/6 us resolution.
+ *
+ * The timer clock frequency (before prescaling):
+ * STM32F1:
+ *   TIM1 -> APB2 = HCLK = 72MHz
+ *   TIM2 -> 2 * APB1 = 2 * 36MHz = 72MHz
+ * STM32F4:
+ *   TIM1 -> 2 * APB2 = 2 * 84MHz = 168MHz
+ *   TIM2 -> 2 * APB1 = 2 * 42MHz = 84MHz
  */
-#define RC_PPM_TICKS_PER_USEC 8
+#define RC_PPM_TICKS_PER_USEC 6
 
 #define RC_PPM_TICKS_OF_USEC(_v)        ((_v)*RC_PPM_TICKS_PER_USEC)
 #define RC_PPM_SIGNED_TICKS_OF_USEC(_v) (int32_t)((_v)*RC_PPM_TICKS_PER_USEC)

sw/airborne/boards/krooz_sd.h

@@ -3,7 +3,7 @@
 
 #define BOARD_KROOZ
 
-/* Krooz/M has a 12MHz external clock and 168MHz internal. */
+/* KroozSD has a 12MHz external clock and 168MHz internal. */
 #define EXT_CLK 12000000
 #define AHB_CLK 168000000
 
@@ -210,7 +210,7 @@
 #define BOARD_HAS_BARO 1
 
 /* PWM */
-#define PWM_USE_TIM2 1
+//#define PWM_USE_TIM2 1
 #define PWM_USE_TIM3 1
 #define PWM_USE_TIM4 1
 #define PWM_USE_TIM5 1
@@ -383,7 +383,7 @@
 #define USE_PPM_TIM2 1
 
 #define PPM_CHANNEL         TIM_IC2
-#define PPM_TIMER_INPUT     TIM_IC_IN_TI1
+#define PPM_TIMER_INPUT     TIM_IC_IN_TI2
 #define PPM_IRQ             NVIC_TIM2_IRQ
 //#define PPM_IRQ2            NVIC_TIM2_UP_TIM10_IRQ
 // Capture/Compare InteruptEnable and InterruptFlag