From c36d8848992ef1eda659ec23918e49e5e36ec595 Mon Sep 17 00:00:00 2001
From: Dominic Clifton <me@dominicclifton.name>
Date: Thu, 3 Apr 2014 12:40:17 +0100
Subject: [PATCH] Change softserial timer calculation code to support baud
 rates in the range 600 to 19200 (tested)

---
 src/cli.c            |  2 +-
 src/drv_softserial.c | 26 +++++++++++++++++++++-----
 src/drv_timer.c      | 11 ++++++++---
 src/drv_timer.h      |  4 ++--
 4 files changed, 32 insertions(+), 11 deletions(-)

diff --git a/src/cli.c b/src/cli.c
index 94bda8a425..7b70d90280 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -126,7 +126,7 @@ const clivalue_t valueTable[] = {
     { "flaps_speed", VAR_UINT8, &mcfg.flaps_speed, 0, 100 },
     { "fixedwing_althold_dir", VAR_INT8, &mcfg.fixedwing_althold_dir, -1, 1 },
     { "serial_baudrate", VAR_UINT32, &mcfg.serial_baudrate, 1200, 115200 },
-    { "softserial_baudrate", VAR_UINT32, &mcfg.softserial_baudrate, 9600, 19200 },
+    { "softserial_baudrate", VAR_UINT32, &mcfg.softserial_baudrate, 600, 19200 },
     { "softserial_inverted", VAR_UINT8, &mcfg.softserial_inverted, 0, 1 },
     { "gps_type", VAR_UINT8, &mcfg.gps_type, 0, 3 },
     { "gps_baudrate", VAR_INT8, &mcfg.gps_baudrate, -1, 4 },
diff --git a/src/drv_softserial.c b/src/drv_softserial.c
index 2cae1f2817..cec19ee6e2 100644
--- a/src/drv_softserial.c
+++ b/src/drv_softserial.c
@@ -1,8 +1,5 @@
 #include "board.h"
 
-// There needs to be a table of timerMhz per baud rate so the system can set the timer correctly.
-// See http://www.wormfood.net/avrbaudcalc.php?postbitrate=19200&postclock=72
-// Currently defaulting to 3Mhz to support 19200.
 #define SOFT_SERIAL_TIMER_MHZ 3
 #define SOFT_SERIAL_1_TIMER_RX_HARDWARE 4
 #define SOFT_SERIAL_1_TIMER_TX_HARDWARE 5
@@ -74,10 +71,29 @@ void serialInputPortConfig(const timerHardware_t *timerHardwarePtr)
 
 #define TICKS_PER_BIT 3
 
+bool isTimerPeriodTooLarge(uint32_t timerPeriod)
+{
+    return timerPeriod > 0xFFFF;
+}
+
 void serialTimerConfig(const timerHardware_t *timerHardwarePtr, uint32_t baud, uint8_t reference, timerCCCallbackPtr callback)
 {
-    uint16_t timerPeriod = (SOFT_SERIAL_TIMER_MHZ * 1000000) / (baud * TICKS_PER_BIT);
-    timerInConfig(timerHardwarePtr, timerPeriod, SOFT_SERIAL_TIMER_MHZ);
+    uint32_t clock = SystemCoreClock;
+    uint32_t timerPeriod;
+    do {
+        timerPeriod = clock / (baud * TICKS_PER_BIT);
+        if (isTimerPeriodTooLarge(timerPeriod)) {
+            if (clock > 1) {
+                clock = clock / 2;
+            } else {
+                // TODO unable to continue, unable to determine clock and timerPeriods for the given baud
+            }
+
+        }
+    } while (isTimerPeriodTooLarge(timerPeriod));
+
+    uint8_t mhz = SystemCoreClock / 1000000;
+    timerInConfig(timerHardwarePtr, timerPeriod, mhz);
     configureTimerCaptureCompareInterrupt(timerHardwarePtr, reference, callback);
 }
 
diff --git a/src/drv_timer.c b/src/drv_timer.c
index 0619b0baaf..2c2a884880 100644
--- a/src/drv_timer.c
+++ b/src/drv_timer.c
@@ -145,19 +145,24 @@ void timerNVICConfig(uint8_t irq)
     NVIC_Init(&NVIC_InitStructure);
 }
 
-void configTimeBase(TIM_TypeDef *tim, uint32_t period, uint8_t mhz)
+void configTimeBase(TIM_TypeDef *tim, uint16_t period, uint8_t mhz)
 {
     TIM_TimeBaseInitTypeDef  TIM_TimeBaseStructure;
 
     TIM_TimeBaseStructInit(&TIM_TimeBaseStructure);
-    TIM_TimeBaseStructure.TIM_Period = period - 1;
+    TIM_TimeBaseStructure.TIM_Period = period - 1; // AKA TIMx_ARR
+
+    // "The counter clock frequency (CK_CNT) is equal to f CK_PSC / (PSC[15:0] + 1)." - STM32F10x Reference Manual 14.4.11
+    // Thus for 1Mhz: 72000000 / 1000000 = 72, 72 - 1 = 71 = TIM_Prescaler
     TIM_TimeBaseStructure.TIM_Prescaler = (SystemCoreClock / ((uint32_t)mhz * 1000000)) - 1;
+
+
     TIM_TimeBaseStructure.TIM_ClockDivision = 0;
     TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
     TIM_TimeBaseInit(tim, &TIM_TimeBaseStructure);
 }
 
-void timerInConfig(const timerHardware_t *timerHardwarePtr, uint32_t period, uint8_t mhz)
+void timerInConfig(const timerHardware_t *timerHardwarePtr, uint16_t period, uint8_t mhz)
 {
     configTimeBase(timerHardwarePtr->tim, period, mhz);
     TIM_Cmd(timerHardwarePtr->tim, ENABLE);
diff --git a/src/drv_timer.h b/src/drv_timer.h
index ab99059aa3..cae2e20a02 100644
--- a/src/drv_timer.h
+++ b/src/drv_timer.h
@@ -13,8 +13,8 @@ typedef struct {
 
 extern const timerHardware_t timerHardware[];
 
-void configTimeBase(TIM_TypeDef *tim, uint32_t period, uint8_t mhz);
-void timerInConfig(const timerHardware_t *timerHardwarePtr, uint32_t period, uint8_t mhz);
+void configTimeBase(TIM_TypeDef *tim, uint16_t period, uint8_t mhz);
+void timerInConfig(const timerHardware_t *timerHardwarePtr, uint16_t period, uint8_t mhz);
 void timerNVICConfig(uint8_t irq);
 
 void configureTimerInputCaptureCompareChannel(TIM_TypeDef *tim, const uint8_t channel);