From 976ba2d44af3d9dafd9edad1a50b4a12571cf625 Mon Sep 17 00:00:00 2001
From: Elliot Williams <elliot@pre.postero.us>
Date: Mon, 16 Dec 2019 16:11:07 +0100
Subject: [PATCH] minimalistic framing for UART command

copied the uart framing idea from
https://github.com/EmanuelFeru/hoverboard-firmware-hack-FOC

to use:
send $AAAA to start frame
then steer, speed as usual
and XOR($AAAA, steer, speed) as checksum

commands that don't match are ignored, and UART DMA is reset
hopefully this will set it up to catch the next one
re-send frequently.
---
 Inc/config.h |  1 +
 Src/main.c   | 26 ++++++++++++++++----------
 2 files changed, 17 insertions(+), 10 deletions(-)

diff --git a/Inc/config.h b/Inc/config.h
index 319ae81..dfe17d2 100644
--- a/Inc/config.h
+++ b/Inc/config.h
@@ -9,6 +9,7 @@
 #define DELAY_IN_MAIN_LOOP 5        // in ms. default 5. it is independent of all the timing critical stuff. do not touch if you do not know what you are doing.
 
 #define TIMEOUT          5          // number of wrong / missing input commands before emergency off
+#define START_FRAME      0xAAAA     // serial command start-of-frame magic word
 
 // ############################### GENERAL ###############################
 
diff --git a/Src/main.c b/Src/main.c
index 4cc2624..8c53302 100644
--- a/Src/main.c
+++ b/Src/main.c
@@ -41,9 +41,10 @@ int cmd2;
 int cmd3;
 
 typedef struct{
-   int16_t steer;
-   int16_t speed;
-   //uint32_t crc;
+	uint16_t start_of_frame;
+	int16_t  steer;
+	int16_t  speed;
+	uint16_t checksum;
 } Serialcommand;
 
 volatile Serialcommand command;
@@ -157,7 +158,7 @@ int main(void) {
 
   #ifdef CONTROL_SERIAL_USART2
     UART_Control_Init();
-    HAL_UART_Receive_DMA(&huart2, (uint8_t *)&command, 4);
+    HAL_UART_Receive_DMA(&huart2, (uint8_t *)&command, sizeof(command));
   #endif
 
   #ifdef DEBUG_I2C_LCD
@@ -218,12 +219,17 @@ int main(void) {
       timeout = 0;
     #endif
 
-    #ifdef CONTROL_SERIAL_USART2
-      cmd1 = CLAMP((int16_t)command.steer, -1000, 1000);
-      cmd2 = CLAMP((int16_t)command.speed, -1000, 1000);
-
-      timeout = 0;
-    #endif
+#ifdef CONTROL_SERIAL_USART2
+	  if (command.start_of_frame == START_FRAME && command.checksum == 
+			  (command.start_of_frame ^ command.steer ^ command.speed)) {
+		  cmd1 = CLAMP((int16_t)command.steer, -1000, 1000);
+		  cmd2 = CLAMP((int16_t)command.speed, -1000, 1000);
+	  } else {                                  // restart DMA to hopefully get back in sync
+		  HAL_UART_DMAStop(&huart2);
+		  HAL_UART_Receive_DMA(&huart2, (uint8_t *)&command, sizeof(command));
+	  }
+	  timeout = 0;
+#endif
 
     #ifdef CONTROL_MOTOR_TEST
       if (motor_test_direction == 1) cmd2 += 1;