Merge pull request #9354 from jeromecoutant/PR_H7CAN

STM32H7 : add CAN support

Author: Cruz Monrreal

targets/TARGET_STM/TARGET_STM32H7/TARGET_STM32H743xI/TARGET_NUCLEO_H743ZI/system_clock.c

@@ -127,10 +127,10 @@ uint8_t SetSysClock_PLL_HSE(uint8_t bypass)
     RCC_OscInitStruct.HSI48State = RCC_HSI48_ON;
     RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
     RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSE;
-    RCC_OscInitStruct.PLL.PLLM = 4;
-    RCC_OscInitStruct.PLL.PLLN = 400;
-    RCC_OscInitStruct.PLL.PLLP = 2;
-    RCC_OscInitStruct.PLL.PLLQ = 32;
+    RCC_OscInitStruct.PLL.PLLM = 4;   // 2 MHz
+    RCC_OscInitStruct.PLL.PLLN = 400; // 800 MHz
+    RCC_OscInitStruct.PLL.PLLP = 2;   // PLLCLK = SYSCLK = 400 MHz
+    RCC_OscInitStruct.PLL.PLLQ = 80;  // PLL1Q used for FDCAN = 10 MHz
     RCC_OscInitStruct.PLL.PLLR = 2;
     RCC_OscInitStruct.PLL.PLLFRACN = 0;
     RCC_OscInitStruct.PLL.PLLVCOSEL = RCC_PLL1VCOWIDE;