uart.c

   1 #include "uart.h"
   2 #include "stm32f411.h"
   3
   4 /* Wait for data to be transferred into the shift register.  */
   5 static void
   6 busywait_txe (void)
   7 {
   8   /* TXE: Transmit data register empty  */
   9   while (!(*USART1_SR & (1u << 7))); /* 0: Data is not transferred to the shift
  10                                         register  */
  11 }
  12
  13 /* Wait for transmission of the last frame.  */
  14 static void
  15 busywait_tc (void)
  16 {
  17   /* TC: Transmission complete  */
  18   while (!(*USART1_SR & (1u << 6))); /* 0: Transmission is not complete  */
  19 }
  20
  21 static void
  22 tx (char c)
  23 {
  24   /* DR[8:0]: Data value  */
  25   *USART1_DR = c;
  26 }
  27
  28 void
  29 uart_init (void)
  30 {
  31   /* GPIOAEN: IO port A clock enable  */
  32   *RCC_AHB1ENR |= (1u); /* 1: IO port A clock enabled  */
  33
  34   /* MODERy[1:0]: Port x configuration bits (y = 0..15)  */
  35   *GPIOA_MODER &= ~(0b1111u << 18); /* Clear PA9 and PA10 bits  */
  36   *GPIOA_MODER |= (0b1010u << 18); /* 10: Alternate function mode  */
  37
  38   /* AFRHy: Alternate function selection for port x bit y (y = 8..15)  */
  39   *GPIOA_AFRH &= ~(0b11111111u << 4); /* Clear PA9 and PA10 bits  */
  40   *GPIOA_AFRH |= (0b01110111u << 4); /* 0111: AF7  */
  41
  42   /* PA9: USART1_TX
  43    * PA10: USART1_RX  */
  44
  45   /* USART1EN: USART1 clock enable  */
  46   *RCC_APB2ENR |= (1u << 4); /* 1: USART1 clock enabled  */
  47
  48   /* UE: USART enable  */
  49   *USART1_CR1 |= (1u << 13); /* 1: USART enable  */
  50
  51   /* M: Word length  */
  52   *USART1_CR1 &= ~(1u << 12); /* 0: 1 Start bit, 8 Data bits, n Stop bit  */
  53
  54   /* PCE: Parity control enable  */
  55   *USART1_CR1 &= ~(1u << 10); /* 0: Parity control disabled  */
  56
  57   /* STOP: STOP bits  */
  58   *USART1_CR2 &= ~(0b11u << 12); /* 00: 1 Stop bit  */
  59
  60   /* Clock frequency: 25MHz
  61    * Desired baud rate: 1.2 KBps */
  62
  63   /* Mantissa and fraction (/16)  */
  64   *USART1_BRR = (1302u << 4) | 1u; /* USARTDIV = 1302.0625  */
  65
  66   /* TE: Transmitter enable  */
  67   *USART1_CR1 |= (1u << 3); /* 1: Transmitter is enabled  */
  68 }
  69
  70 void
  71 kputs_busytc (const char *str)
  72 {
  73   while (*str)
  74   {
  75     busywait_txe ();
  76     tx (*(str++));
  77   }
  78
  79   busywait_tc ();
  80 }