{"id":347,"date":"2021-08-23T05:53:29","date_gmt":"2021-08-23T05:53:29","guid":{"rendered":"https:\/\/blog.embeddedexpert.io\/?p=347"},"modified":"2021-08-23T05:53:33","modified_gmt":"2021-08-23T05:53:33","slug":"working-with-stm32-and-uart-part-3-receive-a-character-in-interrupt-mode","status":"publish","type":"post","link":"https:\/\/blog.embeddedexpert.io\/?p=347","title":{"rendered":"Working with STM32 and UART part 3: Receive a character in interrupt mode"},"content":{"rendered":"\n

In the pervious two guides (part 1<\/a> & part 2<\/a> ) how to send a single character and string using UART. In this guide, we shall discuss how to receive a character from UART using interrupt handler and echo back the sent character and blink the internal led number of times according to the sent number.<\/p>\n\n\n\n

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UART<\/figcaption><\/figure><\/div>\n\n\n\n

In this guide, we will cover the following:<\/p>\n\n\n\n

  • Configure UART for full duplex (TX and RX).<\/li>
  • Code <\/li>
  • Demo<\/li><\/ul>\n\n\n\n

    1. Configure UART for full duplex <\/h2>\n\n\n\n

    In part 1 we discussed how to set the uart to send a single character, we shall use the same initialization sequence.<\/p>\n\n\n\n

    We started by enabling clock access to USART2 and GPIOA port<\/p>\n\n\n\n

    RCC->APB1ENR|=RCC_APB1ENR_USART2EN;\nRCC->AHB1ENR|=RCC_AHB1ENR_GPIOAEN;<\/pre><\/div>\n\n\n\n
    From part one, we concluded that PA2 is the TX pin and PA3 is the RX pin. Hence, we configure them as alternate function and which alternate function as following:<\/p>\n\n\n\n
    GPIOA->MODER|=(1<<5);\/\/set bit5\nGPIOA->MODER&=~(1<<4);\/\/reset bit4\n\nGPIOA->MODER|=(1<<7);\/\/set bit7\nGPIOA->MODER&=~(1<<6);\/\/reset bit6\n\nGPIOA->AFR[0]=0x07700; \/\/ALT7 for UART2 (PA2 and PA3)<\/pre><\/div>\n\n\n\n
    Now we need to configure the UART<\/p>\n\n\n\n
    We starting by setting the baud rate to 9600 as following:<\/p>\n\n\n\n
    USART2->BRR  = 0x0681;    \/\/9600 @16MHz<\/pre><\/div>\n\n\n\n
    then selecting the UART to enable TX, RX and RXNEIE (Receiver interrupt) in Control Register 1 (CR1) and enabling UART module as following:<\/p>\n\n\n\n
    \"\"
    CR1<\/figcaption><\/figure><\/div>\n\n\n\n
    Hence, the configuration shall be as following:<\/p>\n\n\n\n
    USART2->CR1 |= (1<<2)|(1<<3)|(1<<5)|(1<<13);<\/pre><\/div>\n\n\n\n
    After than we shall enable the interrupt of USART in NVIC as following<\/p>\n\n\n\n
    NVIC_EnableIRQ(USART2_IRQn);<\/pre><\/div>\n\n\n\n
    Now for the interrupt handler <\/p>\n\n\n\n
    void USART2_IRQHandler(void){\n\nif(USART2->SR&0x0020) \/\/check if the read data register is not empty\n{\nc=USART2->DR; \/\/store current data in variable c\n}\n}<\/pre><\/div>\n\n\n\n
    2. Code<\/h2>\n\n\n\n
    #include "stm32f4xx.h"                  \/\/ Device header\nvoid delay(int delayms);\nvoid led_play(int value);\nvoid UART2_Init();\nvoid USART2_IRQHandler(void);\nvoid USART2_write(int ch);\nchar c;\nint main ()\n{\n__disable_irq();\nUART2_Init();\nGPIOA->MODER|= GPIO_MODER_MODER5_0; \/\/PA5 as output\nUSART2->CR1|=0x0020;\n\n__enable_irq();\n\nwhile(1)\n{\nif(c!=' '){\nled_play(c);\nUSART2_write(c);\nUSART2_write('\\r');\nUSART2_write('\\n');\t\nc=' ';\n}\n}\n\n}\n\nvoid led_play(int value)\n{\nvalue %=16; \n\tfor (;value>0;value--){\n\tGPIOA->BSRR|=GPIO_BSRR_BS5; \/\/turn on ledd\n\t\tdelay(100);\n\t\tGPIOA->BSRR|=GPIO_BSRR_BR5; \/\/turn off the led\t\t\n\t\tdelay(100);\n\t}\n}\n\nvoid delay(int delayms){\nSysTick->LOAD=16000-1;\nSysTick->VAL=0;\nSysTick->CTRL=0x5;\nfor (int i=0;i<delayms;i++)\n{\nwhile(!(SysTick->CTRL &0x10000)){}\n\t\t}\n\tSysTick->CTRL=0;\n}\n\nvoid UART2_Init(){\n\tRCC->AHB1ENR |=1; \/\/Enable GPIOA clock\n\tRCC->APB1ENR|=0x20000; \/\/Enable USART clock\n\t\/\/configure PA3 as alternative function \n\tGPIOA->AFR[0]=0x07700; \/\/ALT7 for UART2\n\tGPIOA->AFR[1]=0x07000;\n\tGPIOA->MODER|=0x0080;  \/\/enable PA3 as alternate function\n\tGPIOA->MODER|=0x0020; \/\/enable PA2 as alterate fuction\n\tUSART2->BRR=0x008B; \/\/ set baud rate of 115200 for 16MHz\n\tUSART2->CR1|=0x0020;\n\tUSART2->CR1|=0x200C; \/\/enable RX\n\tNVIC_EnableIRQ(USART2_IRQn);\n}\n\nvoid USART2_IRQHandler(void){\n\nif(USART2->SR&0x0020){\nc=USART2->DR;\n}\n}\nvoid USART2_write(int ch){\n\t\nwhile(!(USART2->SR&0x0080)){\n}\nUSART2->DR=(ch&0xff);\t\n}<\/pre><\/div>\n\n\n\n
    3. Demo<\/h2>\n\n\n\n