基于UART串口的stm32 modbusRTU实现（软件方式）详解

1. 实现思路

 此程序中, 串口通信方式: 115200-n-8-1, modbus协议要求帧与帧之间的间隔必须大于3.5个字符时间间隙作为帧与帧之间的分割. 

字符时间计算公式: interval_time = character_interval * 8 / baud_speed * 10 ^ 6 (微秒)

1.1 设置定时器, 超时时间为interval_time.

1.2 设置stm32的uart串口接收数据中断, 每次读取数据都重置定时器计数为0

1.3 定时器超时后, 说明此时modbus帧已经传输结束, 在定时器超时函数中处理响应.

2.编程

2.1 设置定时器超时

使用的波特率为115200bit/s, 则字符间隔时间大约为2431微妙.

在cubemx中选择一个定时器设置超时时间为2431微妙.

这里我们使用的时钟是16MHz, 定时器prescaler设置为16, 每隔1us触发一次计时器计时, 共计时2430次. 

2.2 设置UART接收字符中断

//main.c文件中修改
HAL_TIM_Base_Start_IT(&htim14);//启动定时器
huart1.Instance->CR1 |= 1 << 5;//enable reception interruption


//stm32_it.c文件中修改UART1中断入口函数USART1_IRQHandler
if(huart1.Instance->ISR & 1 << 5)
{
//	uint8_t data = huart1.Instance->RDR;
//	while( (huart1.Instance->ISR & 1 << 7) == 0);
//	huart1.Instance->TDR = data;
	uart_buf[uart_buf_len++] = huart1.Instance->RDR;;
	TIM14->CNT = 0;
}

2.3定时器中断函数实现modbusRTU协议解析

2.3.1 crc校验

modbus协议使用crc16-modbus标准校验.

此处通过直接计算方法生成crc.

#define POLYNOMIAL 0xa001

uint16_t generate_crc(uint8_t* buf, uint8_t len)
{
	uint16_t crc = 0xffff;
	uint8_t i, j;
	for(i = 0; i < len; i++)
	{
		crc ^= buf[i];
		for(j = 0; j < 8; j++)
		{
			uint8_t tmp = crc & 0x0001;
			crc >>= 1;
			if( tmp == 1)
				crc ^= POLYNOMIAL;
		}
	}
	return crc;

}

这是从modbuspoll中获取到的一个帧序列, 可以通过此序列进行crc校验, 后两个字节为校验码.

低地址在前, 高地址在后. CD C5 为其crc值 

2.3.2 判断是否解析帧

首先判断帧中的请求地址是否为此设备, 如果不是, 将第二个字节的高位置1, 重新生成crc, 发送给主机.

uint8_t check_receive_packet()
{
	
	if( uart_buf[0] != local_address) return 3;
	uint16_t crc = generate_crc(uart_buf, uart_buf_size - 2);
	if(crc != (uart_buf[uart_buf_size - 1] << 8 | uart_buf[uart_buf_size - 2]) )
			return 8;
	return 0;

}

2.3.3 帧解析

if(uart_buf_size > 0)
		{
			uint8_t i = 0;
			
			if( (ret = check_receive_packet()) )
			{
				send_buf[i++] = uart_buf[0];
				send_buf[i++] = uart_buf[1] | 0x80;
				send_buf[i++] = ret; //exception code
				crc = generate_crc(send_buf, i);
				send_buf[i++] = crc & 0xff;
				send_buf[i++] = crc >> 8;
				send_data(send_buf, i);
				uart_buf_size = 0;
				processed = 1;
				return;
			}
			uint8_t function_code = uart_buf[1];
			uint16_t reg_addr = uart_buf[2] << 8 | uart_buf[3];
			//uint16_t val;
			switch(function_code)
			{
				case 0x03:
					if( reg_addr == 0x01)
					{
						send_buf[i++] = uart_buf[0];
						send_buf[i++] = uart_buf[1];
						send_buf[i++] = 2;
						send_buf[i++] = (uint8_t)(g_device_id >> 8);
						send_buf[i++] = (uint8_t)g_device_id;
						crc = generate_crc(send_buf, i);
						send_buf[i++] = crc & 0xff;
						send_buf[i++] = crc >> 8;
						send_data(send_buf, i);
					}else if( reg_addr == 0x02)
					{
						send_buf[i++] = uart_buf[0];
						send_buf[i++] = uart_buf[1];
						send_buf[i++] = sizeof(uint8_t) * 2;
						send_buf[i++] = g_temperature_int;
						send_buf[i++] = g_temperature_dot;
						crc = generate_crc(send_buf, i);
						send_buf[i++] = crc & 0xff;
						send_buf[i++] = crc >> 8;
						send_data(send_buf, i);
					} else if( reg_addr == 0x03)
					{
						send_buf[i++] = uart_buf[0];
						send_buf[i++] = uart_buf[1];
						send_buf[i++] = sizeof(uint8_t) * 2;
						send_buf[i++] = g_humidity_int;
						send_buf[i++] = g_humidity_dot;
						crc = generate_crc(send_buf, i);
						send_buf[i++] = crc & 0xff;
						send_buf[i++] = crc >> 8;
						send_data(send_buf, i);
					} else if( reg_addr == 0x04)
					{
						send_buf[i++] = uart_buf[0];
						send_buf[i++] = uart_buf[1];
						send_buf[i++] = sizeof(uint8_t) * 2;
						
						uint8_t _int = (int)g_humidity_start;
						uint8_t x = g_humidity_start - _int;
						uint8_t _dot = (int)(x * 100); //keep 2 decimals
						
						send_buf[i++] = _int;
						send_buf[i++] = _dot;
						crc = generate_crc(send_buf, i);
						send_buf[i++] = crc & 0xff;
						send_buf[i++] = crc >> 8;
						send_data(send_buf, i);
					}
					else if(reg_addr == 0x05)
					{
						send_buf[i++] = uart_buf[0];
						send_buf[i++] = uart_buf[1];
						send_buf[i++] = sizeof(uint8_t) * 2;
						
						uint8_t _int = (int)g_temperature_start;
						uint8_t x = g_temperature_start - _int;
						uint8_t _dot = (int)(x * 100); //keep 2 decimals
						
						send_buf[i++] = _int;
						send_buf[i++] = _dot;
						crc = generate_crc(send_buf, i);
						send_buf[i++] = crc & 0xff;
						send_buf[i++] = crc >> 8;
						send_data(send_buf, i);
					}
					
					break;
				case 0x06:
					 i = 1;
					//val = uart_buf[4] << 8 | uart_buf[5];
					if( reg_addr == 0x04)
					{
						//register_val = val;
						//send_data(uart_buf, uart_buf_len);
						uint8_t _dot = uart_buf[4];
						uint8_t _int = uart_buf[5];
						g_humidity_start = uint2float(_int, _dot);
						
					}else if(reg_addr == 0x05)
					{
						uint8_t _dot = uart_buf[4];
						uint8_t _int = uart_buf[5];
						g_temperature_start = uint2float(_int, _dot);
					}
					send_data(uart_buf, uart_buf_size);
					break;
				}
			
				if(i == 0)
				{
					while(i < uart_buf_size)
					{
						fputc(uart_buf[i], (FILE*)100);
						i++;
					}
				}
				
				processed = 1;
				uart_buf_size = 0;
			}