最近有客户在使用极海APM32F072CBT6时，遇到串口波特率不能配置到3M以上的问题，因为在数据手册中有明确指示072的串口波特率最高可以配置到6M。客户使用极海当前SDK例程库（V1.7）串口波特率最多只能配置到3M，这是怎么回事呢？APM32F072的串口波特率无法配置到更高的速率吗？
答案当然是否定的。经过我们内测试验，发现这个问题是由于当前SDK例程库（V1.7）的底层软件配置有缺失，当前的例程库在配置USART_Config这个函数底层的时候，其配置方式只能讲串口波特率配置到3M，如下所示：
void USART_Config(USART_T* uart, USART_Config_T* configStruct)
{
uint32_t temp, fCLK, intDiv, fractionalDiv;
/** Disable USART */
uart->CTRL1_B.UEN = 0x00;
/** WLS, PCEN, TXEN, RXEN */
temp = uart->CTRL1;
temp &= 0xE9F3;
temp |= (uint32_t)configStruct->mode | \
(uint32_t)configStruct->parity | \
(uint32_t)configStruct->wordLength;
uart->CTRL1 = temp;
/** STOP bits */
temp = uart->CTRL2;
temp &= 0xCFFF;
temp |= configStruct->stopBits;
uart->CTRL2 = temp;
/** Hardware Flow Control */
temp = uart->CTRL3;
temp &= 0xFCFF;
temp |= (uint32_t)configStruct->hardwareFlowCtrl;
uart->CTRL3 = temp;
if (uart == USART1)
{
fCLK = RCM_ReadUSART1CLKFreq();
}
else if (uart == USART2)
{
fCLK = RCM_ReadUSART2CLKFreq();
}
else
{
fCLK = RCM_ReadPCLKFreq();
}
intDiv = ((25 * fCLK) / (4 * (configStruct->baudRate)));
temp = (intDiv / 100) << 4;
fractionalDiv = intDiv - (100 * (temp >> 4));
temp |= ((((fractionalDiv * 16) + 50) / 100)) & ((uint8_t)0x0F);
uart->BR = temp;
}
那么要想配置到更高波特率，需要对此配置软件做哪些修改呢？
显然这里我们需要使用8倍的采样率，才能配置更高的波特率，因为48M/8=6M。那么USART_Config的底层驱动软件需要更改为：
void USART_Config(USART_T* uart, USART_Config_T* configStruct)
{
uint32_t temp, fCLK, intDiv, fractionalDiv;
uint32_t divider = 0, tmpreg = 0;
/** Disable USART */
uart->CTRL1_B.UEN = 0x00;
/** WLS, PCEN, TXEN, RXEN */
temp = uart->CTRL1;
temp &= 0xE9F3;
temp |= (uint32_t)configStruct->mode | \
(uint32_t)configStruct->parity | \
(uint32_t)configStruct->wordLength;
uart->CTRL1 = temp;
/** STOP bits */
temp = uart->CTRL2;
temp &= 0xCFFF;
temp |= configStruct->stopBits;
uart->CTRL2 = temp;
/** Hardware Flow Control */
temp = uart->CTRL3;
temp &= 0xFCFF;
temp |= (uint32_t)configStruct->hardwareFlowCtrl;
uart->CTRL3 = temp;
if (uart == USART1)
{
fCLK=RCM_ReadUSART1CLKFreq();
}
else if (uart == USART2)
{
fCLK=RCM_ReadUSART2CLKFreq();
}
else
{
fCLK=RCM_ReadPCLKFreq();
}
/* Determine the integer part */
//if ((USARTx->CR1 & USART_CR1_OVER8) != 0)
if (uart->CTRL1_B.OSMCFG != RESET)/*!<Ovensampling by 8*/
{
/* (divider * 10) computing in case Oversampling mode is 8 Samples */
divider = (uint32_t)((2 * fCLK) / (configStruct->baudRate));
tmpreg = (uint32_t)((2 * fCLK) % (configStruct->baudRate));
}
else /* if ((USARTx->CR1 & CR1_OVER8_Set) == 0) */
{
/* (divider * 10) computing in case Oversampling mode is 16 Samples */
divider = (uint32_t)((fCLK) / (configStruct->baudRate));
tmpreg = (uint32_t)((fCLK) % (configStruct->baudRate));
}
/* round the divider : if fractional part i greater than 0.5 increment divider */
if (tmpreg >= (configStruct->baudRate) / 2)
{
divider++;
}
/* Implement the divider in case Oversampling mode is 8 Samples */
//if ((USARTx->CR1 & USART_CR1_OVER8) != 0)
if (uart->CTRL1_B.OSMCFG != RESET)
{
/* get the LSB of divider and shift it to the right by 1 bit */
tmpreg = (divider & (uint16_t)0x000F) >> 1;
/* update the divider value */
divider = (divider & (uint16_t)0xFFF0) | tmpreg;
}
uart->BR = (uint16_t)divider;
}
以上就是如何配置APM32F072的软件底层来实现高速串口波特率通讯的方法，不过极海在下一版SDK中会将此配置方式更新进去，届时请各位工程师留意下载最新的SDK例程库。
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