如何在低速MSP430上获得高速串口数据

Question

我的项目有一个通过UART连接到Bluegiga蓝牙模块的MSP430。MCU必须能够从BG模块接收可变长度的消息。在当前架构中，每个接收到的字节都会产生一个通用异步收发器中断，以允许进行消息处理，并且功率限制会对MSP430的时钟速度施加限制。这使得MSP430很难跟上高于9600bps的任何通用异步收发器的速度。结果是通信接口很慢。加快数据速率会导致溢出错误、字节丢失和通信中断。

在这种情况下，如何在不牺牲数据完整性的情况下提高通信速度？

Answer 1

通过在MSP430上使用3个可用DMA通道中的2个来填充可由CPU处理的环形缓冲区，我能够实现12倍的速度提升。这有点棘手，因为只有当大小寄存器达到零时才会产生MSP430 DMA中断，所以我不能直接填充环形缓冲区，因为消息大小是可变的。

使用一个DMA通道作为单字节缓冲区，UART接收到的每个字节都会触发该缓冲区，然后触发第二个DMA通道填充环形缓冲区。

下面是演示该方法的示例C代码。请注意，它合并了来自MSP430库的引用。

#include "dma.h"

#define BLUETOOTH_RXQUEUE_SIZE <size_of_ring_buffer>

static int headIndex = 0;
static int tailIndex = 0;

static char uartRecvByte;
static char bluetoothRXQueue[BLUETOOTH_RXQUEUE_SIZE];

/*!********************************************************************************
  * \brief Initialize DMA hardware
  *
  * \param none
  *
  * \return none
  *
  ******************************************************************************/
void init(void)
{
    // This is the primary buffer.
    //  It will be triggered by UART Rx and generate an interrupt.
    //  It's purpose is to service every byte recieved by the UART while
    //    also waking up the CPU to let it know something happened.
    //  It uses a single address of RAM to store the data, so it needs to be
    //    serviced before the next byte is received from the UART.
    //  This was done so that any byte received triggers processing of the data.
    DMA_initParam dmaSettings;
    dmaSettings.channelSelect = DMA_CHANNEL_2;
    dmaSettings.transferModeSelect = DMA_TRANSFER_REPEATED_SINGLE;
    dmaSettings.transferSize = 1;
    dmaSettings.transferUnitSelect = DMA_SIZE_SRCBYTE_DSTBYTE;
    dmaSettings.triggerSourceSelect = DMA_TRIGGERSOURCE_20; // USCA1RXIFG, or any UART recieve trigger
    dmaSettings.triggerTypeSelect = DMA_TRIGGER_RISINGEDGE;
    DMA_init(&dmaSettings);
    DMA_setSrcAddress(DMA_CHANNEL_2, (UINT32)&UCA1RXBUF, DMA_DIRECTION_UNCHANGED);
    DMA_setDstAddress(DMA_CHANNEL_2, (UINT32)&uartRecvByte, DMA_DIRECTION_UNCHANGED);

    // This is the secondary buffer.
    //  It will be triggered when DMA_CHANNEL_2 copies a byte and will store bytes into a ring buffer.
    //  It's purpose is to pull data from DMA_CHANNEL_2 as quickly as possible
    //    and add it to the ring buffer.
    dmaSettings.channelSelect = DMA_CHANNEL_0;
    dmaSettings.transferModeSelect = DMA_TRANSFER_REPEATED_SINGLE;
    dmaSettings.transferSize = BLUETOOTH_RXQUEUE_SIZE;
    dmaSettings.transferUnitSelect = DMA_SIZE_SRCBYTE_DSTBYTE;
    dmaSettings.triggerSourceSelect = DMA_TRIGGERSOURCE_30; // DMA2IFG
    dmaSettings.triggerTypeSelect = DMA_TRIGGER_RISINGEDGE;
    DMA_init(&dmaSettings);
    DMA_setSrcAddress(DMA_CHANNEL_0, (UINT32)&uartRecvByte, DMA_DIRECTION_UNCHANGED);
    DMA_setDstAddress(DMA_CHANNEL_0, (UINT32)&bluetoothRXQueue, DMA_DIRECTION_INCREMENT);

    DMA_enableTransfers(DMA_CHANNEL_2);
    DMA_enableTransfers(DMA_CHANNEL_0);
    DMA_enableInterrupt(DMA_CHANNEL_2);
}

/*!********************************************************************************
  * \brief DMA Interrupt for receipt of data from the Bluegiga module
  *
  * \param none
  *
  * \return none
  *
  * \par Further Detail
  * \n Dependencies:   N/A
  * \n Processing:     Clear the interrupt and update the circular buffer head
  * \n Error Handling: N/A
  * \n Tests:          N/A
  * \n Special Considerations: N/A
  *
  ******************************************************************************/
void DMA_Interrupt(void)
{
    DMA_clearInterrupt(DMA_CHANNEL_2);
    headIndex = BLUETOOTH_RXQUEUE_SIZE - DMA_getTransferSize(DMA_CHANNEL_0);

    if (headIndex == tailIndex)
    {
        // This indicates ring buffer overflow.
    }
    else
    {
        // Perform processing on the current state of the ring buffer here.
        // If only partial data has been received, just leave.  Either set a flag
        // or generate an event to process the message outside of the interrupt.
        // Once the message is processed, move the tailIndex.
    }
}