根据 SCI 输入信号自动校准波特率

作者：Terry Deng

本文档概述了一种基于 SCI/UART 输入信号，可以自动校准本设备SCI/UART波特率的方法，该方法适用与所有第三代C2000芯片，比如F2807x/37x，F28004x，F28002x等等。

一 原理说明

假设有2块电路板通过SCI进行通信。“Transmitter”向“Receiver”发送未知波特率的数据，“ Receiver”则使用 eCAP 测量未知的波特率，然后修改其自身的波特率和“Transmitter”匹配。

下面款图是一种情况，其中“Transmitter” 的波特率设置为 9889，而“Receiver”的初始波特率设置为 9601 ，相比之下“Receiver”的波特率为 -3% 偏差。 经过算法的自动校准以后，“Receiver”将会把自身波特率校正为与“Transmitter”相同的9889。

下面框图则是另一种情况，假如“Receiver”和“Transmitter”的初始波特率都是9889，但“Receiver”的内部晶振INTOSC有-3%的偏差。使用上述完全相同的方法原理和步骤，“Receiver”波特率设置将会从9889校准成9601，这样“Receiver”的波特率设置被自动校准抵消内部晶振的偏差。在测量实际信号时，“Receiver”输出到“Transmitter”的信号会是正确的 9889 波特率。

二 Receiver 的校准代码

1. 初始化

需要配置以下模块来校准波特率：

• 时钟：使用 INTOSC2 并选择 100MHz 的 LSPCLK

#define DEVICE_SETCLOCK_CFG      (SYSCTL_OSCSRC_OSC2 | SYSCTL_IMULT(20) |  \
                                     SYSCTL_FMULT_NONE | SYSCTL_SYSDIV(2) |   \
                                     SYSCTL_PLL_ENABLE)
    //
    // Set up PLL control and clock dividers
    //
    SysCtl_setClock(DEVICE_SETCLOCK_CFG);

    //
    // Make sure the LSPCLK divider is set to the default (divide by 4)
    //
    SysCtl_setLowSpeedClock(SYSCTL_LSPCLK_PRESCALE_1);

• SCI 模块：通讯数据使用，发出校准以后的波形

    // Initialize SCIA and its FIFO.
    //
    SCI_performSoftwareReset(SCIA_BASE);

    //
    // Configure SCIA for communications.
    //
    SCI_setConfig(SCIA_BASE, DEVICE_LSPCLK_FREQ, TARGETBAUD, (SCI_CONFIG_WLEN_8 |
                                                        SCI_CONFIG_STOP_ONE |
                                                        SCI_CONFIG_PAR_NONE));
    SCI_resetChannels(SCIA_BASE);
    SCI_resetRxFIFO(SCIA_BASE);
    SCI_resetTxFIFO(SCIA_BASE);
    SCI_clearInterruptStatus(SCIA_BASE, SCI_INT_TXFF | SCI_INT_RXFF);
    SCI_enableFIFO(SCIA_BASE);
    SCI_enableModule(SCIA_BASE);
SCI_performSoftwareReset(SCIA_BASE);

• Xbar 输入：将 GPIO28/SCI 内部连接到 INPUTXBAR7 与 ECAP1 配合使用

//
    // Configure GPIO 28 as eCAP input
    //
    XBAR_setInputPin(XBAR_INPUT7, 28);

• ECAP 模块：监控接收到的 SCI 通信脉冲宽度

//
    // Disable ,clear all capture flags and interrupts
    //
    ECAP_disableInterrupt(ECAP1_BASE,
                          (ECAP_ISR_SOURCE_CAPTURE_EVENT_1  |
                           ECAP_ISR_SOURCE_CAPTURE_EVENT_2  |
                           ECAP_ISR_SOURCE_CAPTURE_EVENT_3  |
                           ECAP_ISR_SOURCE_CAPTURE_EVENT_4  |
                           ECAP_ISR_SOURCE_COUNTER_OVERFLOW |
                           ECAP_ISR_SOURCE_COUNTER_PERIOD   |
                           ECAP_ISR_SOURCE_COUNTER_COMPARE));
    ECAP_clearInterrupt(ECAP1_BASE,
                        (ECAP_ISR_SOURCE_CAPTURE_EVENT_1  |
                         ECAP_ISR_SOURCE_CAPTURE_EVENT_2  |
                         ECAP_ISR_SOURCE_CAPTURE_EVENT_3  |
                         ECAP_ISR_SOURCE_CAPTURE_EVENT_4  |
                         ECAP_ISR_SOURCE_COUNTER_OVERFLOW |
                         ECAP_ISR_SOURCE_COUNTER_PERIOD   |
                         ECAP_ISR_SOURCE_COUNTER_COMPARE));

    //
    // Disable CAP1-CAP4 register loads
    //
    ECAP_disableTimeStampCapture(ECAP1_BASE);

    //
    // Configure eCAP
    //    Enable capture mode.
    //    One shot mode, stop capture at event 4.
    //    Set polarity of the events to rising, falling, rising, falling edge.
    //    Set capture in time difference mode.
    //    Select input from XBAR7.
    //    Enable eCAP module.
    //    Enable interrupt.
    //
    ECAP_stopCounter(ECAP1_BASE);
    ECAP_enableCaptureMode(ECAP1_BASE);
    ECAP_setCaptureMode(ECAP1_BASE, ECAP_ONE_SHOT_CAPTURE_MODE, ECAP_EVENT_4);

    ECAP_setEventPolarity(ECAP1_BASE, ECAP_EVENT_1, ECAP_EVNT_FALLING_EDGE);
    ECAP_setEventPolarity(ECAP1_BASE, ECAP_EVENT_2, ECAP_EVNT_RISING_EDGE);
    ECAP_setEventPolarity(ECAP1_BASE, ECAP_EVENT_3, ECAP_EVNT_FALLING_EDGE);
    ECAP_setEventPolarity(ECAP1_BASE, ECAP_EVENT_4, ECAP_EVNT_RISING_EDGE);

    ECAP_enableCounterResetOnEvent(ECAP1_BASE, ECAP_EVENT_1);
    ECAP_enableCounterResetOnEvent(ECAP1_BASE, ECAP_EVENT_2);
    ECAP_enableCounterResetOnEvent(ECAP1_BASE, ECAP_EVENT_3);
    ECAP_enableCounterResetOnEvent(ECAP1_BASE, ECAP_EVENT_4);

    ECAP_selectECAPInput(ECAP1_BASE, ECAP_INPUT_INPUTXBAR7);

    ECAP_enableLoadCounter(ECAP1_BASE);
    ECAP_setSyncOutMode(ECAP1_BASE, ECAP_SYNC_OUT_DISABLED);
    ECAP_startCounter(ECAP1_BASE);
    ECAP_enableTimeStampCapture(ECAP1_BASE);
    ECAP_reArm(ECAP1_BASE);

    ECAP_enableInterrupt(ECAP1_BASE, ECAP_ISR_SOURCE_CAPTURE_EVENT_4);

2. 中断

捕获传入 SCI 通信的脉冲宽度，每捕获 4 次就中断一次。 将这 4 个捕获添加到阵列中。

__interrupt void ecap1ISR(void)
{
    if(stopCaptures==0)
    {
        //
        // Get the capture counts, interrupt every 4. Can be 1-bit or more wide.
        // add one to account for partial eCAP counts at higher baud rates
        // (e.g. count = 40, but if had higher resolution, this would be 40.5)
        //
        capCountArr[0] = 1+ECAP_getEventTimeStamp(ECAP1_BASE, ECAP_EVENT_1);
        capCountArr[1] = 1+ECAP_getEventTimeStamp(ECAP1_BASE, ECAP_EVENT_2);
        capCountArr[2] = 1+ECAP_getEventTimeStamp(ECAP1_BASE, ECAP_EVENT_3);
        capCountArr[3] = 1+ECAP_getEventTimeStamp(ECAP1_BASE, ECAP_EVENT_4);

        //
        // Add samples to a buffer. Get average baud and tune INTOSC if buffer filled.
        //
        capCountIter = 0;
        for (capCountIter=0; capCountIter<4; capCountIter++)
        {
            //
            // if we still have samples left to capture, add it to the samples array
            //
            if(samplesArrIter<NUMSAMPLES)
            {
                samplesArr[samplesArrIter] = capCountArr[capCountIter];
                samplesArrIter++;
            }

            //
            // else, all samples were received, break to begin tuning
            //
            else
            {
                stopCaptures=1;
                break;
            }
        }
    }

    //
    // Clear interrupt flags for more interrupts.
    //
    ECAP_clearInterrupt(ECAP1_BASE,ECAP_ISR_SOURCE_CAPTURE_EVENT_4);
    ECAP_clearGlobalInterrupt(ECAP1_BASE);

    //
    // Start eCAP
    //
    ECAP_reArm(ECAP1_BASE);

    //
    // Acknowledge the group interrupt for more interrupts.
    //
    Interrupt_clearACKGroup(INTERRUPT_ACK_GROUP4);
}

3. 主循环

捕获阵列满后，计算阵列的平均脉冲宽度 (也就是波特率)，并更新SCI波特率寄存器，使其尽可能接近计算的平均值。

//
    // Loop forever. Suspend or place breakpoints to observe the buffers.
    //
    for(;;)
    {
        //
        // Array is filled, begin tuning
        //
        if(stopCaptures==1)
        {
            //
            // Get an average baud rate from the array of samples
            //
            uint32_t avgBaud = getAverageBaud(samplesArr,NUMSAMPLES,TARGETBAUD);

            //
            // if the baud function returns the error code '0', then flag an error
            //
            if(avgBaud==0)
            {
                ESTOP0;
            }

            //
            // Update the device's baud rate to match the measured baud rate
            //
            SCI_setBaud(SCIA_BASE, DEVICE_LSPCLK_FREQ, avgBaud);

            //
            // (OPTIONAL) Continuously send data to SCITX once tuning
            // is complete for external observation (by logic analyzer or scope)
            //
            //unsigned char *msg;
            //while(1)
            //{
            //    msg = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\0";
            //    SCI_writeCharArray(SCIA_BASE, (uint16_t*)msg, 91);
            //}

            //
            // Wait for user to view the results in "Expressions" window
            //
            ESTOP0;

            //
            // If continuing, reset the array iterator and unlock the ISR for new captures
            //
            samplesArrIter=0;
            stopCaptures=0;
        }
}

4. 平均脉冲宽度

对于许多应用的SCI 通信，传输的数据 (例如 0xA5)是变化不固定的，因此SCI的高低电平脉冲宽度就是变化的。所以必须对样本阵列进行如下的预处理，然后才能计算平均脉冲宽度。

a) 丢弃大于 10 位宽的脉冲宽度 (丢弃空闲时间)

b) 将 n 位值除以 n

c) 对修改后的样本数组进行平均化

uint32_t getAverageBaud(volatile float arr[], int size, float targetBaudRate)
{
    //
    // clean up variable width array to single-bit-width array
    //
    uint16_t pass = arrTo1PulseWidth(arr, size, (float)DEVICE_SYSCLK_FREQ/targetBaudRate);

    //
    // pass only if enough good samples provided
    //
    if(pass == 0)
    {
        return 0;
    }

    //
    // convert 2-bit width, 3-bit width, etc. to 1-bit width values by dividing, and average these values.
    // skip unrelated values
    //
    float averageBitWidth = computeAvgWidth(arr, size);

    //
    // get the rounded baud rate from the average number of clocks and the sysclk frequency
    //
    return (uint32_t)(((float)DEVICE_SYSCLK_FREQ/(float)averageBitWidth)+0.5);
}

以下是平均脉宽计算的原理和代码流程图

三 结果

按照以下设置进行测试，结果详见表格，校准以后的误差从3% 改善为0.1%左右甚至更小。

1. “Transmitter”设置为正确的波特率 (我们尝试匹配的波特率)
2. “Receiver”设置为错误波特率 (-3% 或 +3%)
3. “Receiver”运行校准程序以匹配“Transmitter”