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。
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);
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);
- }
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;
- }
- }
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);
- }
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。
- “Transmitter”设置为正确的波特率 (我们尝试匹配的波特率)
- “Receiver”设置为错误波特率 (-3% 或 +3%)
- “Receiver”运行校准程序以匹配“Transmitter”
| 100K 波特率 | 9601波特率 | ||||
| -3% | +3% | -3% | +3% | ||
| Transmitter (我们正在尝试匹配的内容) | 理想波特率 (仅供参考) | 103306 | 96899 | 9889 | 9314. |
| 实际波特率 (必须与此匹配) | 104174. | 96906 | 9890 | 9315. | |
| Receiver (初始错误波特率) | 波特率 (校准前) | 100154. | 100157. | 9622. | 9622. |
| 出错百分比 (校准前) | -3.859% | 3.355% | -2.706% | 3.296% | |
| Receiver (校准后波特率) | 波特率 (校准后) | 104336. | 97047. | 9888 | 9314. |
| 出错百分比 (校准后) | 0.156% | 0.146% | -0.016% | -0.012% | |
来源：TI
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