根据 SCI 输入信号自动校准波特率

品慧电子讯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。

根据 SCI 输入信号自动校准波特率

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。

根据 SCI 输入信号自动校准波特率

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?";

// 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

根据 SCI 输入信号自动校准波特率

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”

根据 SCI 输入信号自动校准波特率

免责声明：本文为转载文章，转载此文目的在于传递更多信息，版权归原作者所有。本文所用视频、图片、文字如涉及作品版权问题，请联系小编进行处理。

根据 SCI 输入信号自动校准波特率

品慧电子讯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。

相关文章

用户评论

发表评论

最新内容