BetaFlight模块设计之三十六：SoftSerial

1. 源由

鉴于Betaflight关于STM32F405 SBUS协议兼容硬件电气特性问题，从程序代码上看，软串口应该能够采用定时器、中断的方式进行电平协议的解析。

但是从实测Betaflight4.4.2固件的角度看，又无法使用，怀疑可能存在以下问题：

1. 配置问题
2. 代码移植BUG(unified_target ==> config)
3. 代码不支持

所以尝试整理下SoftSerial代码结构，通过对整体代码的了解，能否找出其中的一些深层次原因。

2. API接口

从对外接口的角度看，主要有以下API：

• 打开软件串口openSoftSerial
• 底层串行信号电平变更处理onSerialRxPinChange
• 底层串行信号超市处理onSerialTimerOverflow
• 后端Tx状态处理processTxState
• 后端Rx状态处理processRxState

serialPort_t *openSoftSerial(softSerialPortIndex_e portIndex, serialReceiveCallbackPtr rxCallback, void *rxCallbackData, uint32_t baud, portMode_e mode, portOptions_e options)
void onSerialRxPinChange(timerCCHandlerRec_t *cbRec, captureCompare_t capture)
void onSerialTimerOverflow(timerOvrHandlerRec_t *cbRec, captureCompare_t capture)
void processTxState(softSerial_t *softSerial)
void processRxState(softSerial_t *softSerial)

2.1 openSoftSerial

根据资源进行配置：

• 【Hardware】GPIO：Tx/Rx/SERIAL_INVERTED
• 【Hardware】TIMER
• 【Hardware】Interrupt：ICPOLARITY_RISING/ICPOLARITY_FALLING
• 【Software】Buffer
• 【Software】Callback：onSerialRxPinChange(edgeCb)/onSerialTimerOverflow(overCb)/rxCallback

openSoftSerial││   // get serial port description├──> softSerial_t *softSerial = &(softSerialPorts[portIndex]);││   // get serial port rx/tx ioTag├──> ioTag_t tagRx = softSerialPinConfig()->ioTagRx[portIndex];├──> ioTag_t tagTx = softSerialPinConfig()->ioTagTx[portIndex];││   // one wire(Sbus etc.) or two wire softserial(UART etc.)├──> const timerHardware_t *timerTx = timerAllocate(tagTx, OWNER_SOFTSERIAL_TX, RESOURCE_INDEX(portIndex));├──> const timerHardware_t *timerRx = (tagTx == tagRx) ? timerTx : timerAllocate(tagRx, OWNER_SOFTSERIAL_RX, RESOURCE_INDEX(portIndex));││   // get serial port rx/tx IO_t├──> IO_t rxIO = IOGetByTag(tagRx);├──> IO_t txIO = IOGetByTag(tagTx);││   // timer & io set├──> <options & SERIAL_BIDIR> // bi-direction configuration│   ├──> <!timerTx || (timerTx->output & TIMER_OUTPUT_N_CHANNEL)>│   │   │   // If RX and TX pins are both assigned, we CAN use either with a timer.│   │   │   // However, for consistency with hardware UARTs, we only use TX pin,│   │   │   // and this pin must have a timer, and it should not be N-Channel.│   │   └──> return NULL;│   ├──> softSerial->timerHardware = timerTx;│   ├──> softSerial->txIO = txIO;│   ├──> softSerial->rxIO = txIO;│   └──> IOInit(txIO, OWNER_SOFTSERIAL_TX, RESOURCE_INDEX(portIndex));├──> < else > // unidirection configuration│   ├──> <mode & MODE_RX>│   │   ├──> <!timerRx || (timerRx->output & TIMER_OUTPUT_N_CHANNEL)>│   │   │   │   // Need a pin & a timer on RX. Channel should not be N-Channel.│   │   │   └──> return NULL;│   │   ├──> softSerial->rxIO = rxIO;│   │   ├──> softSerial->timerHardware = timerRx;│   │   └──> <!((mode & MODE_TX) && rxIO == txIO)>│   │       └──> IOInit(rxIO, OWNER_SOFTSERIAL_RX, RESOURCE_INDEX(portIndex));│   └──> <mode & MODE_TX>│       ├──> <!tagTx>│       │   │   // Need a pin on TX│       │   └──> return NULL;│       ├──> softSerial->txIO = txIO;│       ├──> <!(mode & MODE_RX)>│       │   ├──> <!timerTx> return NULL;│       │   │   // TX Simplex, must have a timer│       │   └──> softSerial->timerHardware = timerTx;│       ├──> < else >  // Duplex│       │   └──> softSerial->exTimerHardware = timerTx;│       └──> IOInit(txIO, OWNER_SOFTSERIAL_TX, RESOURCE_INDEX(portIndex));││   // port configuration├──> softSerial->port.vTable = &softSerialVTable;├──> softSerial->port.baudRate = baud;├──> softSerial->port.mode = mode;├──> softSerial->port.options = options;├──> softSerial->port.rxCallback = rxCallback;├──> softSerial->port.rxCallbackData = rxCallbackData;│├──> resetBuffers(softSerial);│├──> softSerial->softSerialPortIndex = portIndex;│├──> softSerial->transmissionErrors = 0;├──> softSerial->receiveErrors = 0;│├──> softSerial->rxActive = false;├──> softSerial->isTransmittingData = false;││   // Configure master timer (on RX); time base and input capture├──> serialTimerConfigureTimebase(softSerial->timerHardware, baud);├──> timerChConfigIC(softSerial->timerHardware, (options & SERIAL_INVERTED) ? ICPOLARITY_RISING : ICPOLARITY_FALLING, 0);││   // Initialize callbacks├──> timerChCCHandlerInit(&softSerial->edgeCb, onSerialRxPinChange);├──> timerChOvrHandlerInit(&softSerial->overCb, onSerialTimerOverflow);││   // Configure bit clock interrupt & handler.│   // If we have an extra timer (on TX), it is initialized and configured│   // for overflow interrupt.│   // Receiver input capture is configured when input is activated.├──> <(mode & MODE_TX) && softSerial->exTimerHardware && softSerial->exTimerHardware->tim != softSerial->timerHardware->tim>│   ├──> softSerial->timerMode = TIMER_MODE_DUAL;│   ├──> serialTimerConfigureTimebase(softSerial->exTimerHardware, baud);│   ├──> timerChConfigCallbacks(softSerial->exTimerHardware, NULL, &softSerial->overCb);│   └──> timerChConfigCallbacks(softSerial->timerHardware, &softSerial->edgeCb, NULL);├──> < else >│   ├──> softSerial->timerMode = TIMER_MODE_SINGLE;│   └──> timerChConfigCallbacks(softSerial->timerHardware, &softSerial->edgeCb, &softSerial->overCb);│├──> <USE_HAL_DRIVER>│   └──> softSerial->timerHandle = timerFindTimerHandle(softSerial->timerHardware->tim);││   // antivate port├──> <!(options & SERIAL_BIDIR)>│   ├──> serialOutputPortActivate(softSerial);│   └──> setTxSignal(softSerial, ENABLE);├──> serialInputPortActivate(softSerial);└──> return &softSerial->port;

2.2 onSerialRxPinChange

通过边沿中断记录bit数据流。

onSerialRxPinChange├──> softSerial_t *self = container_of(cbRec, softSerial_t, edgeCb);├──> bool inverted = self->port.options & SERIAL_INVERTED;│├──> <(self->port.mode & MODE_RX) == 0>│   └──> return;  // 无接收模式，直接返回│├──> <self->isSearchingForStartBit>│   │  // Synchronize the bit timing so that it will interrupt at the center│   │  // of the bit period.│   ├──> <USE_HAL_DRIVER>│   │   └──> __HAL_TIM_SetCounter(self->timerHandle, __HAL_TIM_GetAutoreload(self->timerHandle) / 2);│   ├──> <else>│   │   └──> TIM_SetCounter(self->timerHardware->tim, self->timerHardware->tim->ARR / 2);│   ││   │  // For a mono-timer full duplex configuration, this may clobber the│   │  // transmission because the next callback to the onSerialTimerOverflow│   │  // will happen too early causing transmission errors.│   │  // For a dual-timer configuration, there is no problem.│   ├──> <(self->timerMode != TIMER_MODE_DUAL) && self->isTransmittingData>│   │   └──> self->transmissionErrors++;│   ││   ├──> timerChConfigIC(self->timerHardware, inverted ? ICPOLARITY_FALLING : ICPOLARITY_RISING, 0);│   ├──> <defined(STM32F7) || defined(STM32H7) || defined(STM32G4)>│   │   └──> serialEnableCC(self);│   ││   ├──> self->rxEdge = LEADING;│   ││   ├──> self->rxBitIndex = 0;│   ├──> self->rxLastLeadingEdgeAtBitIndex = 0;│   ├──> self->internalRxBuffer = 0;│   ├──> self->isSearchingForStartBit = false;│   └──> return;││   // handle leveled signal├──> <self->rxEdge == LEADING>│   └──> self->rxLastLeadingEdgeAtBitIndex = self->rxBitIndex;├──>  applyChangedBits(self);│├──> <self->rxEdge == TRAILING>│   ├──> self->rxEdge = LEADING;│   └──> timerChConfigIC(self->timerHardware, inverted ? ICPOLARITY_FALLING : ICPOLARITY_RISING, 0);├──> < else >│   ├──> self->rxEdge = TRAILING;│   └──> timerChConfigIC(self->timerHardware, inverted ? ICPOLARITY_RISING : ICPOLARITY_FALLING, 0);└──> <defined(STM32F7) || defined(STM32H7) || defined(STM32G4)>└──> serialEnableCC(self);

2.3 onSerialTimerOverflow

串行数据从原理上属于字符流协议，从实际应用角度，还是一包一包的数据(通常不会密集到头尾相连)。

因此，超时机制相当于处理：

• 数据帧
• 异常中断

onSerialTimerOverflow├──> softSerial_t *self = container_of(cbRec, softSerial_t, overCb);├──> <self->port.mode & MODE_TX> processTxState(self);└──> <self->port.mode & MODE_RX> processRxState(self);

2.4 processTxState

Tx数据处理存在三种情况：

• 发送数据前处理
• 发送数据
• 发送数据后处理

processTxState│   // 发送数据前处理├──> <!softSerial->isTransmittingData>│   ├──> <isSoftSerialTransmitBufferEmpty((serialPort_t *)softSerial)>│   │   │   // Transmit buffer empty.│   │   │   // Start listening if not already in if half-duplex│   │   ├──> <!softSerial->rxActive && softSerial->port.options & SERIAL_BIDIR) {│   │   │   ├──> serialOutputPortDeActivate(softSerial);│   │   │   └──> serialInputPortActivate(softSerial);│   │   └──> return;│   │    │   │   // data to send│   ├──> uint8_t byteToSend = softSerial->port.txBuffer[softSerial->port.txBufferTail++];│   ├──> <softSerial->port.txBufferTail >= softSerial->port.txBufferSize>│   │   └──> softSerial->port.txBufferTail = 0;│   │   │   │   // build internal buffer, MSB = Stop Bit (1) + data bits (MSB to LSB) + start bit(0) LSB│   ├──> softSerial->internalTxBuffer = (1 << (TX_TOTAL_BITS - 1)) | (byteToSend << 1);│   ├──> softSerial->bitsLeftToTransmit = TX_TOTAL_BITS;│   ├──> softSerial->isTransmittingData = true;│   └──> <softSerial->rxActive && (softSerial->port.options & SERIAL_BIDIR)>│       │   // Half-duplex: Deactivate receiver, activate transmitter│       ├──> serialInputPortDeActivate(softSerial);│       ├──> serialOutputPortActivate(softSerial);│       ││       │   // Start sending on next bit timing, as port manipulation takes time,│       │   // and continuing here may cause bit period to decrease causing sampling errors│       │   // at the receiver under high rates.│       │   // Note that there will be (little less than) 1-bit delay; take it as "turn around time".│       │   // XXX We may be able to reload counter and continue. (Future work.)│       └──> return;││   // 发送bit数据：高/低 电平├──> <softSerial->bitsLeftToTransmit>│   ├──> mask = softSerial->internalTxBuffer & 1;│   ├──> softSerial->internalTxBuffer >>= 1;│   ││   ├──> setTxSignal(softSerial, mask);│   ├──> softSerial->bitsLeftToTransmit--;│   └──> return;││   // 发送数据后处理└──> softSerial->isTransmittingData = false;

2.5 processRxState

RX_TOTAL_BITS 10 bits format: start bit + 8 bits for one byte + stop bit

processRxState│   //Start bit处理├──> <softSerial->isSearchingForStartBit>│   └──> return;├──> softSerial->rxBitIndex++;││   //1 Byte数据处理├──> <softSerial->rxBitIndex == RX_TOTAL_BITS - 1>│   ├──> applyChangedBits(softSerial);│   └──> return;│   //Stop bit处理└──> <softSerial->rxBitIndex == RX_TOTAL_BITS>├──> softSerial->rxEdge == TRAILING>│   └──> softSerial->internalRxBuffer |= STOP_BIT_MASK;├──> extractAndStoreRxByte(softSerial);└──> prepareForNextRxByte(softSerial);

注：上述函数过程存在10bit缺损卡死的情况，代码还不够robust。

3. 辅助函数

3.1 applyChangedBits

1~9 bit数据将通过该函数进行存储，最后10bit数据将在processRxState中进行保存。

applyChangedBits└──> <softSerial->rxEdge == TRAILING>└──> for (bitToSet = softSerial->rxLastLeadingEdgeAtBitIndex; bitToSet < softSerial->rxBitIndex; bitToSet++)└──> softSerial->internalRxBuffer |= 1 << bitToSet;

3.2 extractAndStoreRxByte

从10 bit格式中抽取1Byte有效数据。

extractAndStoreRxByte│   //仅TX模式，无需进行任何接收字节的保存工作├──> <(softSerial->port.mode & MODE_RX) == 0>│   └──> return;│   ├──> uint8_t haveStartBit = (softSerial->internalRxBuffer & START_BIT_MASK) == 0;├──> uint8_t haveStopBit = (softSerial->internalRxBuffer & STOP_BIT_MASK) == 1;│  │   //起止bit位，若一项不符合规格，则丢弃数据├──> <!haveStartBit || !haveStopBit>│   ├──> softSerial->receiveErrors++;│   └──> return;│  │   //保存1Byte数据├──> uint8_t rxByte = (softSerial->internalRxBuffer >> 1) & 0xFF;│  ├──> <softSerial->port.rxCallback> //回调接收函数│   └──> softSerial->port.rxCallback(rxByte, softSerial->port.rxCallbackData);└──> < else > //无接收注册函数情况下，将数据存入缓冲buffer中，并采用循环方式覆盖保存├──> softSerial->port.rxBuffer[softSerial->port.rxBufferHead] = rxByte;└──> softSerial->port.rxBufferHead = (softSerial->port.rxBufferHead + 1) % softSerial->port.rxBufferSize;

3.3 prepareForNextRxByte

收录下一字节数据做预处理工作。

prepareForNextRxByte├──> softSerial->rxBitIndex = 0;├──> softSerial->isSearchingForStartBit = true;└──> <softSerial->rxEdge == LEADING>├──> softSerial->rxEdge = TRAILING;├──> timerChConfigIC(softSerial->timerHardware, (softSerial->port.options & SERIAL_INVERTED) ? ICPOLARITY_RISING : ICPOLARITY_FALLING, 0);└──> serialEnableCC(softSerial);

4. 总结

从SoftSerial代码角度，采用定时器、边沿中断的方式，随机使用CPU资源。如果应用在高速、大数据量通信场景，将会影响和打扰CPU正常业务逻辑，尤其是在CPU资源紧张时。