BetaFlight模块设计之三十六:SoftSerial
- 1. 源由
- 2. API接口
- 2.1 openSoftSerial
- 2.2 onSerialRxPinChange
- 2.3 onSerialTimerOverflow
- 2.4 processTxState
- 2.5 processRxState
- 3. 辅助函数
- 3.1 applyChangedBits
- 3.2 extractAndStoreRxByte
- 3.3 prepareForNextRxByte
- 4. 总结
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:
- 配置问题
- 代码移植BUG(unified_target ==> config)
- 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。