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Improved usb cdc interface for stm32

Advantages

  • Same API as provided as by ST but with additional and improved functionality
  • const correct
  • easy to use with automatic buffer management for sending and receiving data
    • achieves up to 1MB/s (USB FS) when transmitting to Linux host (tested on STM32F4 with fcpu > 100MHz)
  • Added functionality:
    • functions to check if packets were dropped both for transmission and reception
    • CDC_IsComPortOpen() can be used to check if transmitted packets are actually read by computer side
  • Attempts to avoid zero length packets (small latency vs speed tradeoff)
  • Can be configured as reentrant safe via #define

How to use

  • replace USB_DEVICE/App/usbd_cdc_if.c and USB_DEVICE/App/usbd_cdc_if.h with the ones provided in the repository
    • adjust APP_RX_DATA_SIZE and APP_TX_DATA_SIZE if needed
  • Send data via CDC_Transmit(const void* Buf, uint32_t Len) or CDC_TransmitString(const char *string)
  • Receive data by polling uint32_t CDC_RXQueue_Dequeue(void* Dst, uint32_t MaxLen) or by overriding uint8_t CDC_DataReceivedHandler(const uint8_t *Data, uint32_t len) and handling data immediately upon reception

Configuration

There are several configuration options in the header:

  • If you want to use CDC_Transmit from both main context and interrupt context (or generally in a reentrant way), #define CDC_REENTRANT
    • Beware that reentrancy is ensured by using critical sections (disabling interrupts), which means one should be careful when using reentrancy in combination with CDC_TransmitTimed as interrupts can get disabled up to the provided timeout
  • Transmit and receive buffer size can be configured via APP_TX_DATA_SIZE and APP_RX_DATA_SIZE
    • If you send large bursts of data, increase transmit buffer accordingly. Otherwise, use the CDC_TransmitTimed function.
    • Likewise APP_TX_DATA_SIZE can be reduced to save RAM.
  • If your USB runs in full speed, keep the #define USE_USB_FS as is. If your USB runs in high speed, comment the #define.
    • If your USB runs in full speed, but ST uses the xxx_HS definitions, uncomment #define USB_FS_HS_PERIPHERAL

TODO

  • check if the physical limit of 4096 bytes for the USB packet area really applies to all devices

Tested on

  • STM32F4

API

uint8_t CDC_Transmit(const void* Buf, uint32_t Len);
uint8_t CDC_TransmitTimed(const void* Buf, uint32_t Len, uint32_t TimeoutMs);
uint8_t CDC_TransmitString(const char *string);
uint8_t CDC_IsBusy();
uint32_t CDC_RXQueue_Dequeue(void* Dst, uint32_t MaxLen);
uint32_t CDC_TXQueue_GetReadAvailable();
uint32_t CDC_TXQueue_GetWriteAvailable();
uint32_t CDC_RXQueue_GetReadAvailable();
uint32_t CDC_RXQueue_GetWriteAvailable();
uint32_t CDC_GetDroppedTxPackets();
uint32_t CDC_GetDroppedRxPackets();
void CDC_ResetDroppedTxPackets();
void CDC_ResetDroppedRxPackets();
uint8_t CDC_DataReceivedHandler(const uint8_t *Data, uint32_t len);
uint32_t CDC_GetLastTransmitStartTick();
uint32_t CDC_GetLastTransmitCompleteTick();
uint8_t CDC_IsComportOpen();