6 Software Flowchart

Figure 6-1 shows the code flow for this example.

USB to SPI direction:

1. TinyUSB invokes tud_cdc_rx_cb() each time the USB host sends data. The callback calls usb2spi_pushTask(), which reads up to 64 bytes from the USB CDC interface and queues them as a task in gUSB2SPITask.
2. In the main loop, when gUSB2SPITask.fifoCount > 0, usb2spi_popTask() flushes the SPI TX FIFO, configures DMA channel CH1 with the task buffer address and length, enables the DMA_DONE_TX interrupt, and starts the SPI write transfer.
3. The DMA_DONE_TX interrupt fires when DMA finishes loading the TX FIFO. The handler clears and enables the TX_EMPTY interrupt. The TX_EMPTY interrupt fires when the last byte has been clocked out. The handler calls fifoCompleteRead() to release the task and clears spiInProgress.

SPI to USB direction:

1. The SPI target device asserts the data-ready GPIO pin (rising edge). The GPIO interrupt handler sets SPISlaveDataReady = true and returns immediately.
2. In the main loop, when SPISlaveDataReady is set and spiInProgress is false, spi2usb_pushTask() flushes both RX and TX FIFOs, configures DMA channel CH0 for the receive buffer, configures DMA channel CH1 to send dummy data on MOSI, and starts both DMAs. The TX_DONE_TX interrupt is explicitly disabled so that only DMA_DONE_RX signals completion.
3. The DMA_DONE_RX interrupt fires when all 64 bytes have been received. The handler calls fifoCompleteWrite() to mark the task ready for USB transmission, clears SPISlaveDataReady, and clears spiInProgress.
4. In the main loop, when gSPI2USBTask.fifoCount > 0, spi2usb_popTask() checks that the TinyUSB CDC TX FIFO has sufficient space, writes the buffered data, flushes it to the host, and calls fifoCompleteRead() to release the task.