8 Data Flow Examples

Scenario A: Host writes 5 bytes to target

1. Host sends HELLO via COM port
2. tud_cdc_rx_cb() queues data
   1. usb2spi_pushTask()
   2. Queue [H, E, L, L, O] with length = 5
3. Main loop: usb2spi_popTask()
   1. Get task from queue
   2. SPI write: data=[H,E,L,L,O]
   3. DMA transfers to SPI TX FIFO
4. SPI hardware executes
5. SPI_0_INST_IRQHandler()
   1. DL_SPI_IIDX_DMA_DONE_TX: DMA finished
   2. DL_SPI_IIDX_TX_EMPTY: Transmission complete
   3. fifoCompleteRead() marks task consumed

Scenario B: host reads 64 bytes from target

1. SPI target has data, pulls GPIO high
2. GPIO interrupt fires
   1. SPISlaveDataReady = true
3. Main loop detects: spi2usb_pushTask()
   1. Flush SPI RX/TX FIFOs
   2. Setup DMA TX: send dummy data (clock generation)
   3. Setup DMA RX: SPI RX → buffer
   4. Start SPI transfer (64 bytes)
4. SPI Hardware executes
5. SPI_0_INST_IRQHandler()
   1. DL_SPI_IIDX_DMA_DONE_RX: All data received
   2. fifoCompleteWrite(&gSPI2USBTask), and mark ready for USB transmission
6. Main loop: spi2usb_popTask()
   1. Send 64 bytes to USB
   2. fifoCompleteRead() marks task done
7. Host receives 64 bytes on COM port