6 Software Flowchart

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

USB to I2C direction:

1. TinyUSB invokes tud_cdc_rx_cb() each time the USB host sends data. The callback calls usb2i2c_pushTask(), which reads up to 64 bytes from the USB CDC interface and queues them as a task in gUSB2I2CTask.
2. In the main loop, when gUSB2I2CTask.fifoCount > 0, usb2i2c_popTask() configures DMA channel CH1 with the task buffer address and length, then starts the I²C write transfer to the target device.
3. The TX DMA done interrupt fires when DMA finishes loading the I²C TX FIFO. The handler clears the STOP interrupt status flag and enables the STOP interrupt. The STOP interrupt fires when the I²C STOP condition appears on the bus. The handler calls fifoCompleteRead() to release the task and clears i2cInProgress.

I2C to USB direction:

1. The I2C target device asserts the data-ready GPIO pin (rising edge). The GPIO interrupt handler sets i2ctargetDataReady
2. In the main loop, when i2ctargetDataReady is set and i2cInProgress is false, i2c2usb_pushTask() configures DMA channel CH0 for the receive buffer, starts the I2C read transfer for 64 bytes, and sets i2cInProgress. The data length is always fixed to 64 bytes.
3. The RX DMA done interrupt fires when all 64 bytes have been received. The handler calls fifoCompleteWrite() to mark the task ready for USB transmission, clears i2ctargetDataReady, and clears i2cInProgress.
4. In the main loop, when gI2C2USBTask.fifoCount > 0, i2c2usb_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.