Tuesday, March 2, 2021

Writing USB firmware on the CH55x MCUs

Over the last several months, I've been familiarizing myself with the CH552 and CH551 MCUs.  Most recently, I've been learning how to program the USB serial interface engine on these devices.  The USB interface is powerful and flexible enough to implement many different kinds of USB devices, from HID to CDC serial.  The highlights are:

  • support for endpoints 0 through 4, both IN and OUT
  • 64-byte maximum packet size
  • DMA to/from xram only
  • multiple USB interrupt triggers
One of the first requirements for writing USB firmware is writing the descriptors.  The examples from WCH are difficult to use as a template due to the descriptors being uint8_t arrays instead of structures.  There are USB structure and constant definitions in ch554_usb.h, which I recommend using instead of arrays.  For instance, I changed the CDC serial example from :

__code uint8_t DevDesc[] = {0x12,0x01,0x10,0x01,0x02,0x00,0x00,DEFAULT_ENDP0_SIZE,

__code USB_DEV_DESCR DevDesc = {
.bLength = 18,
.bDescriptorType = USB_DESCR_TYP_DEVICE,
.bcdUSBH = 0x01, .bcdUSBL = 0x10,
.bDeviceSubClass = 0,
.bDeviceProtocol = 0,
.bMaxPacketSize0 = DEFAULT_ENDP0_SIZE,
.idVendorH = 0x1a, .idVendorL = 0x86,
.idProductH = 0x57, .idProductL = 0x22,
.bcdDeviceH = 0x01, .bcdDeviceL = 0x00,
.iManufacturer = 1, // string descriptors
.iProduct = 2,
.iSerialNumber = 0,
.bNumConfigurations = 1

Once the descriptors are written, the code to handle device enumeration is mostly boilerplate and can be copied from one of the examples.  During the firmware development stage, I recommend adding a call to disconnectUSB() near the start of main().  It's a function I added to debug.h which forces the host to re-enumerate the device.  This way I don't have to unplug and re-connect the USB module after flashing new firmware.

Setting up the DMA buffer pointers requires special attention when multiple IN and OUT endpoints are used.  Even though five endpoints are supported, there are only four DMA buffer pointer registers: UEP[0-3]_DMA.  When the bits bUEP4_RX_EN and bUEP4_TX_EN are set in the UEP4_1_MOD SFR, the EP4 OUT buffer is UEP0_DMA + 64, and the EP4 IN buffer is UEP0_DMA + 128.  Endpoints 1-3 have even more complex buffer configurations, with optional double-buffering for IN and OUT using 256 bytes for four buffers starting from the UEPn_DMA pointer.

When I first started writing USB firmware for the CH551 and CH552, I was concerned that it may be difficult to meet the tight timing requirements, particularly for control and bulk packets that can have multiple in a single 1ms frame.  For example, with small data packets, the time between the end of one OUT transfer and the end of the next OUT transfer can be less than 20uS.  If the USB interrupt handler is too slow, the 2nd OUT transfer could overwrite the DMA buffer before processing of the first has completed.  This situation is avoided by setting bUC_INT_BUSY in the USB_CTRL SFR.  When this bit is set, the SIE will NAK any packets while the UIF_TRANSFER flag is set.  Therefore I recommend setting bUC_INT_BUSY, and clear UIF_TRANSFER at the end of the interrupt handler.

I am currently working on the CMSIS_DAP example.  It implements the DAPv1 (HID) protocol supporting SWD transfers, and works well with OpenOCD and pyOCD.  I'm working on adding CDC/ACM for serial UART communication.  The first step is creating the descriptors for the composite CDC + HID device.  The second step will be integrating the usb_device_cdc code.  The final step, although not absolutely necessary, will be optimizing the CDC code for baud rates up to 1mbps.  The current code uses transmit and receive ring buffers with data copied to and from the IN and OUT DMA buffers.  With double-buffering, the transmit and receive ring buffers can be omitted.  The UART interrupt will copy directly between SBUF and the appropriate USB DMA buffer.