Instead of purchasing the bare chips for pennies at LCSC and putting together a breakout board, I bought a couple modules from Electrodragon. I had learned that the CH551, CH552, and CH554 all used the same die. I bought the CH551 and CH552 modules with the intention of eventually trying to hack them into working as a CH554.
For testing the modules, in addition to the CH554 SDK for SDCC on Linux, I've used Ch55xduino on Windows. One thing not in the Ch55xduino documentation is driver setup. The windoze version I'm using is 7E, and when I first inserted the CH551 module, I got a driver error.
Using Zadig to set the driver to libusb-win32 solved the problem.
The CH55xduino documenation also lacks pinout documentation for anything other than the reverence board. To help, I've copied the pinouts from the CH552 datahseet.
The CH55x bootloader supports DFU, which is what the CH55xduino uploader uses the first time code is uploaded to the module. Once the first sketch is uploaded, the CH55xduino core includes a CDC serial stack. With my CH551 module no longer appearing as a DFU device, I had to use Zadig again to change the CDC Serial device to use the USB Serial (CDC) driver. After that, the module appears as a COM port.
With the COM port selected in the Arduino IDE, subsequent uploads enter the bootloader by switching the baud rate to 1200bps. If no COM port is selected, the upload tool looks for a CH55x device in DFU bootloader mode. To enter the bootloader, it is necessary to pull the USB D+ pin up to 3.3V when power is applied. The Electrodragon boards have a pinout for an upload jumper, which when shorted will connect the D+ pin (P3.6/UDP)to 3.3V through a 10k resistor. On one of my modules I soldered pin headers and use a jumper to force it into upload mode. On the other, I just used a low-value (270Ohm) through-hole resistor pushed into the holes.
Currently CH55xduino is not optimized for size, with a basic blink sketch requiring 5333 bytes of flash. Officially, the CH551 is only supposed to have 10kB of available flash, so the CH55xduino overhead means less than 5kB is left for user code. The CH551 actually seems to have 12kB available for flashing user code, which I think will be plenty if the CH55xduino core gets some optimization work. Since I like to do low-level embedded coding, I'll be using SDCC from the command line most of the time. The blink example in the CH554 SDK for SDCC compiles to 700 bytes, and I was able to get that down to 232 bytes after leaving out the UART initialization in debug.c. With a bit more optimization I think I can get the blink example down to 100 bytes or so.
One small surprise I found during my testing is that the Electrodragon CH551 and CH552 modules use different pins for the user LED. On the CH551, use P3.0, working in open-drain mode so the LED light up when P3.0 is low. On the CH552, drive P1.4 high to light the LED. This is documented on the Electrodragon web site, but it is easy to forget when switching between the two modules.
I've already started to learn how to configure the standard MCS-51 UART, and have figured out how to directly manipulate the ports using the SFRs (Special Function Registers). Once I've mastered how to program these cheap little devices, I'll follow up with another blog post revealing the details.
Postscript
I recently found out that these modules do not fit well in a solderless breadboard. The row spacing for the 0.1" headers on the CH551 is about 0.47", so the pins have to bend out slightly to plug into the breadboard. On the CH552 module, the row spacing is about 0.52", so the pins have to bend out slightly to fit.
