I'm having problems programming an STC15F2K60S2. It's programmed through the RxD and TxD pins of one of the UARTs. These pins are "converted" to USB through a Microchip MCP2221.

When programming the STC15 micros, the programming software IDs the chip, calibrates the RC-oscillator and then erases/writes the flash. When going through the MCP, only the chip ID works - so reading/writing data does work. But the calibration step does not complete, and the software aborts the programming attempt.

I found the "free programmer" that came with the first STC chips I bought. This programmer is just a USB/UART bridge using a Prolific PL2303HX. When going through the Prolific bridge, programming does work. But why not through the Microchip bridge? What difference could the bridge chip make?

The only reason I went with the MCP originally, is the easy to hand-solder SOIC package...