The maximum clock frequency for the ST62T20CM6 is 20 MHz, but it can be overclocked up to 24 MHz with some limitations.
The watchdog timer on the ST62T20CM6 can be configured using the WDTCR register. Set the WDTS bit to enable the watchdog timer, and set the WDT period using the WDTP bits.
Yes, the ST62T20CM6 can operate with a 3.3V power supply, but the maximum clock frequency may be limited to 16 MHz. Consult the datasheet for specific voltage and frequency limitations.
The UART on the ST62T20CM6 is configured using the UARTCR register. Set the UARTEN bit to enable the UART, and configure the baud rate using the UARTBRG register.
The ST62T20CM6 has 20KB of RAM, but 1KB is reserved for the stack, leaving 19KB available for user applications.