The microcontroller I am using is the Philips P89LPC935.
Using Flash Magic and the Keil MCB900 board, I select "Watchdog Oscillator (400 kHz +- 10%)" as the Clock source in the UCFG1 popup then click "Reprogram" on that popup, it is not programmed and I can never communicate with the microcontroller again.
I have tried selecting the internal 400 kHz oscillator as clock in 2 different microcontrollers. Both are now non-programmable.
How do I use the 400 kHz clock as the system clock?
Because 400kHz is very slow, the baud rate you use also has to be very low to match. I tested using the LPC935 on an MCB900 board clocked from the Watchdog oscillator and found that I could connect only at 600 baud. To use this speed you will need to directly enter "600" into the Baud Rate box in section 1 of the Flash Magic window. The value in the Oscillator Frequency box is not used so can be anything.
Let me know if this solves your problem.
Yes, Andrew's suggestion works. Thank you Andres