Abstract: An in-system debugging (ISD) capability is incorporated into a production microcontroller. The ISD capability is incorporated without the costly addition of any extra pins to read out the data for debugging by using the oscillator pins of the production microcontroller to read out the data. Building such an ISD capability into the microcontroller, enables debugging to be performed on the actual production board (instead of a special debug board) having the actual production microcontroller (instead of a bond-out microcontroller). This allows designers to debug programming using the actual production system instead of an emulation system.

Abstract: A method and apparatus for In-System Programming which overcomes the above-described disadvantages. The method and apparatus of the ISP system interfaces with the two oscillator (instead of I/O) pins on the microcontroller. By interfacing with the two oscillator pins, the need for extra isolation circuitry to isolate other circuits from the ISP circuits is avoided in most circumstances, without incurring the expense of an expensive JTAG tester or extra dedicated pins. The amount of isolation circuitry necessary is reduced because the two oscillator pins are usually connected to passive components (registers, capacitors, or crystals) which cannot be damaged by the relatively high programming voltages and which do not produce signals that would interfere with the ISP programming signals.