How does the Microchip ICD Work?

• The ICD spec from Microchip has been released! (cached 20010504121901 in CCS HTML)
• PICList post "ICD Debugger Specification"
• PICList post "In Circuit Debugging.... 16F877" By: Craig Peacock  http://www.infosite.com/%7Ejkeyzer/piclist/2000/Jan/1552.html
• http://www.beyondlogic.org/pic/icd.htm

Myke Predko says:

When I was doing the second edition of my PICmicro microcontroller book, I did some hacking of the ICD Interface and I found that in "ICD mode", all the hardware registers could be written to, but any changes to them didn't take effect until *after* a "return" instruction was executed.

While the PICmicro MCU was in ICD mode the only hardware resources that could be accessed correctly were:

1. RB6/RB7 via 0x0106. Only the Programming "Data" pin's TRIS bit could be changed. The Clock bit stayed in input mode.
2. Program memory flash read/write controls.
3. ICKBUG and BIGBUG (according to DS51242A - I didn't have this document when I was playing around with it so I called the registers "Reserve_E" and "Reserve_F") could be read and written before executing a "return" to return to the executing application.
4. STATUS, PCLATH, FSR
5. All File Registers

If you are interested in seeing what I found, Take a look at pages 650 to 652 of the book. On the CD-ROM, in the EMU-II directory, you will find the source code that I created from the 256 instructions inserted into the ICD by the controller.

Questions:

• do you have any pictures?
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• Kevin Curley asks:

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