51795B datasheet(23/100 Pages) MICROCHIP

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Theory of Operation

 2010 Microchip Technology Inc.

DS51795B-page 23

2.6.1

Sequence of Operations Leading to Debugging

Given that the requirements for debugging (see previous section) are met, these

actions can be performed when the PICkit 3 programmer/debugger is set as the current

debugger from the MPLAB IDE menu (Debugger>Select Tool>PICkit 3):

• The application code is compiled/assembled by selecting Project>Build

Configuration>Debug.

• When Debugger>Program is selected, the application code is programmed into

the device’s memory via the ICSP protocol as described above.

• A small “debug executive” program is loaded into the high area of program

memory of the target device automatically by MPLAB IDE. Since the debug

executive must reside in program memory, the application program must not use

this reserved space. Some devices have special memory areas dedicated to the

debug executive. Check your device data sheet for details.

• Special “in-circuit debug” registers in the target device are enabled. These allow

the debug executive to be activated by the debugger.

• The target device is held in Reset by keeping the VPP/MCLR line low.

2.6.2

Debugging Details

Figure 2-7 illustrates the PICkit 3 programmer/debugger system when it is ready for

debugging.

FIGURE 2-7:

PICkit™ 3 DEBUGGER READY FOR DEBUGGING

Typically, in order to find out if an application program will run correctly, a breakpoint is

set early in the program code. When a breakpoint is set from the user interface of

MPLAB IDE, the address of the breakpoint is stored in the special internal debug

registers of the target device. Commands on PGC and PGD communicate directly to

these registers to set the breakpoint address.

Next, the Debugger>Run function or the Run icon (forward arrow) is usually pressed

from MPLAB IDE. The debugger will then tell the debug executive to run. The target

will start from the Reset vector and execute until the Program Counter reaches the

breakpoint address previously stored in the internal debug registers.

After the instruction at the breakpoint address is executed, the in-circuit debug

mechanism of the target device “fires” and transfers the device’s Program Counter to

the debug executive (much like an interrupt) and the user’s application is effectively

+5V

+12V

4.7 k



4.7 k



Internal Circuits

Program

Memory

File

Registers

Internal

Debug

Registers

VPP/MCLR

PGC

PGD

Executive

Debug

Area Used by

Target

Running

must

for Debug

Executive

to Function

Area

VDD

Hardware

Stack Shared

by Debug Exec

Debug Exec

Reserved

for Debug

Executive

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