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Theory of Operation

ISD51 adds a serial interrupt function (the ISD51 Interrupt) for the 8051 UART to your user program. When ISD51 connects to the µVision Debugger, the 8051 enters the ISD51 interrupt function. While program execution is stopped, the 8051 program runs only the ISD51 interrupt function. When the µVision Debugger issues a Go command, the 8051 exits the ISD51 interrupt function and executes the user program.

If no breakpoints are set or if only hardware breakpoints or Flash breakpoints are set, the 8051 executes the user program at full speed. Flash breakpoints are easily configured for many 8051 devices with IAP and Flash block sizes 128 bytes and smaller.

If software breakpoints are set, the 8051 enters the ISD51 interrupt function after each 8051 CPU instruction. The ISD51 interrupt checks to see if the 8051 program reached a breakpoint address and, if so, begins communication with the µVision Debugger. 8051 programs execute considerably slower (about 100x) when software breakpoints are used.

The µVision Debugger sends an 0xA5 character to the ISD51 interrupt function to halt the user program and start communication with the debugger. Therefore, the 8051 program must be started when the µVision Debugger is invoked.

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