SLAS191A datasheet(57/86 Pages) TI

57 / 86 page

3–1

3 Monitor Restrictions

3.1

The Monitor Program uses the R4 register internally to return data from the user application to the

Hyperterminal. Modifications to the value stored in R4 will result in unexpected behavior of the Monitor

Program. The Monitor command r, to change the register contents, is not supported for R4. The register R4

can be modified by the user application code. Consequently, the user should exercise caution when using

this register.

It is possible to interrupt the user application by pressing the ESC key on the keyboard. This works only if

the GIE bit has not been cleared. One exception is possible in spite of a cleared GIE flag. If the user

application runs on a breakpoint, program execution will branch to the Monitor as the program runs.

Problems may occur if the user application uses most of its time in interrupt routines. While the application

is servicing an interrupt, a new interrupt can only occur if the GIE bit has been set. The GIE bit is set only

if the first interrupt routine explicitly sets it within the routine itself, thereby allowing for nested interrupts.

Starting the user application with the Monitor causes the Monitor Program to set the GIE bit. If the GIE-bit

was reset before in the user application, it will remain set. This ensures the availability of the RS232

communication but influences the user’s software application. The particular interrupts can be enabled with

their associated interrupt flags.

3.2

The Instruction CALL R4

In the user application, it is possible to return to the Monitor with the instruction CALL R4. The monitor should

be started prior to this so that the contents in R4 are valid and to initialize the Monitor.

A single step should not be executed over the CALL R4 instruction because this will cause unpredictable

behavior of the Monitor. The following code is an example that uses the instruction CALL R4.

NOTE:

Execute the single step command only if the program counter points to an instruction in

the RAM.

WDTCTL

.equ

0120h

WDTHold

.equ

80h

WDT_wrkey

.equ

05a00h

.text

0240h

RESET:

MOV

#03DEh,SP

MOV

# (WDTHold+WDT_wrkey),&WDTCTL

; stop Watchdog

; Timer

MOV

#0h,R7

WAIT:

INC

CALL

.sect

“Int_Vect” ,03FEh

.word

RESET

.end

Figure 3–1. CALL R4 Instruction Code

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