7630 datasheet(186/238 Pages) RENESAS

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3-15

APPENDIX

3.3 Notes on use

7630 Group User’s Manual

3.3.5 Notes on programming

(1) Initialization of processor status register

Flags which affect program execution must be

initialized after a reset.

In particular, it is essential to initialize the T

and D flags because they have an important

effect on calculations.

Reason

After a reset, the contents of the processor

status register (PS) are undefined except for

the I flag which is “1”.

Fig. 3.3.1 Stack memory contents after instruction

execution

(2) How to reference the processor status register

To reference the contents of the processor status

then read the contents of (S)+1. If necessary,

execute the PLP instruction to return the PS to

its original status. A NOP instruction must be

executed after every PLP instruction.

Reset

Main program

Initializing of flags

NOP

PLP instruction execution

Stored PS

(S)

(S) + 1

(2) Modifying output data with bit managing

When the data register (port latch) of an I/O port is modified with the bit managing instruction52, the

value of the unspecified bit may be changed.

Reason

The bit managing instructions are read-modify-write form instructions for reading and writing data by

a byte unit. Accordingly, when these instructions are executed on a bit of the port latch of an I/O port,

the following is executed to all bits of the data register.

• As for a bit which is set for an input port:

The pin state is read in the CPU, and is written to this bit after bit managing.

• As for a bit which is set for an output port:

The bit value is read in the CPU, and is written to this bit after bit managing.

Note the following :

• Even when a port which is set as an output port is changed for an input port, its data register holds

the output data.

• As for a bit of which is set for an input port, its value may be changed even when not specified

with a bit managing instruction in case where the pin state differs from its data register contents.

52 bit managing instructions : SEB, and CLB instructions

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