38K2 datasheet(145/358 Pages) RENESAS

145 / 358 page

NOTES ON PROGRAMMING

Rev.2.00

Oct 15, 2006

page 126 of 130

38K2 Group

REJ09B0338-0200

HARDWARE

NOTES ON PROGRAMMING

Processor Status Register

The contents of the processor status register (PS) after a reset are

undefined, except for the interrupt disable flag (I) which is “1.” Af-

ter a reset, initialize flags which affect program execution. In

particular, it is essential to initialize the index X mode (T) and the

decimal mode (D) flags because of their effect on calculations.

Interrupts

The contents of the interrupt request bits do not change immedi-

ately after they have been written. After writing to an interrupt

request register, execute at least one instruction before perform-

ing a BBC or BBS instruction.

Decimal Calculations

• To calculate in decimal notation, set the decimal mode flag (D)

to “1”, then execute an ADC or SBC instruction. After executing

an ADC or SBC instruction, execute at least one instruction be-

fore executing a SEC, CLC, or CLD instruction.

• In decimal mode, the values of the negative (N), overflow (V),

and zero (Z) flags are invalid.

Timers

• When n (0 to 255) is written to a timer latch, the frequency divi-

sion ratio is 1/(n+1).

• When a count source of timer X is switched, stop a count of timer

Multiplication and Division Instructions

• The index X mode (T) and the decimal mode (D) flags do not af-

fect the MUL and DIV instruction.

• The execution of these instructions does not change the con-

tents of the processor status register.

Ports

The contents of the port direction registers cannot be read. The

following cannot be used:

• The data transfer instruction (LDA, etc.)

• The operation instruction when the index X mode flag (T) is “1”

• The addressing mode which uses the value of a direction regis-

ter as an index

• The bit-test instruction (BBC or BBS, etc.) to a direction register

• The read-modify-write instructions (ROR, CLB, or SEB, etc.) to

a direction register.

Use instructions such as LDM and STA, etc., to set the port direc-

tion registers.

A/D Converter

The comparator uses capacitive coupling amplifier whose charge

will be lost if the clock frequency is too low.

Therefore, make sure that f(system clock) in the middle/high-

speed mode is at least on 500 kHz during an A/D conversion.

Do not execute the STP or WIT instruction during an A/D conver-

sion.

Instruction Execution Time

The instruction execution time is obtained by multiplying the fre-

quency of the internal clock

φ by the number of cycles needed to

execute an instruction. However, When using the USB function or

EXB function, an occurrence of one-wait due to the multichannel

RAM will double an internal clock

φ cycle.

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