38C3 datasheet(59/224 Pages) RENESAS

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38C3 Group User’s Manual

HARDWARE

1-44

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.” After 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 immediately

after they have been written. After writing to an interrupt request reg-

ister, execute at least one instruction before performing 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 before

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

If a value n (between 0 and 255) is written to a timer latch, the fre-

quency division ratio is 1/(n+1).

Multiplication and Division Instructions

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

the MUL and DIV instruction.

• The execution of these instructions does not change the contents

of the processor status register.

Ports

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

lowing 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 register

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 direction

registers.

Serial I/O

• Using an external clock

When using an external clock, input “H” to the external clock input

pin and clear the serial I/O interrupt request bit before executing

serial I/O transfer and serial I/O automatic transfer.

• Using an internal clock

When using an internal clock, set the synchronous clock to the in-

ternal clock, then clear the serial I/O interrupt request bit before

executing a serial I/O transfer and serial I/O automatic transfer.

A-D Converter

The comparator uses internal capacitors whose charge will be lost if

the clock frequency is too low.

Therefore, make sure that f(XIN) is at least on 500 kHz during an A-D

conversion.

Do not execute the STP or WIT instruction during an A-D conversion.

Instruction Execution Time

The instruction execution time is obtained by multiplying the frequency

of the internal system clock by the number of cycles needed to ex-

ecute an instruction.

The number of cycles required to execute an instruction is shown in

the list of machine instructions.

The frequency of the internal system clock is the same half of the XIN

frequency in high-speed mode.

At STP Instruction Release

At the STP instruction release, all bits of the timer 12 mode register

are cleared.

NOTES ON USE

Notes on Built-in EPROM Version

The P51 pin of the One Time PROM version or the EPROM version

functions as the power source input pin of the internal EPROM.

Therefore, this pin is set at low input impedance, thereby being af-

fected easily by noise.

To prevent a malfunction due to noise, insert a resistor (approx. 5 k

Ω)

in series with the P51 pin.

NOTES ON PROGRAMMING/NOTES ON USE

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