3819 datasheet(65/218 Pages) RENESAS

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3819 Group

SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER

MITSUBISHI MICROCOMPUTERS

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 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. Only the ADC

and SBC instructions yield proper decimal results. After execut-

ing 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) flag are invalid.

The carry flag can be used to indicate whether a carry or borrow

has occurred. Initialize the carry flag before each calculation.

Clear the carry flag before an ADC and set the flag before an

SBC.

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 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 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 direc-

tion registers.

Serial I/O

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.

When using the internal clock, set the synchronous clock to inter-

nal 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.

Make sure that f(XIN) is 500 kHz or more during an A-D conver-

sion.

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.

The number of cycles required to execute an instruction is shown

in the list of machine instructions. The frequency of the internal

clock

φ is half of the XIN or XCIN frequency.

At the STP Instruction Release

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

ter are cleared.

The XCOUT drivability selection bit (the CPU mode register) is set

to “1” (high drive) in order to start oscillating.

3819 Datasheet(PDF) 65 Page - Renesas Technology Corp

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