30
3850 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.” After
a reset, initialize flags which affect program execution. In particu-
lar, 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. 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
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 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.
Serial I/O
In clock synchronous serial I/O, if the receive side is using an ex-
ternal clock and it is to output the S
RDY
signal, set the transmit
enable bit, the receive enable bit, and the S
RDY
output enable bit
to “1.”
Serial I/O continues to output the final bit from the T
X
D pin after
transmission is completed.
When an external clock is used as synchronous clock in serial I/O,
write transmission data to the transmit buffer register while the
transfer clock is “H.”
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(X
IN
) 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.
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 X
IN
frequency in
high-speed mode.