38K2 Datasheet, PDF (125/151 Pages) Renesas Technology Corp – SINGLE-CHIP 8-BIT CMOS MICROCOMPUTER

38K2 Group
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
X.
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.
Rev.3.00 Oct 15, 2006 page 125 of 147
REJ03B0193-0300