3800 Datasheet, PDF (46/174 Pages) Renesas Technology Corp – 8-BIT SINGLE-CHIP MICROCOMPUTER 740 FAMILY / 38000 SERIES

HARDWARE
NOTES ON PROGRAMMING
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 re-
quest register, execute at least one instruction before executing 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 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.
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 contents
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 instruction (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 SRDY signal, set the transmit en-
able bit, the receive enable bit, and the SRDY output enable bit to
“1”.
Serial I/O continues to output the final bit from the TXD pin after
transmission is completed.
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 frequency.
When the ONW function is used in modes other than single-chip
mode, the frequency of the internal clock φ may be one fourth the
XIN frequency.
Memory Expansion Mode and Microproces-
sor Mode
Execute the LDM or STA instruction for writing to port P3 (address
000616) in memory expansion mode and microprocessor mode.
Set areas which can be read out and write to port P3 (address
000616) in a memory, using the read-modify-write instruction
(SEB, CLB).
3800 GROUP USER’S MANUAL
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