3858 Group
NOTES
NOTES ON PROGRAMMING
The carry flag (C) is set to “1” if a carry is generated as a result of
the calculation, or is cleared to “0” if a borrow is generated. To de-
termine whether a calculation has generated a carry, the C flag
must be initialized to “0” before each calculation. To check for a
borrow, the C flag must be initialized to “1” before each calcula-
tion.
1. Processor Status Register
(1) Initializing of processor status register
Flags which affect program execution must be initialized after a reset.
In particular, it is essential to initialize the T and D flags because
they have an important effect on calculations.
<Reason>
Set D flag to “1”
↓
After a reset, the contents of the processor status register (PS)
are undefined except for the I flag which is “1”.
ADC or SBC instruction
↓
NOP instruction
Reset
↓
↓
SEC, CLC, or CLD instruction
Initializing of flags
↓
Fig 65. Execution of decimal calculations
Main program
4. JMP instruction
When using the JMP instruction in indirect addressing mode, do
Fig 63. Initialization of processor status register
not specify the last address on a page as an indirect address.
(2) How to reference the processor status register
To reference the contents of the processor status register (PS),
execute the PHP instruction once then read the contents of (S+1).
If necessary, execute the PLP instruction to return the PS to its
original status.
5. 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.
6. Ports
(S)
The contents of the port direction registers cannot be read. The
(S)+1
Stored PS
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
Fig 64. Stack memory contents after PHP instruction execution
• 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.
2. BRK instruction
(1) Interrupt priority level
Use instructions such as LDM and STA, etc., to set the port direc-
tion registers.
When the BRK instruction is executed with the following condi-
tions satisfied, the interrupt execution is started from the address
of interrupt vector which has the highest priority.
• Interrupt request bit and interrupt enable bit are set to “1”.
• Interrupt disable flag (I) is set to “1” to disable interrupt.
7. Instruction Execution Time
The instruction execution time can be obtained by multiplying the
frequency of the internal clock φ by the number of cycles men-
tioned in the 740 Family Software Manual.
3. Decimal calculations
(1) Execution of decimal calculations
The frequency of the internal clock φ is the twice the XIN cycle in
high-speed mode, 8 times the XIN cycle in middle-speed mode,
and the twice the XCIN in low-speed mode.
The ADC and SBC are the only instructions which will yield proper
decimal notation, set the decimal mode flag (D) to “1” with the
SED instruction. After executing the ADC or SBC instruction, ex-
ecute another instruction before executing the SEC, CLC, or CLD
instruction.
8. Reserved Area, Reserved Bit
Do not write any data to the reserved area in the SFR area and
the special page. (Do not change the contents after reset.)
(2) Notes on status flag in decimal mode
When decimal mode is selected, the values of three of the flags in
the status register (the N, V, and Z flags) are invalid after a ADC or
SBC instruction is executed.
9. CPU Mode Register
Be sure to fix bit 3 of the CPU mode register (address 003B16) to
“1”.
Rev.1.10 Apr 3, 2006 page 70 of 75
REJ03B0139-0110
RENESAS [ RENESAS TECHNOLOGY CORP ]
