Semiconductor Group
4-31
1998-04-01
Instruction Set
C500 Family
DJNZ
<byte>, <rel-addr>
Function:
Decrement and jump if not zero
Description:
DJNZ decrements the location indicated by 1, and branches to the address
indicated by the second operand if the resulting value is not zero. An original value
of 00H will underflow to 0FFH. No flags are affected. The branch destination would
be computed by adding the signed relative-displacement value in the last instruction
byte to the PC, after incrementing the PC to the first byte of the following instruction.
The location decremented may be a register or directly addressed byte.
Note:
When this instruction is used to modify an output port, the value used as the original
port data will be read from the output data latch, not the input pins.
Example:
Internal RAM locations 40H, 50H, and 60H contain the values, 01H, 70H, and 15H,
respectively. The instruction sequence
DJNZ 40H,LABEL_1
DJNZ 50H,LABEL_2
DJNZ 60H,LABEL_3
will cause a jump to the instruction at label LABEL_2 with the values 00H, 6FH, and
15H in the three RAM locations. The first jump was
not
taken because the result was
zero.
This instruction provides a simple way of executing a program loop a given number
of times, or for adding a moderate time delay (from 2 to 512 machine cycles) with a
single instruction. The instruction sequence
MOV
R2, #8
TOGGLE: CPL
P1.7
DJNZ
R2,TOGGLE
will toggle P1.7 eight times, causing four output pulses to appear at bit 7 of output
port 1. Each pulse will last three machine cycles; two for DJNZ and one to alter the
pin.
