C501_1 Datasheet, PDF (46/121 Pages) Siemens Semiconductor Group – 8-Bit Single-Chip Microcontroller

On-Chip Peripheral Components
C501
Table 6-5
Read-Modify-Write"- Instructions
Instruction
ANL
ORL
XRL
JBC
CPL
INC
DEC
DJNZ
MOV Px.y,C
CLR Px.y
SETB Px.y
Function
Logic AND; e.g. ANL P1, A
Logic OR; e.g. ORL P2, A
Logic exclusive OR; e.g. XRL P3, A
Jump if bit is set and clear bit; e.g. JBC P1.1, LABEL
Complement bit; e.g. CPL P3.0
Increment byte; e.g. INC P1
Decrement byte; e.g. DEC P1
Decrement and jump if not zero; e.g. DJNZ P3, LABEL
Move carry bit to bit y of port x
Clear bit y of port x
Set bit y of port x
The reason why read-modify-write instructions are directed to the latch rather than the pin is to avoid
a possible misinterpretation of the voltage level at the pin. For example, a port bit might be used to
drive the base of a transistor. When a “1” is written to the bit, the transistor is turned on. If the CPU
then reads the same port bit at the pin rather than the latch, it will read the base voltage of the
transistor (approx. 0.7 V, i.e. a logic low level!) and interpret it as “0”. For example, when modifying
a port bit by a SETB or CLR instruction, another bit in this port with the above mentioned
configuration might be changed if the value read from the pin were written back to the latch.
However, reading the latch rater than the pin will return the correct value of “1”.
Semiconductor Group
6-12