C508_01 Datasheet, PDF (80/267 Pages) Infineon Technologies AG – 8-Bit CMOS Microcontroller

C508
On-Chip Peripheral Components
6.1.6 Read-Modify-Write Feature of Ports 0 to 5 (Except Port 4)
Some port-reading instructions read the latch and others read the pin. The instructions
reading the latch read a value, possibly change it, and then rewrite it to the latch. These
are called “read-modify-write” instructions. They are listed in Table 6-3. If the destination
is a port or a port pin, these instructions read the latch rather than the pin. Note that all
other instructions which can be used to read a port, exclusively read the port pin. In any
case, reading from latch or pin, respectively, is performed by reading the SFR P0, P2
and P3; for example, “MOV A, P3” reads the value from Port 3 pins, while “ANL P3,
#0AAH” reads from the latch, modifies the value, and writes it back to the latch.
It may not be obvious that the last three instructions in Table 6-3 are read-modify-write
instructions, but they are. This is because they read the port byte, all 8 bits, modify the
addressed bit, then write the complete byte back to the latch.
Table 6-3 “Read-Modify-Write”-Instructions
Instruction Function
ANL
Logic AND; for example: ANL P1, A
ORL
Logic OR; for example: ORL P2, A
XRL
Logic exclusive OR; for example: XRL P3, A
JBC
Jump if bit is set and clear bit; for example: JBC P1.1, LABEL
CPL
Complement bit; for example: CPL P3.0
INC
Increment byte; for example: INC P4
DEC
Decrement byte; for example: DEC P5
DJNZ
Decrement and jump if not zero; for example: DJNZ P3, LABEL
MOV Px.y, C Move carry bit to bit y of Port x
CLR Px.y Clear bit y of Port x
SETB Px.y Set bit y of Port x
Read-modify-write instructions are directed to the latch rather than the pin 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, that is, 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’.
User’s Manual
6-15
2001-05