sak-c868p-1rr Infineon Technologies Corporation, sak-c868p-1rr Datasheet - Page 52

• Manufacturer Part Number: sak-c868p-1rr
• Description: 8 - Bit Cmos Microcontroller
• Manufacturer: Infineon Technologies Corporation

4.4.1
Some port-reading instructions read the latch and others read the pin. The instructions
reading the latch rather than the pin read a value, possibly change it, and then rewrite it
to the latch. These are called "read-modify-write"- instructions, which are listed in 4-3. 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, resp., is performed by reading the SFR 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 is not obvious that the last three instructions in
instructions, but they are. The reason is that they read the port byte, all 8 bits, modify the
addressed bit, then write the complete byte back to the latch.
Table 4-3
Instruction
ANL
ORL
XRL
JBC
CPL
INC
DEC
DJNZ
MOV Px.y,C Move carry bit to bit y of port x
CLR Px.y
SETB Px.y
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 transitor (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 th latch. However, reading
the latch rater than the pin will return the correct value of "1".
User’s Manual
Read-Modify-Write Feature of Ports
"Read-Modify-Write"-Instructions
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 P4
Decrement byte; e.g. DEC P5
Decrement and jump if not zero; e.g. DJNZ P3, LABEL
Clear bit y of port x
Set bit y of port x
4-8
On-Chip Peripheral Components
Figure 4-3
are read-modify-write
V 1.0, 2003-01
C868