N79E855 Datasheet, PDF (28/180 Pages) List of Unclassifed Manufacturers – Microcontroller

N79E855/854 Data Sheet
peripheral-clock time to pull the port pin high quickly. Then it turns off and “weak: pull-up continues remaining the port
pin high. The quasi bidirectional port structure is shown below.
V_DD
Port Latch
2-peripheral-
clock delay
P
Strong ^P
Very P
Weak
Weak
Port Pin
N
Input
Figure 9–1 Quasi Bi-direction I/O Structure
9.1.1 Read-Modify-Write
In the standard 8051 instruction set, user should watch out for one kind of instructions, read-modify-write instructions.
Instead of the normal instructions, the read-modify-write instructions read the internal port latch (Px in SFRs) rather than
the external port pin state. This kind of instructions read the port SFR value, modify it and write back to the port SFR.
Read-modify-write instructions are listed as follows.
Instruction
ANL
ORL
XRL
JBC
CPL
INC
DEC
DJNZ
MOV
CLR
SETB
Description
Logical AND. (ANL Px,A and ANL Px,direct)
Logical OR. (ORL Px,A and ORL Px,direct)
Logical exclusive OR. (XRL Px,A and XRL Px,direct)
Jump if bit = 1 and clear it. (JBC Px.y,LABEL)
Complement bit. (CPL Px.y)
Increment. (INC Px)
Decrement. (DEC Px)
Decrement and jump if not zero. (DJNZ Px,LABEL)
Px.y,C Move carry bit to Px.y.
Px.y Clear bit Px.y.
Px.y Set bit Px.y.
The last three seems not obviously read-modify-write instructions but actually they are. They read the entire port latch
value, modify the changed bit, and then write the new value back to the port latch.
April 23, 2014
Page 28 of 180
Revision A2.5