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MS51
Nov. 28, 2019
Page
274
of 491
Rev 1.00
MS51
32K
SE
RIES
TE
CHNICAL RE
F
EREN
CE MA
N
UAL
Port Pin
Input
Figure 6.4-3 Input-Only Mode Structure
Open-Drain Mode
6.4.1.4
The open-drain mode turns off all pull-high transistors and only drives the pull-low of the port pin when
the port latch is given by logic 0. If the port latch is logic 1, it behaves as if in input-only mode. To be
used as an output pin generally as I
2
C lines, an open-drain pin should add an external pull-high,
typically a resistor tied to V
DD
. User needs to take care that an open-drain pin with its port latch as
logic 1 should be given with a determined voltage level by external devices or resistors. A floating pin
will induce leakage current especially in Power-down mode.
Port Pin
Input
Port Latch
N
Figure 6.4-4 Open-Drain Mode Structure
6.4.2
Read-Modify-Write Instructions
Instructions that read a byte from SFR or internal RAM, modify it, and rewrite it back, are called “Read-
Modify-
Write” instructions. When the destination is an I/O port or a port bit, these instructions read the
internal output latch rather than the external pin state. This kind of instructions read the port SFR
value, modify it and write back to the port SFR. All “Read-Modify-Write” instructions are listed as
follows.
Instruction
Description
ANL
Logical AND. (ANL direct, A and ANL direct, #data)
ORL
Logical OR. (ORL direct, A and ORL direct, #data)
XRL
Logical exclusive OR. (XRL direct, A and XRL direct, #data)
JBC
Jump if bit = 1 and clear it. (JBC bit, rel)
CPL
Complement bit. (CPL bit)
INC
Increment. (INC direct)
DEC
Decrement. (DEC direct)
DJNZ
Decrement and jump if not zero. (DJNZ direct, rel)
MOV
bit, C Move carry to bit. (MOV bit, C)
CLR
bit
Clear bit. (CLR bit)
SETB
bit
Set bit. (SETB bit)
The last three seem not obviously “Read-Modify-Write” instructions but actually they are. They read