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Mini57
Apr. 06, 2017
Page 165 of 475
Rev.1.00
MINI5
7
S
E
RI
E
S
TECH
NIC
A
L R
E
F
E
RE
N
CE
MA
N
UA
L
Port Pin
Port Pin
N
N
Port Latch Data
Port Latch Data
Input Data
Input Data
Figure 6.5-4 Open-Drain Output
6.5.5.4
Quasi-bidirectional Mode
Set MODEn (Px_MODE[2n+1:2n]) to 11 as the Px.n pin is in Quasi-bidirectional mode and the I/O
pin supports digital output and input function at the same time but the source current is only up to
hundreds uA. Before the digital input function is performed the corresponding DOUT
(Px_DOUT[n]) bit must be set to 1. The quasi-bidirectional output is common on the 80C51 and
most of its derivatives. If the bit value in the corresponding DOUT (Px_DOUT[n]) bit is 0, the pin
drive a low output on the pin. If the bit value in the corresponding DOUT (Px_DOUT[n]) bit is 1,
the pin will check the pin value. If pin value is high, no action takes. If pin state is low, the pin will
drive strong high with 2 clock cycles on the pin and then disable the strong output drive.
Meanwhile, the pin status is controlled by internal pull-up resistor. Note that the source current
capability in quasi-bidirectional mode is only about 200 uA to 30 uA for V
DD
is from 5.0 V to 2.5 V.
Port Pin
Port Pin
N
N
P
P
VDD
VDD
Port Latch Data
Port Latch Data
Input Data
Input Data
2 CPU
Clock Delay
P
P
P
P
Strong
Strong
Very
Weak
Very
Weak
Weak
Weak
Figure 6.5-5 Quasi-Bidirectional I/O Mode