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Standard 8051 8-Bit Flash MCU
HT85F2260/HT85F2270/HT85F2280
Input/Output Ports
Input/Output Ports
I/O Pin Structures
The accompanying diagrams illustrate the internal structures of some generic I/O pin types. As
the exact logical construction of the I/O pin will differ from these drawings, they are supplied as a
guide only to assist with the functional understanding of the I/O pins.
Quasi-bidirectional I/O – All Ports
This is the traditional 8051 type I/O port type, constructed from an NMOS FET transistor and
three pull high resistors, so called Strong, Weak, Very Weak pull high resistors. This structure can
be used to reduce the power consumption and the output switching state respond time.
A Very Weak pull high resistor will be turned on whenever the I/O port registers, associated with
the I/O pins, contain a high level.
When the I/O port registers has a high level and the corresponding I/O pins stay at high level as
well, the Weak pull high resistor will be turned on. However, if the I/O port registers are high and
the corresponding I/O pins are pulled low by the external devices, then the Weak pull high resistor
will be disabled by hardware. These weak pull-high resistor enable/disable function are dependant
on the voltage level after the I/O pin is connected to the external circuit.
The Strong pull high resistor is used to enhance the output response time. When the output state
changes from low to high, the Strong resistor will be turned on after two system clock delay times.
A Quasi-bidirectional pin also provides a Schmitt Trigger input.
Input
Data
Q
Port Register Data
VCC
Strong
Two System
Clock Delay
VCC
Very Weak
VCC
Weak
Port
Pin
Quasi-bidirectional I/O Structure