gc80c520a CORERIVER Semiconductor, gc80c520a Datasheet - Page 32

• Manufacturer Part Number: gc80c520a
• Description: Midas1.0 Family Mask Rom/eprom/romless 8-bit Turbo Microcontrollers
• Manufacturer: CORERIVER Semiconductor

Functional Description
port pin is determined by the DC current flowing out from it.
For status transition from logic 0 to logic 1, strong pull-ups are used. Since the pull-down drivers are
strong, no driver for status transition from logic 1 to logic 0 is needed. The transition current is applied for
changing from logic 0 to logic 1. This transition current is used for two clock cycles. The absolute current
is approximately -650uA at 5V.
Now the operation an I/O port will be explained. A very strong pull-down FET is always activated if logic 0
is programmed to a port pin. A short circuit to Vcc must be avoided if the transistor is turned on, since the
high current might destroy the FET. This also means that no logic 0 must be programmed into the latch of
a pin that is used as input. A very strong pull-up FET is activated for two clock cycles if a 0-to-1 transition
is programmed to a port pin, i.e. logic 1 is programmed to a port latch which contained logic 0. This
provides a fast transition of the logic levels at the pin. A weak pull-up transistor is always activated when
logic 1 is in the port latch, thus providing the logic high output level. This pull-up FET sources a much
lower current than the pull-up FET for a 0-to-1 transition; therefore the pin may also be tied to ground, e.g.
when used as input with logic low input level.
Substantial DC current can be supplied for both logic 1 and 0. However, it is not recommended to drive
many heavy loads. It is because power dissipation may exceed the limitation.
6.2.1.8 Input Functions
To be used as an input, the port latch must contain logic 1, which turns off all the output drivers except
the weak pull-up resistors. So the port pin is pulled high weakly by the internal pull-up resistors and can
be pulled low by an external source. Thus when the port latch contains logic 1, the port pin will be
configured as an input. When externally pulled high, it will maintain logic 1. When externally pulled low, it
will source current and its logic state will drop to 0. So it can be used as an input. All instructions except
the read-modify-write instructions read the port pin.
6.2.1.9 Read-Modify-Write Instructions
Some instructions that read a port read the latch and others read the pin. The instructions that read the
latch rather than the pin are the ones that read a value, possibly change it, and then rewrite it to the latch.
These are called “read-modify-write” instructions. The instructions listed below are read-modify-write
instructions.
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