gc80c520a CORERIVER Semiconductor, gc80c520a Datasheet - Page 29

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

MiDAS1.0 Family
external memory. During accesses to external data memory that use 8-bit addresses (MOVX @Ri), Port 2
emits the contents of the P2 special function register as the high address byte.
Alternative Function Output
P2SEL.0
Pull-
up
2 OSC
Pulse
Q
CPU BUS
P2.0
SFR
QB
Digital Input
Figure 6-7 Configuration of PORT2
The ROMless devices of the MiDAS1.0 family use Ports 2 only for accesses to external memory. In this
application, it uses strong driver all the time immediately after the rising edge of /PSEN.
6.2.1.4 PORT 3
Port 3 is an 8-bit bi-directional I/O port with internal pull-up resistors. The pull-up resistors are switched
on/off by the value of the SFR P3SEL. Port 3 pins that have 1s written to them are pulled high weakly by
the internal pull-up resistors and can be used as inputs. Port 3 pins can be also used alternate functions.
To use the alternate functions, the corresponding bit of the Port 3 pin should be set to high. The alternate
functions will be selected according to the combination of the SFR values.
The drive characteristics of an alternate function are the same with those of general I/O. The logic 0
output pulls a pin low strongly. The exceptions are pins P3.6 and P3.7, which employ the current-limited
transition drivers described later when used as /RD and /WR signals. The logic 1 output changes a pin
status with a strong pull-up and maintains the logic 1 status with a weak pull-up. More than one port pins
are assigned to alternate functions without changing the functions of the other pins.
As an example, P3.6 is used as the alternate function of /WR and P3.7 as the alternate function of /RD.
These strobes are used for accesses to external data memory. If only the /RD signal is used, P3.6 would
still function as an I/O pin.
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