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3
4.
Port 2
Port 2 is an output-only port that also serves as the external bus control pins. In single-
chip mode only, the port functions as an output-only port. In external bus mode, the
port pins become external bus control pins. However, the external bus pin control
register (BCTR) can be used to specify a subset of the pins to function as output-only
ports in external bus mode. This section principally describes the port functions when
operating as an output-only port.
The section describes the port structure and pins, the pin block diagram, and the port
registers for port 2.
n Structure of Port 2
Port 2 consists of the following three components:
•
Output-only pins/external bus control pins (P20/BUFC to P27/ALE)
•
Port 2 data register (PDR2)
•
External bus pin control register (BCTR)
n Port 2 Pins
Port 2 consists of eight output pins of a CMOS output type.
As the output pins function as external bus control pins in external bus mode, the pins cannot
be used as output-only ports in this mode. However, if the external bus control pin functions of
P20 to P22 are not used, the external bus pin control register (BCTR) can set P20 to P22 to
function as output-only ports.
Table 4.3a lists the port 2 pins.
Table 4.3a Port 2 Pin
Port
Port 2
Note: P22/HRQ and P23/RDY function as input pins in external bus mode.
Reference: See Section 1.7, "I/O Pins and Pin Functions" for a description of the circuit type.
94
CHAPTER 4 I/O PORTS
Function
Pin name
(Single-chip
mode)
P20/BUFC
P20 Output-only
P21/HAK
P21 Output-only
P22/HRQ
P22 Output-only
P23/RDY
P23 Output-only
P24/CLK
P24 Output-only
P25/WR
P25 Output-only
P26/RD
P26 Output-only
P27/ALE
P27 Output-only
Shared peripheral
(External bus mode)
BUFC Buffer control output
HAK Hold acknowledge output
HRQ Hold request input
RDY Ready input
CLK Clock output
WR Write strobe output
RD Read strobe output
ALE Address latch enable output
I/O type
Circuit
type
Input
Output
—
F
CMOS
D
CMOS
—
F
MB89620 series
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