MC68HC12 MOTOROLA [Motorola, Inc], MC68HC12 Datasheet - Page 108

• Manufacturer Part Number: MC68HC12
• Description: The MC68HC912DT128A microcontroller unit (MCU) is a 16-bit device composed of standard on-chip peripherals including a 16-bit central processing unit
• Manufacturer: MOTOROLA [Motorola, Inc]

PEAR — Port E Assignment Register
Bus Control and Input/Output
Technical Data
108
RESET:
RESET:
RESET:
RESET:
RESET:
NDBE
BIT 7
0
0
1
1
0
CGMTE
6
0
0
1
0
0
Port E serves as general purpose I/O lines or as system and bus
control signals. The PEAR register is used to choose between the
general-purpose I/O functions and the alternate bus control functions.
When an alternate control function is selected, the associated DDRE
bits are overridden.
The reset condition of this register depends on the mode of operation
because bus control signals are needed immediately after reset in
some modes.
In normal single-chip mode, no external bus control signals are
needed so all of port E is configured for general-purpose I/O.
In normal expanded modes, the reset vector is located in external
memory. The DBE and E clock are required for de-multiplexing
address and data but LSTRB and R/W are only needed by the system
when there are external writable resources. Therefore in normal
expanded modes, the DBE and the E clock are configured for their
alternate bus control functions and the other bits of port E are
configured for general-purpose I/O. If the normal expanded system
needs any other bus control signals, PEAR would need to be written
before any access that needed the additional signals.
In special expanded modes, DBE, IPIPE1, IPIPE0, E, LSTRB, and
R/W are configured as bus-control signals.
In special single chip modes, DBE, IPIPE1, IPIPE0, E, LSTRB, R/W,
and CALE are configured as bus-control signals.
PIPOE
5
0
1
0
0
1
Bus Control and Input/Output
NECLK
4
0
0
1
1
0
LSTRE
3
0
1
0
0
1
RDWE
2
0
1
0
0
1
MC68HC912DT128A — Rev 4.0
CALE
1
0
0
0
0
0
DBENE
BIT 0
0
0
0
0
0
MOTOROLA
Normal sin-
Special sin-
Normal Ex-
Special Ex-
Peripheral
gle chip
gle chip
panded
panded
$000A