Advanced Parallel I/O - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 307

The PE0 pin has two functions.
1. A general-purpose I/O pin
2. The XIRQ input
In addition to the DDRE and PORTE registers, Port E also has the following registers:
• Port E assignment register (PEAR)
• Mode register (MODE)
• Pull-up control register (PUCR)
• Reduced drive register (RDRIV)
• External bus interface control register (EBICTL)

P ORT E A SSIGNMENT R EGISTER (PEAR)
In expanded mode, this register assigns the function of each Port E pin. The contents of the
register PEAR are shown in Figure 7.2. The PEAR register is not accessible for reads or writes in
peripheral mode.

7 6 5 4 3 2 1 0

Reset value
= $000A

NOACCE 0 PIPOE NECLK LSTRE RDWE

0 0

NOACCE: . Can be read/written any time.

0 = PE7 is used as general-purpose I/O pin.
1 = PE7 is output and indicates whether the cycle is a CPU free cycle.
PIPOE:

No Access output enable

.
In normal modes, write once. Special modes, write anytime except the first
time. This bit has no effect in single-chip modes.
0 = PE[6:5] are general-purpose I/O.
1 = PE[6:5] are outputs and indicate the state of the instruction queue.
NECLK:

Pipe signal output enable

. Can be read anytime.
In expanded modes, writes to this bit have no effect. E-clock is required for
demultiplexing the external address. NECLK can be written once in
normal single-chip mode and can be written anytime in special single-chip mode.
0 = PE4 is the external E-clock.
1 = PE4 is a general-purpose I/O pin.
LSTRE: Low strobe (LSTRB) enable . Can be read anytime.
In normal modes, write once. Special modes, write anytime. This bit has no
effect in single-chip modes or normal expanded narrow mode.
0 = PE3 is a general-purpose I/O pin.
1 = PE3 is configured as the LSTRB bus-control output, provided the
HCS12 is not in single-chip or normal expanded narrow modes.
RDWE: Read/write enable . Can be read anytime.
In normal modes, write once. Special modes, write anytime except the
first time. This bit has no effect in single-chip modes.
0 = PE2 is a general-purpose I/O pin.
1 = PE2 is configured as the R/W pin. In single-chip modes, RDWE has
no effect and PE2 is a general-purpose I/O pin.
R/W is used for external writes. After reset in normal expanded mode, it
is disabled. If needed, it should be enabled before any external writes.

No external E-clock

Figure 7.2 ■ PEAR register