MC68331
MOTOROLA
MC68331TS/D
35
3.6 General-Purpose Input/Output
SIM pins can be configured as two general-purpose I/O ports, E and F. The following paragraphs de-
scribe registers that control the ports.
A write to the port E data register is stored in the internal data latch and, if any port E pin is configured
as an output, the value stored for that bit is driven on the pin. A read of the port E data register returns
the value at the pin only if the pin is configured as a discrete input. Otherwise, the value read is the value
stored in the register.
The port E data register is a single register that can be accessed in two locations. When accessed at
$YFFA11, the register is referred to as PORTE0; when accessed at $YFFA13, the register is referred
to as PORTE1. The register can be read or written at any time. It is unaffected by reset.
The bits in this register control the direction of the pin drivers when the pins are configured as I/O. Any
bit in this register set to one configures the corresponding pin as an output. Any bit in this register
cleared to zero configures the corresponding pin as an input. This register can be read or written at any
time.
The bits in this register control the function of each port E pin. Any bit set to one configures the corre-
sponding pin as a bus control signal, with the function shown in the following table. Any bit cleared to
zero defines the corresponding pin to be an I/O pin, controlled by PORTE and DDRE.
Data bus bit 8 controls the state of this register following reset. If DATA8 is set to one during reset, the
register is set to $FF, which defines all port E pins as bus control signals. If DATA8 is cleared to zero
during reset, this register is set to $00, configuring all port E pins as I/O pins.
Any bit cleared to zero defines the corresponding pin to be an I/O pin. Any bit set to one defines the
corresponding pin to be a bus control signal.
PORTE0, PORTE1 —Port E Data Register
$YFFA11, $YFFA13
15
8
7
6
5
4
3
2
1
0
NOT USED
PE7
PE6
PE5
PE4
PE3
PE2
PE1
PE0
RESET:
U
DDRE — Port E Data Direction Register
$YFFA15
15
8
7
6
5
4
3
2
1
0
NOT USED
DDE7
DDE6
DDE5
DDE4
DDE3
DDE2
DDE1
DDE0
RESET:
0
PEPAR — Port E Pin Assignment Register
$YFFA17
15
8
7
6
5
4
3
2
1
0
NOT USED
PEPA7
PEPA6
PEPA5
PEPA4
PEPA3
PEPA2
PEPA1
PEPA0
RESET:
DATA8
Table 16 Port E Pin Assignments
PEPAR Bit
Port E Signal
Bus Control Signal
PEPA7
PE7
SIZ1
PEPA6
PE6
SIZ0
PEPA5
PE5
AS
PEPA4
PE4
DS
PEPA3
PE3
RMC
PEPA2
PE2
AVEC
PEPA1
PE1
DSACK1
PEPA0
PE0
DSACK0