
Chapter 18 Multiplexed External Bus Interface (MEBIV3)
MC9S12E128 Data Sheet, Rev. 1.07
Freescale Semiconductor
519
18.3.2
Register Descriptions
18.3.2.1
Port A Data Register (PORTA)
Read: Anytime when register is in the map
Write: Anytime when register is in the map
Port A bits 7 through 0 are associated with address lines A15 through A8 respectively and data lines
D15/D7 through D8/D0 respectively. When this port is not used for external addresses such as in
single-chip mode, these pins can be used as general-purpose I/O. Data direction register A (DDRA)
determines the primary direction of each pin. DDRA also determines the source of data for a read of
PORTA.
This register is not in the on-chip memory map in expanded and special peripheral modes. Therefore, these
accesses will be echoed externally.
NOTE
To ensure that you read the value present on the PORTA pins, always wait
at least one cycle after writing to the DDRA register before reading from the
PORTA register.
76543210
R
Bit 7
6
5
4
3
2
1
Bit 0
W
Reset
0
Single Chip
PA7
PA6
PA5
PA4
PA3
PA2
PA1
PA0
Expanded Wide,
Emulation Narrow with
IVIS, and Peripheral
AB/DB15
AB/DB14
AB/DB13
AB/DB12
AB/DB11
AB/DB10
AB/DB9
AB/DB8
Expanded Narrow AB15 and
DB15/DB7
AB14 and
DB14/DB6
AB13 and
DB13/DB5
AB12 and
DB12/DB4
AB11 and
DB11/DB3
AB10 and
DB10/DB2
AB9 and
DB9/DB1
AB8 and
DB8/DB0
Figure 18-2. Port A Data Register (PORTA)