Input/Output (I/O) Ports
DDRA[7:0] — Data Direction Register A Bits
These read/write bits control port A data direction. Reset clears DDRA[7:0], configuring all port A pins
as inputs.
1 = Corresponding port A pin configured as output
0 = Corresponding port A pin configured as input
NOTE
Avoid glitches on port A pins by writing to the port A data register before
changing data direction register A bits from 0 to 1.
Figure 16-4 shows the port A I/O logic.
READ DDRA ($0004)
WRITE DDRA ($0004)
DDRAx
RESET
WRITE PTA ($0000)
PTAx
PTAx
READ PTA ($0000)
Figure 16-4. Port A I/O Circuit
When DDRAx is a logic 1, reading address $0000 reads the PTAx data latch. When DDRAx is a logic 0,
reading address $0000 reads the voltage level on the pin. The data latch can always be written,
regardless of the state of its data direction bit.
Table 16-2 summarizes the operation of the port A pins.
Table 16-2. Port A Pin Functions
Accesses to DDRA
Read/Write
Accesses to PTA
DDRA
Bit
PTA Bit
I/O Pin Mode
Read
Pin
Write
X(1)
X
Input, Hi-Z(2)
Output
PTA[7:0](3)
PTA[7:0]
0
1
DDRA[7:0]
DDRA[7:0]
PTA[7:0]
1. X = don’t care.
2. Hi-Z = high impedance.
3. Writing affects data register, but does not affect input.
MC68HC908AP Family Data Sheet, Rev. 4
262
Freescale Semiconductor
FREESCALE [ Freescale ]
