Input/Output (I/O) Ports
Port A
MC68HC908MR16/MC68HC908MR32 — Rev. 4.0 Advance Information
MOTOROLA Input/Output (I/O) Ports 311
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 regi ster before
changing data direction register A bits from 0 to 1.
Figure 15-4 shows the port A I/O logic.
Figure 15-4. Port A I/O Circuit
When bit DDRAx is a logic 1, reading address $0000 reads the PTAx
data latch. When bit 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 15-1 summarizes
the operation of the port A pins.
Table 15-1. Port A Pin Functions
DDRA
Bit PTA Bit I/O Pin
Mode
Accesses
to DDRA Accesses to PTA
Read/Write Read Write
0X(1)
1. X = don’t care
Input, Hi-Z(2)
2. Hi-Z = high impedance
DDRA[7:0] Pin PTA[7:0](3)
3. Writing affects data register, but does not affect input.
1 X Output DDRA[7:0] PTA[7:0] PTA[7:0]
READ DDRA ($0004)
WRITE DDRA ($0004)
RESET
WRITE PTA ($0000)
READ PTA ($0000)
PTAx
DDRAx
PTAx
INTERNAL DATA BUS