Datasheet

Port B
MC68HC908GR16A Data Sheet, Rev. 1.0
Freescale Semiconductor 121
12.3.2 Data Direction Register B
Data direction register B (DDRB) determines whether each port B pin is an input or an output. Writing a 1
to a DDRB bit enables the output buffer for the corresponding port B pin; a 0 disables the output buffer.
DDRB7–DDRB0 — Data Direction Register B Bits
These read/write bits control port B data direction. Reset clears DDRB7–DDRB0, configuring all port
B pins as inputs.
1 = Corresponding port B pin configured as output
0 = Corresponding port B pin configured as input
NOTE
Avoid glitches on port B pins by writing to the port B data register before
changing data direction register B bits from 0 to 1.
Figure 12-8 shows the port B I/O logic.
Figure 12-8. Port B I/O Circuit
When bit DDRBx is a 1, reading address $0001 reads the PTBx data latch. When bit DDRBx is a 0,
reading address $0001 reads the voltage level on the pin. The data latch can always be written,
regardless of the state of its data direction bit. Table 12-3 summarizes the operation of the port B pins.
Address: $0005
Bit 7654321Bit 0
Read:
DDRB7 DDRB6 DDRB5 DDRB4 DDRB3 DDRB2 DDRB1 DDRB0
Write:
Reset:00000000
Figure 12-7. Data Direction Register B (DDRB)
Table 12-3. Port B Pin Functions
DDRB
Bit
PTB
Bit
I/O Pin
Mode
Accesses to DDRB Accesses to PTB
Read/Write Read Write
0
X
(1)
1. X = Don’t care
Input, Hi-Z
(2)
2. Hi-Z = High impedance
DDRB7–DDRB0 Pin
PTB7–PTB0
(3)
3. Writing affects data register, but does not affect input.
1 X Output DDRB7–DDRB0 PTB7–PTB0 PTB7–PTB0
READ DDRB ($0005)
WRITE DDRB ($0005)
RESET
WRITE PTB ($0001)
READ PTB ($0001)
PTBx
DDRBx
PTBx
INTERNAL DATA BUS