Datasheet

Port B
MC68HC908AZ60A • MC68HC908AS60A • MC68HC908AS60E Data Sheet, Rev. 6
Freescale Semiconductor 253
22.3.2 Data Direction Register B
Data direction register B determines whether each port B pin is an input or an output. Writing a logic 1 to
a DDRB bit enables the output buffer for the corresponding port B pin; a logic 0 disables the output buffer.
DDRB[7:0] — Data Direction Register B Bits
These read/write bits control port B data direction. Reset clears DDRB[7:0], 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 22-7 shows the port B I/O logic.
Figure 22-7. Port B I/O Circuit
When bit DDRBx is a logic 1, reading address $0001 reads the PTBx data latch. When bit DDRBx is a
logic 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 22-2 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 22-6. Data Direction Register B (DDRB)
Table 22-2. Port B Pin Functions
DDRB
Bit
PTB
Bit
I/O Pin
Mode
Accesses to DDRB Accesses to PTB
Read/Write Read Write
0 X Input, Hi-Z DDRB[7:0] Pin
PTB[7:0]
(1)
1 X Output DDRB[7:0] PTB[7:0] PTB[7:0]
X = don’t care
Hi-Z = high impedance
1. Writing affects data register, but does not affect input.
READ DDRB ($0005)
WRITE DDRB ($0005)
RESET
WRITE PTB ($0001)
READ PTB ($0001)
PTBx
DDRBx
PTBx
INTERNAL DATA BUS