I/o ports, Port b, Port b data register – portb – Rainbow Electronics AT90S1200 User Manual
Page 29: Port b data direction register – ddrb, Port b input pin address – pinb
29
AT90S1200
0838H–AVR–03/02
I/O Ports
All AVR ports have true Read-Modify-Write functionality when used as general digital
I/O ports. This means that the direction of one port pin can be changed without uninten-
tionally changing the direction of any other pin with the SBI and CBI instructions. The
same applies for changing drive value (if configured as output) or enabling/disabling of
pull-up resistors (if configured as input).
Port B
Port B is an 8-bit bi-directional I/O port.
Three I/O memory address locations are allocated for the Port B, one each for the Data
Register – PORTB ($18), Data Direction Register – DDRB ($17), and the Port B Input
Pins – PINB ($16). The Port B Input Pins address is read-only, while the Data Register
and the Data Direction Register are read/write.
All port pins have individually selectable pull-up resistors. The Port B output buffers can
sink 20 mA and thus drive LED displays directly. When pins PB0 to PB7 are used as
inputs and are externally pulled low, they will source current if the internal pull-up resis-
tors are activated.
The Port B pins with alternate functions are shown in Table 8.
When the pins are used for the alternate function, the DDRB and PORTB register has to
be set according to the alternate function description.
Port B Data Register – PORTB
Port B Data Direction Register
– DDRB
Port B Input Pin Address –
PINB
The Port B Input Pins address (PINB) is not a register, and this address enables access
to the physical value on each Port B pin. When reading PORTB, the Port B Data Latch
is read, and when reading PINB, the logical values present on the pins are read.
Table 8. Port B Pin Alternate Functions
Port Pin
Alternate Functions
PB0
AIN0 (Analog Comparator positive input)
PB1
AIN1 (Analog Comparator negative input)
PB5
MOSI (Data Input line for memory downloading)
PB6
MISO (Data Output line for memory uploading)
PB7
SCK (Serial Clock input)
Bit
7
6
5
4
3
2
1
0
$18
PORTB7
PORTB6
PORTB5
PORTB4
PORTB3
PORTB2
PORTB1
PORTB0
PORTB
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
$17
DDB7
DDB6
DDB5
DDB4
DDB3
DDB2
DDB1
DDB0
DDRB
Read/Write
R/W
R/W
R/W
R/W
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Bit
7
6
5
4
3
2
1
0
$16
PINB7
PINB6
PINB5
PINB4
PINB3
PINB2
PINB1
PINB0
PINB
Read/Write
R
R
R
R
R
R
R
R
Initial Value
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A