Specifications

63
AT90S8515
0841G–09/01
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 unintention-
ally 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 the enabling/disabling of
pull-up resistors (if configured as input).
Port A Port A is an 8-bit bi-directional I/O port.
Three I/O memory address locations are allocated for the Port A, one each for the Data
Register – PORTA, $1B($3B), Data Direction Register – DDRA, $1A($3A) and the Port
A Input Pins – PINA, $19($39). The Port A 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 A output buffers can
sink 20 mA and thus drive LED displays directly. When pins PA0 to PA7 are used as
inputs and are externally pulled low, they will source current if the internal pull-up resis-
tors are activated.
The Port A pins have alternate functions related to the optional external data SRAM.
Port A can be configured to be the multiplexed low-order address/data bus during
accesses to the external data memory. In this mode, Port A has internal pull-up
resistors.
When Port A is set to the alternate function by the SRE (external SRAM enable) bit in
the MCUCR (MCU Control Register), the alternate settings override the Data Direction
Register.
Port A Data Register – PORTA
Port A Data Direction Register
– DDRA
Port A Input Pins Address –
PINA
The Port A Input Pins address (PINA) is not a register; this address enables access to
the physical value on each Port A pin. When reading PORTA, the Port A Data Latch is
read and when reading PINA, the logical values present on the pins are read.
Port A as General Digital I/O All eight pins in Port A have equal functionality when used as digital I/O pins.
PAn, general I/O pin: The DDAn bit in the DDRA register selects the direction of this pin.
If DDAn is set (one), PAn is configured as an output pin. If DDAn is cleared (zero), PAn
is configured as an input pin. If PORTAn is set (one) when the pin is configured as an
input pin, the MOS pull-up resistor is activated. To switch the pull-up resistor off, the
Bit 76543210
$1B ($3B)
PORTA7 PORTA6 PORTA5 PORTA4 PORTA3 PORTA2 PORTA1 PORTA0 PORTA
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 76543210
$1A ($3A) DDA7 DDA6 DDA5 DDA4 DDA3 DDA2 DDA1 DDA0 DDRA
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 76543210
$19 ($39) PINA7 PINA6 PINA5 PINA4 PINA3 PINA2 PINA1 PINA0 PINA
Read/WriteRRRRRRRR
Initial Value N/A N/A N/A N/A N/A N/A N/A N/A