PA0-7 = port A bit 0-7, PB0-7 = port B bit 0-7, PC0-7 = port C 0-7... these are the main functions. Each pin has an alternate function.
Reset input used to reset device
ADC0-7 analog to digital converter inputs
AREF input to ADC reference voltage
MISO input/output = SPI port Master In Slave Out... input when device is in master mode, ouput when device is in slave mode
MOSI input/output = SPI port Master Out Slave In
SCK input/output = SPI port clk pin output when in master mode, input in slave mode
SS SPI bus master SLAVE mode select
ICP1 input capture pin for timer/counter 1
OC1A/OC1B timer/counter 1 output compare match (A and B)
OC2 timer/counter 2 output compare match
AIN0/AIN1 analog comparator positive/negative input pins
TxD/RxD uart serial transmit and receive pins
SCL/SDA Two wire serial port clock and data in/out pins
XTAL1/XTAL2 external crystal/oscillator inputs/ oscillator output
TOSC1/TOSC2 timer oscillator inputs
T0/T1 timer/counter 0/1 external inputs
XCK USART external clock input
INT0/INT1 external interupt inputs
read pin descriptions starting on page 5. Since each pin has more than one function, the function that pin actually does is dependent on how the device is programmed. For instance, if you are using the internal RC oscillator, pins PB6 and PB7 can be used as IO port pins. If you program the device for external oscillator, there had better be a crystal attached or an external clock signal.
Alternate port pin descriptions start on page 56.
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