Atmel STK500 - Manual (Page 5)

5
Clock Sources and Reset
The Clock Switch
The STK505 has support to easily configure the hardware to support three
different clock configurations
• External
Clock
• On board crystal
• Internal
Oscillator
The XTAL1 and XTAL2 pins are configured as regular I/O port pins when not
used as clock input/output.
The clock selection switch is used to connect the pins to different components
dependant on whether it is used as clock pins or I/O pins.
Note: The device fuses must be programmed to use the selected clock source.
See the device datasheet for more details.
Position Function
XTAL
XTAL1 and XTAL2 pins connected to the XTAL
socket
EXT CLK XTAL1 pin connected to the STK500 clock source
XTAL2 pin connected to PORTB header
INT RC XTAL1 and XTAL2 pins connected to the PORTB
header
The Reset/PORTB Switch
The RESET/PORTB switch connects the reset pin to either the reset signal from
STK500, or to the PORTB header on STK500 to make it accessible for use as a
regular I/O pin.
Note: The RSTDISBL fuse must be programmed to disable reset functionality
and use the pin as a regular I/O port pin. Se the device datasheet for further
details.