The Analog Reference voltage (AREF) can supply the reference voltage to the on-chip A/D converter on the AVR. If the AREF jumper is mounted, the on-board Analog Refer-ence voltage is connected to the AVR’s AREF. The On-board Analog Reference voltage can be adjusted from AVR Studio to 0 - 6.0V, but not above VTARGET.
When the AREF jumper is disconnected, AREF voltage must be supplied from an exter-nal source at the AREF pin on the PORTE/AUX header. The figure below explains AREF jumper options.
When using external an source for AREF, the user must control that VTARGET is at a higher voltage level than AREF. This can be controlled easily by reading the VTG value from AVR Studio before setting AREF.
The STK500 master MCU controls the Analog Reference Voltage using the internal PWM. The AVR’s AREF signal is also accessible on the PORTE header; this pin can also be used for external AREF signal.The figure below shows the internal connection of the AREF signal.
The AVR Studio controlled Analog Reference Voltage can also be used as an input to the analog comparator or for ADC measurements on the AVR. AVR’s AREF signal can then be connected to VTG.
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