AVR STK500 User Guide - Atmel

3.8.2 Analog Reference
Voltage, AREF
Hardware Description
The internal VTARGET has a short circuit protection. If VTARGET is set to be higher
than 0.3V and the master microcontroller measures it to be below 0.3V for a duration of
80 ms, the master microcontroller will shut off the VTARGET and the Internal AREF.
When this happens, the status LED will blink quickly.
If an external VTARGET is used and the external VTARGET is turned off, this will also
be detected by the STK500 as a (false) short circuit. The latter case can be avoided by
lowering the internal VTARGET below 0.3V.
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 reference
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 external
source at the AREF pin on the PORTE/AUX header (Figure 3-6). Figure 3-23
explains AREF jumper options.
When using an external source for AREF, the user must control VTARGET at a higher
voltage level than AREF. This can be controlled easily by reading the VTG value from
AVR Studio before setting AREF.
Figure 3-23. AREF Jumper Options
Jumper Mounted
VTARGET
AREF
RESET
On-board AREF voltage connected (default)
Jumper not Mounted
VTARGET
AREF
RESET
On-board AREF voltage disconnected
The STK500 master microcontroller 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. Figure 3-24 shows the internal connection
of the AREF signal.
AVR STK500 User Guide 3-17
1925C–AVR–3/03