Hardware Description
3-18 AVR STK500 User Guide
1925C–AVR–3/03
Figure 3-24. Internal AREF Connection
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.
The internal AREF has a short circuit protection. If an AREF value is set up 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 AREF. When this happens, the status
LED will blink slowly.
The AREF will also be shut down by the master microcontroller if a short circuit is
detected on VTARGET (in addition to shutting down VTARGET). In this case, the status
LED will blink quickly.
3.8.3 Reset Settings,
RESET
The RESET jumper controls the RESET signal to the STK500. When ISP programming
the target device in the socket, the master microcontroller programs the AVR device
without interfering with the application. When the RESET jumper is mounted, the master
microcontroller controls the RESET signal of the AVR. When the RESET jumper is not
mounted, the RESET signal is disconnected. This is useful for prototype applications
with an external reset system.
The RESET jumper must always be mounted when High-voltage Programming an AVR
device. When using an external reset system, it must allow the reset line to be controlled
by the master microcontroller reset system during programming. The RESET button is
disconnected if the RESET jumper is not mounted. Figure 3-25 explains the RESET
jumper options.
PE1
RST
GND
XT2
VTG
PE0
PE2
REF
XT1
GND
1 2
Master
MCU
AREF
PWM
AREF
PORTE/AUX
Jumper
Voltage
regulator
10 - 15V
VIN
0 - 6V
ADC
Wr
Rd
