Atmel STK525, Hardware User'S Manual

Page: 11 / 40

Using the STK525
STK525 Hardware User Guide 2-11
7608A–AVR–04/06
2.3.3 STK500 RESET
(c.f.
“RESET from STK500” on page 20
)
2.4 AT90USBxxx AVR Microcontroller
2.4.1 Main Clock XTAL
To use the USB interface of the AT90USBxxx, the clock source should always be a
crystal or external clock oscillator (the internal 8MHz RC oscillator cannot be used to
operate with the USB interface). Only the following crystal frequency allows proper USB
operations: 2MHz, 4MHz, 6MHz, 8MHz, 12MHz, 16MHz. The STK525 comes with a
default 8MHz crystal oscillator.
When closing STKX1 and STKX2 switches, and STK525 is connected to an STK500,
the STK525 can operate with the “STK500 Osc” frequency parameter.
2.4.2 Analog Power Supply
AVCC By default, AVCC is equivalent to VCC.
ANA REF By default, AREF is an output of AT90USBxxx.
An external AREF source can be chosen (c.f.
“STK500 Resources” on page 19
).
2.5 Serial Links
2.5.1 USB
The STK525 is supplied with a standard USB mini A-B receptacle. The mini AB
receptacle allows to connect both a mini A plug or a mini B plug connectors.
Figure 2-7 . USB mini A-B Receptacle
When connected to a mini B plug, the AT90USBxxx operates as an “USB device” (the
ID pin of the plug is unconnected) and when connected to a mini A plug, the
AT90USBxxx operates as a “USB host” (the ID pin of the plug is tied to ground).