AT88SCK109STK3

3-8

AT88CK109STK3 User Guide

8699A—CRYPTO—10/09

3.4

AT90USB1287 AVR Microcontroller

To use the USB interface of the AT90USB1287, the clock source should always be a crystal or an

external clock oscillator (the internal 8MHz RC oscillator cannot be used to operate the USB

interface). Only the following crystal frequencies allow proper USB operations: 2MHz, 4MHz,

6MHz, 8MHz, 12MHz, and 16MHz. The AT88Microbase comes with a default 16MHz crystal

oscillator, which is required to obtain the 230.4K USART baud rate needed for

CryptoAuthentication communication.

3.5

Serial Links

3.5.1

USB

The AT88Microbase is supplied with a standard USB Type-A receptacle. The AT88Microbase only

operates as an “USB device”.

Figure 6.

USB Type-A Receptacle

3.5.2

USART

The AT90USB1287 AVR comes with an on-chip USART peripheral (USART1).

Only the

asynchronous mode is supported. See section 3.6.3 for pinout details.

3.5.3

SPI / TWI Selector Switch

The SPI – TWI selector switch (K1) allows either the SPI pins (MOSI and SCLK) or the TWI pins

(SDA and SCL) to be routed to the 10 pin Interface header, J1. See section 3.6.3 for pinout details.

Figure 7.

TWI – SPI Selector Switch