AT91EB55 datasheet(13/37 Pages) ATMEL

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Circuit Description

AT91EB55 Evaluation Board User Guide

4-3

Power can be applied via the 2.1 mm connector to the regulator in either polarity

because of the diode rectifying circuit. Another regulator allows the user to power the

AT91M55800A core with 3.3V or 2V by the mean of the JP8 jumper.

A 3V battery is provided on-board (Figure 8 in "Appendix B – Schematics") to power the

RTC and APMC (V

DDBU). It has been provided to ensure the power supply for approxi-

mately 1 year.

4.6

Push Buttons,

LEDs, Reset and

Serial Interface

The IRQ0, TIOA0, PB17 and PB19 switches are debounced and buffered.

A supervisory circuit has been included in the design to detect and, consequently, reset

the board when the 3.3V supply voltage drops below a typical 3.0V threshold. Note that

the threshold can change, depending on the board production series. The supervisory

circuit also provides a debounced reset signal. This device can also generate the reset

signal in case of watchdog timeout as the pin NWDOVF of the AT91M55800A is con-

nected on its input MR.

The assertion of this reset signal will light the red RESET LED D10 and if the CLEAR

RESET push button is pressed the LED D10 will unlight.

Another supervisory circuit separately initializes the microcontroller embedded

JTAG/ICE interface when the 3.3V supply voltage drops below a typical 3.0V threshold.

Note that this voltage can change depending on the board production series. The sepa-

rated reset lines allow the user to reset the board without resetting the JTAG/ICE

interface while debugging. bill

An RC device has been fitted on-board to ensure a correct power-on reset for the bat-

tery power supply modules (VDDBU) first power up or when VDDBU has been

disconnected. This RC network has been calculated to generate a valid 300 ms mini-

mum pulse width NRSTBU signal.

The schematic Figure 5 in "Appendix B - Schematics" also shows eight general-purpose

LEDs connected to Port B PIO pins PB8 to PB15.

Two 9-way D-type connectors P3/4 are provided for serial port connection.

Serial Port A (P3) is used primarily for host PC communication and is a DB9 female con-

nector. TXD and RXD are swapped so that a straight through cable can be used. CTS

and RTS are connected together as are DCD, DSR and DTR.

Serial Port B (P4) is a DB9 male connector with TXD and RXD obeying the standard

RS-232 pin-out. Apart from TXD, RXD and Ground, the other pins are not connected.

A MAX3223 device U10 and associated bulk storage capacitors provide RS-232 level

conversion.

4.7

Layout Drawing

The layout diagram schematic shows an approximate floorplan for the board. This has

been designed to give the lowest board area, while still providing access to all test

points, jumpers and switches on the board. See Figure 1 in “Appendix B – Schematics.”

The board is provided with four mounting holes, one at each corner, into which feet are

attached. The board has two signal layers and two power planes.

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