ATmega128 datasheet(12/61 Pages) ATMEL

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Hardware Description

3-2

AVR STK500 User Guide

1925C–AVR–3/03

Figure 3-2. Implementation of LEDs and LED Headers

Note:

The AVR can source or sink enough current to drive a LED directly. In the

STK500 design, a transistor with two resistors is used to give the same amount

of light from the LED, whatever the target voltage (VTG) may be and to turn off

the LEDs when VTG is missing.

3.2

Description of

User Switches

The switches connected to the debug headers are implemented as shown in Figure 3-3.

Pushing a switch causes the corresponding SWx to be pulled low, while releasing it will

result in VTG on the appropriate switch header connector. Valid target voltage range is

1.8V < VTG < 6.0V.

Figure 3-3. Implementation of Switches and Switch Headers

Note:

In the AVR, the user can enable internal pull-ups on the input pins, removing the

need for an external pull-up on the push-button. In the STK500 design, we have

added an external 10K pull-up to give all users a logical “1” on SWn when the

push-button is not pressed. The 150R resistor limits the current going into the

AVR.

LED1

LED3

LED5

LED7

VTG

LED0

LED2

LED4

LED6

GND

1 2

LEDn

10K

VTG

+5V

150R

10K

150R

VTG

SWn

SW1

SW3

SW5

SW7

VTG

SW0

SW2

SW4

SW6

GND

1 2

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