DOC1925 datasheet(30/61 Pages) ETC1

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Hardware Description

3-20

AVR STK500 User Guide

1925C–AVR–3/03

When connected to an external system, there is often an external pull-up resistor con-

nected to the reset line. A typical reset connection is shown in Figure 3-27.

Figure 3-27. External Reset Connection

If the external pull-up resistor is too low (<4.7 k

Ω), STK500 will not be able to pull the

RESET line low.

3.8.4

Clock Settings,

XTAL1 and OSCSEL

STK500 includes several clock options for the target AVR. Setting the jumpers XTAL1

and OSCSEL controls the clock selections. OSCSEL determines what signal to route to

the XTAL1 pin of the AVR.

When the XTAL1 jumper is connected, the STK500 internal clock system is used as

main clock to the target AVR. When XTAL1 jumper is not mounted, the internal clock

system is disconnected. This allows external clock signals or crystals to be used as tar-

get clock source for the AVR. Figure 3-28 illustrates the XTAL1 jumper option.

Figure 3-28. XTAL1 Jumper Options

When the XTAL1 jumper is not mounted, an external clock source or crystal can be con-

nected to the PORTE header. This is shown in Figure 3-30.

R (4.7 kohm)

C (10 nF)

RESET

AREF

RESET

XTAL1

OSCSEL

Jumper Mounted

On-board XTAL1 Signal Connected (Default)

AREF

RESET

XTAL1

OSCSEL

Jumper not Mounted

On-board XTAL1 Signal Disconnected

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