What state has pin 11 on a 74HC595 with no vcc ?

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Hi guy's
I have made a design with an atmega and some 74HC595, and I've made it so I can remove the vcc for 74HC595 chips if a specific pin on the atmega is set low.

My problem right now is that my programmer says there is a short circuit on the SCK-line...

So I'm thinking if this is because all the 74HC595's are without power and that this are making them pull the SCK-line down ? (connected to pin 11 on the 74HC595 chip)

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uC's: Atmega16, 32, 64, 128 and Attiny13
Lang.: C
Interests: Small scale robots AND sensor monitoring system

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The pin state of most 74HC logic is undefined in the absence of power.

Most reliable solution is to add an isolating resistor between the 595 and the SCK line. Say 4.7K or so.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Okay thanks :)
I just didn't think of that the input pin could pull in any direction, no matter if the IC is powered or not...

uC's: Atmega16, 32, 64, 128 and Attiny13
Lang.: C
Interests: Small scale robots AND sensor monitoring system

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If the input has protection diodes inside the
chip (most will have !), the diode tied
to the Vcc-pin will conduct for an
input pin that is HIGH and the current will
go to the Vcc-pin.

It's even possible to "power" chips via an input-line.

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Okay thanks for the info guys :)
it's working now ! :D

uC's: Atmega16, 32, 64, 128 and Attiny13
Lang.: C
Interests: Small scale robots AND sensor monitoring system

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I killed the ISP circuit on a STK-500 , by not removing the STK-500 6-pin wire. Before playing with some HC595 attached 7-Segs.

The STK-500 works on everything but ISP.

So now thats the one i use for the STK-501 where i use JTAG programming

Just a little warning ... , to remember to use a resistor on isp lines , or at least disconnect the ISP plug.

/Bingo

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ka7ehk wrote:
Most reliable solution is[...] resistor between the 595 and the SCK line. Say 4.7K or so.
Jim

Isn't 4.7k a little high ? would this not kill the signal ?
And should I also add a resistor to the MOSI-line (data-out) ???

uC's: Atmega16, 32, 64, 128 and Attiny13
Lang.: C
Interests: Small scale robots AND sensor monitoring system

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There should be a resistor with MOSI to. The 4,7 K is quite high, but CMOS inputs are high impedance so a high resistor only slows things down. With about 10 pF at the input the 4,7 K is only about 50 ns of delay. This is fast enough for the typical SPI speed. A smaller resistor down to about 1 K is possible.

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Quote:

A smaller resistor down to about 1 K is possible.

That's what I use. :D

If you think education is expensive, try ignorance.