strange effect with tristate output

I'm using a XC95144XL, two pins are an input for a 5V signal, which have pullup resistors (3k3). Setting the pins to low from my VHDL design works great. Then I thought I could set it to high, too, to avoid the latency of the pullup resistor. But as soon as I switch it back to input (pin

Reply to
Frank Buss
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My guess is that you are setting the output to low in the same cycle that you disable the output driver. That leaves a race between turning off before, during, or after the output has switched to low. It looks like it was during, the output has switched halfway to low.

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Reply to
Hal Murray

9500XL is a 3.3 V logic family that is safe for 5 V signal levels, if series resistance is provided, but it will not drive 5V to an output. So, don't expect anything to work right with 5V pull-ups to an output.

The 5 V tolerance is there only for connection of INPUTS to other circuitry. Anyway, it looks like the pull-up and pull-down transistors don't get cleanly off the pad when you tell it to go to Hi-Z.

Jon

Reply to
Jon Elson

Where did you read that it needs a series resistor? The famlily data sheet says on page 12:

| The I/Os on each XC9500XL device are fully 5V tolerant | even though the core power supply is 3.3 volts. This allows | 5V CMOS signals to connect directly to the XC9500XL | inputs without damage.

And I think it is safe to drive it as output with the pullup resistor. The 3k3 pullup allows a current of max 1.5 mA for the 5V, which should not cause any over voltage at the pin in output mode.

I changed the output slew rate from slow to fast and increased the main clock frequency in the UCF file (someday I'll learn the timing analyzer and how to setup proper timing constraints), now it looks like this:

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Not perfect, but should work for my project.

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Frank Buss, http://www.frank-buss.de 
C64 MIDI interface, preorder: http://www.frank-buss.de/c64/midi/buy.html
Reply to
Frank Buss

The effect of changing the slew rate seems to confirm the suspicion that the output switches low at the same time the tristate happens. Can you look at the fitter output to see if this is the case (look at the generated equations)?

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Gabor
Reply to
Gabor

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