How to check high impedance of a RAM with Logic Analyzer

You could use a logic analyzer if you need a dynamic analysis. You could try adding a resistor between one of the data signals and an address line that is not used to address or select the RAM. Then you can access an address that does not select any device with the address line both high and low. You should see the data line follow the state of the address line. If not, something is driving the bus and you have a problem. I don't see how you can easily tell which device gives you the problem however. The only way to tell if it is the RAM is to isolate the RAM from the bus and see if the problem follows the RAM.

You can't use a logic analyzer as it will only read a 1 or 0 depending on the threshold.

You can use a oscilloscope. Look for midrange values. Or if the bus has pullups or pull downs, look for exponential decays (since a passive pullup will change the signal slower than an active driver).

This assumes no bus keeper circuitry (probably not on an fpga).

Alan Nishioka snipped-for-privacy@nishioka.com

Hi Cheng, Get models of the RAM and simulate the thing in ModelSIM or somesuch. HTH, Syms.

Unless you believe you have a bad device, trust the datasheet. Look at the control signals with a scope (a logic analyser can be good but it only shows whether a signal is above or below a set threshold which is _not_ the same as a signal being a valid high or low).

If the signals in question (which vary according to type of RAM) show deselect, then you can safely assume it has deselected.

On this note, however, you should pay close attention to the datasheet; there is usually (almost always) a delay (thz, tlz etc.) from deselect becoming true and the outputs actually going hi-Z.

Cheers

PeteS

Hi,

I see it. Thanks so much, your guys.

Regards, Cheng