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SN74V293: Static FIFO Output

Manuel Campana
Prodigy 60 points
Part Number: SN74V293

Hello,

I have a problem with my SN74V293 FIFO. I'm not able to readout the memory although I use the timing information for the clock and signals from the data sheet. The inputs of the FIFO are connected to the outputs of a 12-Bit ADC (D0 is LSB of ADC).

!LD is low during master reset. I don't use the programmable offset flags. 

!BE, !OE, FSEL0, FSEL1, IP, IW, OW, PFM are always low. 

!PRS, !RT, !SEN, RM are always high.

Thereby I use the FIFO with the following configuration: Normal latency, no retransmission, big-endian operation, always output enabled, noninterspersed-parity mode, 18-bit bus width for I/O.

This is my write-cycle after master reset:

The !EF is high after the first data input.

This is my read-cycle:

I would have expected to see the signal of D0 from the write-cycle at the output Q0. Additionally, I checked the behaviour of the flags !EF and !FF. They works as desired. 

Do any of you have any idea where my mistake is? Did I do something obvious wrong?

  • 0
    TI__Guru* 96842 points

    Hi Manuel and welcome to the forums!

    I'm going to look over this for a bit and get back to you.

  • 0 in reply to Emrys Maier
    TI__Guru* 96842 points

    Is it possible for you to write in a set number of words, then attempt to read them all out until EF! goes low?  I'd like to see if the device thinks that it's outputting or not.

    My first thought is that maybe there's loading on Q0 that's causing it to not register on the logic analyzer.  From the detail you've provided, I think that's a longshot, but can you get an analog scope shot of the clock and output together during the read operation? Also, can you provide details regarding the circuit connected to Q0?

  • 0 in reply to Emrys Maier
    Prodigy 60 points

    Yes, if I try to readout all words the EF! goes low again. Additionally I tried to write more than 65536 words into the memory and the FF! switched to low correctly. 

    Pin Q0 is only connected to the I/O expander PCA9535ECDTR2G, nothing else. It's still possible that I made a mistake with reading out the input register of the I/O expander, but the word I've seen with the logic analyzer and the readout word are the same. I'd love to use a scope for finding the mistake but I'm in my home office at the moment.

    The output word is either 0x7b2 or 0x762. Sometimes it toggles between these two values. 

  • 0 in reply to Manuel Campana
    TI__Guru* 96842 points

    I understand - I've been in my home office/lab for quite a while, too :)

    Is there any chance that one or two channels on your logic analyzer support an analog mode?

  • 0 in reply to Emrys Maier
    Prodigy 60 points

    My cheap usb logic analyzer doesn't support an analog mode.. (students life is hard :D ) 

    This is the last measurement I have: (blue channel is the clock signal, 1V/div)

    After that measurement I extended the clock frequency. Otherwise I wouldn't be able to measure the signal with my logic analyzer due to its input sample rate of 24Msps.

  • 0 in reply to Manuel Campana
    TI__Guru* 96842 points

    Gotcha.

    Outputs can be damaged, so it's possible that the device is working correctly except for that one pin -- unlikely though.

    Any chance you have a second unit to test with?  That would usually be my next go-to check -- swap the unit and see if the issue follows the unit.  I know that's probably not as easy as I make it sound.

  • 0 in reply to Emrys Maier
    Prodigy 60 points

    Unfortunately, I have only one FIFO available because it's pretty expensive.

    Honestly I already thought about replacing it, but first I wanted to ask you if I made an obvious mistake. I will remove the I/O expander after replacing the FIFO. Maybe I made a mistake during the initialization of the I/O expander which caused a 3.3V signal on the output pins of the FIFO. 

  • 0 in reply to Manuel Campana
    TI__Guru* 96842 points

    I understand that as well.

    I'm afraid there's only limited support I can provide for these FIFO registers.

    My only other thought would be to verify that the Q0 trace is connected where it's supposed to and not shorted to anything else on the board.

  • 0 in reply to Emrys Maier
    Prodigy 60 points

    Omg I just found the mistake:

    I removed the I/O-expander and it works! Seems that I made a mistake with the initialization of the I/O-expander. 

    Fortunately, the FIFO survived that. :D

  • 0 in reply to Manuel Campana
    TI__Guru* 96842 points

    That's great! I'm glad you were able to hunt down the issue.

    Come back any time!