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Altera_Forum's avatar
Altera_Forum
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13 years ago

Unused CPLD Pins

I have two CPLDs on my PCB that connect to a common SPI bus. In order to keep things simple I'm trying to talk to only one of the chips at first but I'm having some trouble with the voltages which lead me to think that there is some sort of bus contention on the SPI bus. Here's a schematic:

http://i.imgur.com/FHZkb.png

The issue is that the MISO_Driver pin only goes upto 2.5V. The pin then goes to a 22 Ohm resistor and then a non-inverting logic buffer. There is a 0.9V drop on the resistor and hence the buffer only sees 1.6V or so. Which leads me to believe that there's a 40mA draw from the CPLD - this has surely damaged the chip! Fortunately, I have a few spares.

Now, the MISO_Receiver pin is unused and my Quartus is setting unused pins to "as outputs driving ground". With my limited experience with CPLDs, my best guess is that this is causing bus contention. Am I correct?

I know that MISO_SD is not causing problems because I don't have a SD Card connected to that pin at all. As there's no card, that point is definitely at Hi-Z.

Do I need to set unused pins to "Inputs with weak pull-ups" in order to solve this issue? (I would just try it but I just came home from work and won't get the chance till tomorrow).

23 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    The problem can't be clearly understood from your schematic snippet, there's e.g. no 22 ohm resistor, so it's unclear which pin you're exactly talking about.

    But input with weak pull-up is the better setting for unused pins, particular if there's a small chance that they may be ever driven by external signals.
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    Oh, I'm so sorry. Long day at work! Yes, there's no 22 Ohm resistor. The schematic is slightly out-dated. I AM using 22 Ohm resistors in place of 100. Considering that, do you think it's bus contention?

    What does "output driving ground" really mean? Does it imply that the pin is LOW?

    Secondly, when I choose "tri-stated but with weak pull up", are the pull-ups internal to the chip or am I supposed to connect them?
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    --- Quote Start ---

    Oh, I'm so sorry. Long day at work! Yes, there's no 22 Ohm resistor. The schematic is slightly out-dated. I AM using 22 Ohm resistors in place of 100. Considering that, do you think it's bus contention?

    --- Quote End ---

    Now you will understand the beauty of having a resistor in your signal path ... remove it, and toggle the source signal, it should be 3.3V logic levels. Probe the destination signal, what voltage is it? Wire things back together, using a larger resistor to protect your circuit, and continue debugging.

    Look at the package diagram for the CPLD. Is your bad signal on a pin next to a power rail? Perhaps you have a short between the destination signal and a power pin next to it.

    Keep in mind that debugging is the fun part, its where you really learn how things work.

    Cheers,

    Dave
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    Hi Dave,

    Will remove the resistor and check those. However, considering the short between that and Vcc - no such thing. I checked that multiple times.

    What thoughts do you have regarding the bus-contention I spoke of? MISO_Receiver is unused and so set to "output as driving ground" - I am guessing this means it's set to LOW. If that's the case, then surely two signals are trying to driving Pin 1 of the Buffer and this is causing problems. Perhaps this is why the pin is never gets to 3.3V because the other pin isn't Hi-Z.

    Would appreciate your thoughts on this.
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    --- Quote Start ---

    What thoughts do you have regarding the bus-contention I spoke of? MISO_Receiver is unused and so set to "output as driving ground" - I am guessing this means it's set to LOW. If that's the case, then surely two signals are trying to driving Pin 1 of the Buffer and this is causing problems. Perhaps this is why the pin is never gets to 3.3V because the other pin isn't Hi-Z.

    Would appreciate your thoughts on this.

    --- Quote End ---

    You definitely do not want to have that setting. It should be set to "As input".

    My Tcl synthesis scripts always have this setting: 

    #  Tri-state unused pins#     set_global_assignment -name RESERVE_ALL_UNUSED_PINS_WEAK_PULLUP "AS INPUT TRI-STATED"
set_global_assignment -name RESERVE_ALL_UNUSED_PINS_WEAK_PULLUP "AS INPUT TRI-STATED WITH WEAK PULL-UP"
    and I uncomment the line I want.

    If you're using a new CPLD, then the default setting should be as input. Did you change it?

    Cheers,

    Dave
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    --- Quote Start ---

    If you're using a new CPLD, then the default setting should be as input. Did you change it?

    Cheers,

    Dave

    --- Quote End ---

    I did not change it, actually. Until recently, I didn't even know how to change it. In the compilation log I always noticed that Qaurtus was warning me that all unused pins were set to as outputs driving ground. Does this mean that they are basically an output set to low?
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    --- Quote Start ---

    I did not change it, actually.

    --- Quote End ---

    Which device are you using?

    The setting is under:

    Assignments->Device, Device and Pin Options button, Unused Pins

    (which can be a tab or selection menu depending on Quartus version).

    --- Quote Start ---

    Does this mean that they are basically an output set to low?

    --- Quote End ---

    Yep.

    Its a stupid default, which has been changed to a sane value for newer devices.

    This is why I always define a top-level entity with every signal pin defined. Then the unused pins really are unused and the unused pin setting is of no consequence.

    Cheers,

    Dave
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    --- Quote Start ---

    Which device are you using?

    The setting is under:

    Assignments->Device, Device and Pin Options button, Unused Pins

    (which can be a tab or selection menu depending on Quartus version).

    --- Quote End ---

    Yep. Just changed it and the warning disappeared. The devices are Max V 5M240ZT100.

    --- Quote Start ---

    Yep.

    Its a stupid default, which has been changed to a sane value for newer devices.

    This is why I always define a top-level entity with every signal pin defined. Then the unused pins really are unused and the unused pin setting is of no consequence.

    Cheers,

    Dave

    --- Quote End ---

    I don't know why it was set to the older default then. I'm 100% sure I did not change this setting and always remember seeing the warning in the compilation log.
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    --- Quote Start ---

    I don't know why it was set to the older default then. I'm 100% sure I did not change this setting and always remember seeing the warning in the compilation log.

    --- Quote End ---

    Stupidity is hard to eradicate from software :)

    What version of Quartus are you using? It must be fairly new to support this device. Create a new project, select that device, and then look at the setting. It sounds like the default will be the stupid value. Repeat for a Cyclone IV device and you'll see that the default is the sane value.

    Cheers,

    Dave
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    Indeed! If I change the device to a Cyclone the default is the same one. If I change it to Max V, it's set to "as outputs driving ground".