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Could someone advise me on what would be a good practice to interface the CoolRunnerXC2C256 with an external clock? I wish to use KC5032A20 crystal and have it to drive global clock GCK2 (pin 38).

  1. Do I connect output pin of the crystal directly to GCK2 pin, or I should have some intermediate components (pull-up resistors, buffers, etc.)?

  2. 20MHz for this CPLD is slow. Would it be a good idea to configure GCK2 pin as Schmitt-triggered?

  3. Should I use separate power supplies for each Vcc, each Vccio1, and each Vccio2 pins, or one power supply per bank is good enough?

  4. Should I use separate power supplies for the CPLD and the Crystal?

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2 Answers 2

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  1. You shouldn't need a load capacitor, but if you're nervous then leave some PCB lands to mount one just in case.
  2. Again, probably not necessary, but I can't think of a reason not to do so. Maybe test it both ways, it's just a fuse setting and you can change it later.
  3. You MUST connect EVERY SINGLE power and ground pin to the appropriate voltage. No exceptions. You don't need a separate supply, just connect all the 3.3V pins to your 3.3V rail, all the 1.8V pins to your 1.8V rail, etc. Also, you should mount decoupling capacitors in close physical proximity to every power pin; Xilinx will have documentation on the recommended power supply decoupling for your specific part and package.
  4. The CPLD and crystal can share the one supply.
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  • \$\begingroup\$ Is it a good practice to place a buffer/gate between the oscillator and the CPLD? \$\endgroup\$
    – Nazar
    Jun 18, 2014 at 15:42
  • \$\begingroup\$ For an oscillator module (as opposed to a crystal), there should be no need for any extra electronics; oscillators generally have TTL- or CMOS-compatible outputs that can be used to directly drive a clock input on a logic device like a CPLD or MCU. \$\endgroup\$
    – markt
    Jun 19, 2014 at 7:47
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First off, what you have is a crystal oscillator, not a crystal.

1.Do I connect output pin of the crystal directly to GCK2 pin, or I should have some intermediate components (pull-up resistors, buffers, etc.)?

If you use the same voltage on the oscillator as you provide for the IO bank which is associated with the GCK pin, you can tie them directly together.

2.20MHz for this CPLD is slow. Would it be a good idea to configure GCK2 pin as Schmitt-triggered?

Nope. The transition times on the oscillator are plenty fast enough.

3.Should I use separate power supplies for each Vcc, each Vccio1, and each Vccio2 pins, or one power supply per bank is good enough?

One supply per separate voltage for the entire chip is adequate. What is important is layout and decoupling.

4.Should I use separate power supplies for the CPLD and the Crystal?

Nope. Just use the appropriate IO bank voltage on the oscillator. Again, it's power distribution, layout and decoupling that count.

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