I want to use a counter MC100EP016A which requires PECL clock input.

What is the difference between LVPECL and PECL?

If a clock source provides LVPECL clock how can I use it with the MC100EP016A IC?


2 Answers 2


I suspect this question is related to your previous one.

The part you are using, the Si530 programmable crystal oscillator, is avaialble in several variants. Depending on the part number you order, you could have an output with 3.3 V LVPECL outputs, 2.5 V LVPECL outputs, or another standard.

ECL logic was initially specified for systems with -5 V supplies. Later, it was remembered that voltages are all relative, and these parts could be used nearly as well in systems with +5 V supplies, and these systems were called PECL systems.

When ECL parts operating on 3.3 V were developed, these were called LVECL or LVPECL parts. Still later, parts were developed to operate from 2.5 V supplies, and these are also generically called LVPECL parts, making the term LVPECL somewhat ambiguous. Furthermore, just plain "ECL" and "PECL" continue to be used as a catch-all term for all parts (5 V, 3.3 V, and 2.5 V) that have a similar differential open-emitter output structure, so saying something is "PECL" doesn't mean it isn't LVPECL.

In my comment, I referred to "3.3 V PECL" to specify this particular voltage level, instead of just "LVPECL" which would not have specified whether the 3.3-V or 2.5-V variant was meant. 3.3 V PECL and 3.3 V LVPECL are two words for the same thing.

Since your oscillator has 3.3-V (LV)PECL output, and the chip you want to use it with (MC100EP016A) has 3.3-V (LV)PECL inputs, you do not need any translator for your situation.

If you did in fact want to connect a 3.3-V (LV)PECL clock signal to a 5-V PECL receiver, you could also do that using AC coupling, and still not require any translator chip. On Semi App Note AN8040 gives a run-down of several ECL termination techniques. You want to connect the receiver as shown in section 6, but remember that the driver also needs some pull-down resistance (~100-200 ohms) which isn't shown in the diagrams in the app note.

  • 1
    \$\begingroup\$ I agree with your answer that you don't need a translator chip but I'd like to chime in on your history. It wasn't "remembered" that voltage potentials are relative. That has been known for 100s of years. Rather decades ago it wasn't so easy and cost effective to place planes in PC boards. Because GND was usually the only plane on a board back then it was used as the "collector" power source for lower inductance and faster speed properties. Now that creating a +5V plane is trivial with current hardware and software PECL became normal as it is more intuitive. \$\endgroup\$
    – ACD
    Aug 11, 2014 at 17:39
  • \$\begingroup\$ @ACD, Thanks for the summary. I couldn't think of a way to summarize that concisely so I put in a bit of a joke instead. \$\endgroup\$
    – The Photon
    Aug 11, 2014 at 18:19

LVPECL is a 3.3V variant of PECL which operates at 5V. The two logic systems have different thresholds for high and low signals, operating voltages etc. They are separate logic families and are such are in compatible for direct connection. You need to use a LVPECL to PECL logic/level translator, Micrel, Onsemi and TI all offer versions for your use.

However, as pointed out by ThePhoton, that device already supports LVPECL and you have a LVPECL clock.

What I think your issue is , is two fold. First off the data sheet uses PECL through out and does not distinguish between LVPECL and PECL, the reason for that is also point 2. In that this device is capable of being configured to run ECL, PECL and LVPECL with different power supply configurations. It can accept a Vcc of 6 V and a Vee of -6 V. So for PECL you'd run the device with Vcc 5V and Vee 0V, for LV PECL you'd run the device with Vcc 3.3 V and Vee 0 V and if you wanted to go to ECL you would run Vee -5.2 V.

  • \$\begingroup\$ 1. The EP parts run on 3.3 V, so they're already LVPECL parts. 2. He's asking about a clock signal so it's easy to shift logic levels without a translator chip. \$\endgroup\$
    – The Photon
    Aug 10, 2014 at 15:04
  • \$\begingroup\$ @ThePhoton so to be clear, The OP has a LVPECL part with a LVPECL Clock input which he erroneously thinks needs a PECL clock input. \$\endgroup\$ Aug 10, 2014 at 15:25
  • \$\begingroup\$ Yes. I confused him with my answer to his last question. But I haven't been online to sort it out (on my phone now). \$\endgroup\$
    – The Photon
    Aug 10, 2014 at 15:29
  • \$\begingroup\$ @ThePhoton I think I understand the issue now, see my edits. \$\endgroup\$ Aug 10, 2014 at 15:36
  • \$\begingroup\$ Only last thing, he needs to buy a different part number to get different output standards, not just hook up a different power supply voltage. \$\endgroup\$
    – The Photon
    Aug 10, 2014 at 17:40

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