Multiple synchronized divide-by-n clocks - Page 1
Forum section: electronics / projects

author: zappedy
author_rank: Regular Contributor
timestamp: July 27, 2016, 07:17:51 am
content: What is a good circuit, or cheap IC, for generating div-by-10, div-by-12, and div-by-15, exactly synchronized so there are no unit delay differences? Starting with a 240MHz clock, and wanting 24, 20 and 16 MHz. All duty cycles must be 50% (45-55 is probably ok).

author: RES
author_rank: Regular Contributor
timestamp: July 27, 2016, 09:42:44 am
content: Think you need to prescale the input frequency down first, and than divide further with e.g a 74HC4040, because a 74VHC4040 max. frequency is 210MHz. A 74HC90 can divide by odd numbers to get to divide by 15.

author: zappedy
author_rank: Regular Contributor
timestamp: July 27, 2016, 02:17:39 pm
content: Thanks!What about the synchronization? (Is it called edge synchronization?)

author: Dave
author_rank: Super Contributor
timestamp: July 27, 2016, 02:31:30 pm
content: How exactly are you supposed to synchronize the edges if the clocks all have different frequencies?

author: zappedy
author_rank: Regular Contributor
timestamp: July 27, 2016, 02:42:09 pm
content: Quote from: Dave on July 27, 2016, 02:31:30 pmThe frequencies are "related", 16, 20 and 24 MHz divided from the same master clock. So every 10th clock on the 24Mhz, all three would "line up", unless there is differing unit delays involved.The purpose of these clocks is to synchronize the logic analyzer with the device under inspection.So I'm asking for a circuit which takes delays into account and produces several clocks that will line up.

author: zappedy
author_rank: Regular Contributor
timestamp: July 27, 2016, 02:58:19 pm
content: By the way, I'm only looking for a way get "synchronized" 16, 20 and 24MHz clocks with 50% duty cycles. If there is an easier or cheaper way than using a 240MHz master oscillator and dividers, I'm all ears!I'm hoping to use a spare Raspberry Pi to generate the master clock, and capture the output of one device over USB at the same time. It does not seem to be possible to generate two or three different high frequency divide-by-n clocks using only settings on the RPi, because I think you are limited to grabbing the output from a PLL clock where you can set an integer and fractional divide, but only as one unit. Please correct me if I misunderstood this part.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 27, 2016, 03:26:55 pm
content: I would use a 4MHz clock and three PLLs to multiply up to 16, 20 and 24.

author: zappedy
author_rank: Regular Contributor
timestamp: July 27, 2016, 03:46:21 pm
content: Quote from: Andy Watson on July 27, 2016, 03:26:55 pmBut the PLLs are analog, how will the edges line up?

author: Kilrah
author_rank: Unknown
timestamp: July 27, 2016, 03:57:32 pm
content: PLL stands for "Phase Locked Loop" - keeping phase in the desired relation is its very job.

author: edavid
author_rank: Super Contributor
timestamp: July 27, 2016, 05:49:20 pm
content: Here's a simple clock generator board that you can buy:http://www.ebay.com/itm/Adafruit-Si5351A-Clock-Generator-Breakout-Board-8KHz-to-160MHz-Arduino-3-3v-5v-/171909660637

author: zappedy
author_rank: Regular Contributor
timestamp: July 27, 2016, 06:13:51 pm
content: Quote from: Kilrah on July 27, 2016, 03:57:32 pmDoh!

author: C
author_rank: Super Contributor
timestamp: July 27, 2016, 06:19:15 pm
content: Quote from: zappedy on July 27, 2016, 07:17:51 amTo get this use the output of a FF and 2x input.Ratio will be change output high specification vs change output low specification of FF.Quote from: zappedy on July 27, 2016, 07:17:51 amSo 48,40,32 as clocks.Quote from: zappedy on July 27, 2016, 07:17:51 amTo get synchronized, you need to know when to sync or reset all counters at same time.In a digital system you are limited to add/remove 1/2 cycle of master clock as correction.Quote from: zappedy on July 27, 2016, 02:42:09 pmYou still need a phase lock to device under inspection.You will need to phase lock to device clock or replace device clock.

author: zappedy
author_rank: Regular Contributor
timestamp: July 27, 2016, 06:20:09 pm
content: Quote from: edavid on July 27, 2016, 05:49:20 pmLooks perfect! However, shipping to my country is $30, and then customs will be at least another $15, kind of steep for an $8 board...I am still interested in a discrete design, but also anything you can do with the Raspberry Pi. Come to think of it, some of the Microchip PICs have multiple programmable PLLs. Got to go check my good-to-have drawer.

author: edavid
author_rank: Super Contributor
timestamp: July 27, 2016, 07:04:59 pm
content: So find a similar board available in your own mysterious country