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I have a LVDS clock signal that is gated about ever 30us. This is a MIPI D-PHY clock that switches from HS mode to LP mode when the data lanes go to LP mode (and are auto-clocked). The problem is that my receiver circuit does not support this and expects the clock to always be running so I wish to build some sort of buffer circuit that "continues" the clock in those periods where it is turned off.

Here is what the input signal looks like: Input clock Zoomed in

In the time when the input clock is on, I wish to have low propagation delay (ideally passing the input clock right through). However, when the input clock is off, I want the output clock to continue at the "last seen frequency"--skew does not matter much. Once the input clock comes back up, I want it to match the phase again. From the D-PHY standards, I think I have 85ns + 6 clock periods to do this.

A potential solution I thought of is to use a 2:n clock buffer with a select pin along with a PLL. The output of the buffer will be the PLL (and my application) and the inputs of the buffer will be the source clock and the PLL output. Then I can set the select pin to my DCLK+ so it will automatically select the PLL when it goes high. Will this work or is there some obvious problems?

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  • \$\begingroup\$ I read this ti.com/lit/an/snaa208/snaa208.pdf and it seems like clock jitter cleanup with two clocks and automatic switching seems to do what I want. Second clock will just be the output. However this solution seems pretty expensive--the cheapest IC that does this seems to be >$6 a piece. \$\endgroup\$ – Yifan Dec 1 '17 at 4:03
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    \$\begingroup\$ Wow, that sounds great if it'll do what you need. As a one off, $6 is a great deal. If you're actually trying to sell a device in large volume from this, then you'd be stuck building your own solution. Definitely weigh engineering costs into your price comparisons though. \$\endgroup\$ – horta Dec 1 '17 at 5:04
  • \$\begingroup\$ Yeah it's actually more like $10 if you only buy one lol. Yeah it's a one-off thing, but the price just makes me wonder if it's an overkill for what I need. \$\endgroup\$ – Yifan Dec 1 '17 at 5:16
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    \$\begingroup\$ If it will do everything you need it to, understand that $10 is extremely cheap. You're asking for an intelligent PLL circuit. Creating that in discretes alone would likely cost much more than that, take up much more space on a pcb, and probably take a lot of engineering hours. Another solution might be to use a fast uController to spoof the clock if its clock is faster than the clock you're spoofing. Even here, although much cheaper and simpler, you're still looking at a decent amount of engineering effort (more from the software side). \$\endgroup\$ – horta Dec 1 '17 at 14:53
  • \$\begingroup\$ I've been reading more on the subject and it seems like I can use a normal PLL with a very low loop bandwidth which will respond slowly to the sudden frequency change (300MHz => 0Hz) and "ignore" it. \$\endgroup\$ – Yifan Dec 2 '17 at 0:25
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Simplest one I would think would be to create a ring oscillator that has an R/C time constant where the R value is actually created by a transistor that is being driven with local voltage. Basically set it up so that you're phase locking onto the remote oscillator while it's up and running and when the remote clock goes down, you keep your local oscillator going without changing the last known voltage that drives the transistor/resistance. As "simple" as this is, you're still creating/using a PLL.

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    \$\begingroup\$ So I don't want to use a FPGA--do I have to put dozens of NOT gates on my board? How does the circuit know "when the remote clock goes down"? \$\endgroup\$ – Yifan Dec 1 '17 at 1:11
  • \$\begingroup\$ @Yifan Dumb question, but is there not a way to prevent the remote side from going into LP mode? That would really be the simplest way. To answer your latest q: it could know that the remote clock is down a few different ways. 1) If it ever receives a 0, assume the clock is down and freeze the voltage controlled resistor (transistor). 2) You could create a averaging circuit and using a comparator trip when the average voltage dropped below a certain threshold. \$\endgroup\$ – horta Dec 1 '17 at 3:26
  • \$\begingroup\$ @Yifan And while you don't have to use an FPGA, you will be creating a significant circuit which could either be done with a lot of discretes or probably easier and cheaper with a uController. \$\endgroup\$ – horta Dec 1 '17 at 3:29
  • \$\begingroup\$ Yeah, I don't have control over the host system unfortunally--it's going to be an adapter that connects to an existing system. \$\endgroup\$ – Yifan Dec 1 '17 at 4:06

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