Sharing an oscillator between devices, semi/mems or crystal? - Page 1
Forum section: electronics / beginners

author: jnz
author_rank: Frequent Contributor
timestamp: October 27, 2020, 11:47:17 pm
content: I'm pretty well aware of sharing a crystal between devices, sometimes works, sometimes doesn't, keep close, might need to adjust the caps. Typically microcontrollers have better crystal drivers than small devices and that's the XTAL1/2 hookup then tap off XTAL1 and get it to the other device. Etc.1. Is this something that should be avoided or is more common than I think it is?2. Is this an application where a semiconductor clock generator makes more sense? (something that is designed to be a single wire clock signal)3. If one device has a clock out, this seems like a better option, but can someone tell me electrically why?My application might be medium volume, so I'm trying to stay under $1 for 20Mhz to two devices, with caps and placement.Thoughts?

author: tggzzz
author_rank: Super Contributor
timestamp: October 28, 2020, 12:06:23 am
content: The question indicates fundamental misunderstandings, is too general, and a dencet answer would take too long.The search terms you are looking for are "signal integrity" and "transmission line".The semiconductor manufacturers have published many application notes on those topics. Alternatively there is Howard Johnsons book, or Bogotins ruls s of thumb.

author: jnz
author_rank: Frequent Contributor
timestamp: October 28, 2020, 05:12:01 pm
content: The next person that may have the same question and get the same answer...At least as it pertains to MEMS... It's complicated combination of series termination and capacitance of the receiving devices. Between the obvious work you'll need to do making the lines match size and length and the experimentation in your circuit, it's define doable, but not great.https://www.sitime.com/sites/default/files/gated/AN10002-Single-ended-clock-termination.pdfIn this case, despite me having two identical devices that are close together, I'll just use the clockout function with the limitation that my device hooked to the real crystal can never be turned off if I need my downstream device on, and that I must init the primary device first, etc.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 29, 2020, 01:43:04 am
content: Pierce Oscillators have an input pin and an output pin, so if you want to attach a wire, then at least connect it to the output pin. But even then wiring capacitance may pull a crystal out of spec, so keep this wiring as short as possible, and use a buffer in between (for example som TTL chip)There are loads of microcontrollers though in which the buffered crystal clock can be output to another pin, and it is also common that the crystal oscillator pins can be configured to be driven by an external clock signal. Fo figure out of this works for you, you have to look into the datasheets of the parts you want to use.

author: jnz
author_rank: Frequent Contributor
timestamp: October 29, 2020, 05:56:59 pm
content: Quote from: Doctorandus_P on October 29, 2020, 01:43:04 amYea, I considered this. If I had three devices, a fan out ic makes a lot of sense. In this case, I'll just deal with using the CLKOUT signal from one to input of another, knowing that limits me to requiring the CLKOUT device to be on and in a higher power mode anytime I need that second device.