Can an oscillator clock two or more ICs? - Page 1
Forum section: electronics / beginners

author: sdim
author_rank: Contributor
timestamp: January 11, 2014, 10:58:30 pm
content: I want to use a single 50MHz or 25MHz oscillator to clock the MCU and the ethernet PHY.I know how to calculate the trace width and the load capacitors for one device but I do not know if it works with two devices.Have you tried something similar?Does it work?Is there anything I should have in mind when I design circuit with one oscillator clocking two or more devices?

author: gxti
author_rank: Frequent Contributor
timestamp: January 11, 2014, 11:14:51 pm
content: If the path is short it will most likely work but oscillators typically have very weak outputs. Check if your PHY has a clock output of its own, maybe you can do OSC -> PHY -> MCU.

author: c4757p
author_rank: Super Contributor
timestamp: January 11, 2014, 11:28:48 pm
content: Quote from: sdim on January 11, 2014, 10:58:30 pmQuote from: gxti on January 11, 2014, 11:14:51 pmQuote from: sdim on January 11, 2014, 10:58:30 pmCareful... looks like it's just a crystal, not an oscillator. Crystals regulate oscillators. If you try to share a crystal between two oscillators, you're just shorting together their outputs.

author: sdim
author_rank: Contributor
timestamp: January 11, 2014, 11:49:34 pm
content: @gxtiThe ethernet phy will be optional, it will not be soldered on every board, so I cannot do OSC->Phy->MCU.It's a great recommendation, I'll keep it in mind for other projects.@c4757pYou are right.I'm always confusing the terminology. Crystal, crystal oscillator, crystal resonator, quartz crystal...

author: Rerouter
author_rank: Super Contributor
timestamp: January 11, 2014, 11:59:19 pm
content: use a oscillator instead and buffer the signal with a clock distribution chip, with each device receiving its own buffered signal

author: gxti
author_rank: Frequent Contributor
timestamp: January 12, 2014, 12:22:19 am
content: Check the microcontroller as well. For example, STM32 has a 'MCO' function that can output many different frequencies, and the higher pin count ones even have Ethernet-specific clock outputs if I'm remembering correctly.

author: miceuz
author_rank: Frequent Contributor
timestamp: January 12, 2014, 12:30:50 am
content: AVR microcontrollers can output their clock too.BTW, are you sure PTY accepts a clock signal, not just uses a crystal?

author: hans
author_rank: Super Contributor
timestamp: January 12, 2014, 09:36:27 am
content: Quote from: gxti on January 12, 2014, 12:22:19 amI've used the STM32F407 in 144QFP package. It has 2 clock outputs, with that I was able to do:- 168MHz CPU clock- USB (internal 12MHz)- External MII Ethernet Phy (25MHz)- External A/D clock (delta sigma, 8-16MHz)From a single crystal. It's very handy.Be sure to look for clock inputs, instead of only saying OSC1/OSC2. In my case the ADC/Phy had OSC1/XI, and the phy had a dedicated 25MHz_out pin.I don't think sharing a Quartz crystal between 2 devices will work. Not only do you have the logistical problem on a PCB (the traces are all over the place), I believe devices does some crystal driving as well, which may mess things up if you connect 2 together.

author: SeanB
author_rank: Super Contributor
timestamp: January 12, 2014, 12:38:41 pm
content: If you want to deliver it from a crystal add a single SO mount gate ( pretty much any single gate will do in this, inverting, buffer, and, nand, or, nor, Xor or whatever as you only use a single CMOS or HCTTL high impedance input) to the oscillator out pin of the MCU to buffer the signal to drive to the other hip. Local decoupling capacitor, a controlled impedance trace and termination at each end feeding the clock in will do. Depending on the receive end you might need a bias circuit and a coupling capacitor, but that should be in the datasheet or appnotes for external clock input, either for the device in question or for the family of devices in that particular process. Might be as simple as a capacitor only with a self biasing clock oscillator.

author: sdim
author_rank: Contributor
timestamp: January 12, 2014, 01:20:17 pm
content: I use STM32F4, LQFP144.I know that there are two clock-out pins but I prefer to use them for other signals.clock-out-1 is available at PA8 pin. I use it for USB-FS-SOFclock-out-2 is available at PC9 pin. I use it for SDIO_D1.I need the USB SOF pin for some timing functionality.I can enable 1-bit SDIO transfer but I do not know yet if the transfer rate will be acceptable.I prefer to use 8-bit sdio transfer and find another way to clock the PHY.I found the attached schematic on the STM32F4 datasheet and I'm trying to find some design information. The schematic is very simple, it's too good to be true. So I'm sure that I'm missing something.

author: AlfBaz
author_rank: Super Contributor
timestamp: January 12, 2014, 01:48:12 pm
content: Quote from: sdim on January 12, 2014, 01:20:17 pmI imaging that oscillator is canned. You supply power and ground and it puts out a buffered signal (maybe) You would have to check it's fan out to see if it's capable of driving the 2 clock inputs.Does your phy need to be on all the time? if so many phy's can be hooked up to a crystal (check the Ethernet specs, it stipulates min ppm frequency wise) and have a clock out that can be used to drive your MCU

author: sdim
author_rank: Contributor
timestamp: January 12, 2014, 05:02:52 pm
content: Yes, the ethernet will be on 24/7 or the IC will not be soldered at all to save cost.I'm trying to get the most out of stm32f4.At the moment the project is expected to have almost everything, USB device port, USB host port, Ethernet port, RS232, OLED, SD, GPIO, wifi, bluetooth, zigbee, GPIO, etc.Maybe later I'll write a driver for a USB 3G data modem connected to the host port but at the moment I think I already have too much work to do.At the moment I guess I'll go for two crystals, on for the MCU and one for the PHY.