vince.cimo
Well-known member
Hi Paul and rest of awesome smart people on this forum. This thread (https://forum.pjrc.com/threads/27397-Teensy-gt-Production?p=60903&viewfull=1#post60903), which outlines some of the design considerations for crystal placement on a custom Teensy board got a bit off topic so I figured I'd start a new thread and try to get a bit more clarity on the topic.
As far as I understand, the 16mhz crystal requires the following considerations:
1. On a 2 layer PCB, there should be a ground plane directly under the crystal.
2. The crystal ground plane should NOT be connected to rest of the circuit's electrical ground.
3. The crystal ground plane should only be connected to the ground pins on the crystal (2 & 4 if using this one https://datasheet.octopart.com/NX3225SA-16.000000MHZ-B3-NDK-datasheet-118964.pdf) and capacitors, if using a crystal with over 10pf capacitance.
4. Route high-speed digital signals as far away from the crystal as possible.
5. Don't run high speed signals in parallel to the crystal.
A few questions come up:
1. What is the logic behind keeping the crystal ground separated from the rest of the circuit ground? Is my interpretation of the design consideration correct?
2. Are there any 'hard' figures regarding distance between crystal traces and other digital signal traces?
3. Any good strategies for breaking up 'parallel' traces? Will a simple 45 degree turn solve the problem?
4. I'm attaching a screenshot of my PCB layout (zoomed in on the crystal), in which I've tried to take these factors into consideration as best as I could. Does this look correct? I'm sending the circuit off to a PCBA house and would love to avoid having to pay the high manufacturing costs twice if I can detect a stupid mistake now.
As far as I understand, the 16mhz crystal requires the following considerations:
1. On a 2 layer PCB, there should be a ground plane directly under the crystal.
2. The crystal ground plane should NOT be connected to rest of the circuit's electrical ground.
3. The crystal ground plane should only be connected to the ground pins on the crystal (2 & 4 if using this one https://datasheet.octopart.com/NX3225SA-16.000000MHZ-B3-NDK-datasheet-118964.pdf) and capacitors, if using a crystal with over 10pf capacitance.
4. Route high-speed digital signals as far away from the crystal as possible.
5. Don't run high speed signals in parallel to the crystal.
A few questions come up:
1. What is the logic behind keeping the crystal ground separated from the rest of the circuit ground? Is my interpretation of the design consideration correct?
2. Are there any 'hard' figures regarding distance between crystal traces and other digital signal traces?
3. Any good strategies for breaking up 'parallel' traces? Will a simple 45 degree turn solve the problem?
4. I'm attaching a screenshot of my PCB layout (zoomed in on the crystal), in which I've tried to take these factors into consideration as best as I could. Does this look correct? I'm sending the circuit off to a PCBA house and would love to avoid having to pay the high manufacturing costs twice if I can detect a stupid mistake now.