MCP2515 and Crystal Error

Question

I'm making a board with MCP2515T-I/ML due to space constraints and it won't start up.

(datasheet) https://ww1.microchip.com/downloads/en/DeviceDoc/MCP2515-Stand-Alone-CAN-Controller-with-SPI-20001801J.pdf

This is the pcb layout and schematic.

The top layer copper pour is 3.3V and the second layer is a solid ground plane.

I have checked for shorts, there are no shorts. I have checked the MISO, MOSI, CLK and CS lines and they all behave appropriately excluding MISO. It seems not only does the MCP2515 not want to turn on but the 8MHz external clock is not oscillating. I checked using a tiny SA and a H field probe (which correctly detects oscillations of other crystals).

I have conducted both manual hardware reset and SPI reset on the device and attempted to communicate with it, it still doesn't communicate.

After replacing the MCP2515T-I/ML multiple times on different boards I'm beginning to wonder if the issue isn't only that the crystal never starts up and the MCP2515 remains in "oscillator startup timer reset mode".

The crystal is ABM3B-8.000MHZ-10-D-1-G-T.

(datasheet) https://4donline.ihs.com/images/VipMasterIC/IC/ABRC/ABRC-S-A0001211322/ABRC-S-A0001211322-1.pdf?hkey=6D3A4C79FDBF58556ACFDE234799DDF0

The crystal load capacitor is 10p and I've replaced the crystal as well, no dice. I don't currently have 0402 capacitors to vary the load capacitance but am wondering if the 3.3V top layer copper pour could be providing so much parasitic capacitance that the board needs to be redesigned. According to C1,2 = 2(CL-Cstray), I should have used C1,2 = 14p if Cstray was 3p. Supposing Cstray is much higher I may not have needed to use any load capacitors. In desperation I've removed the load capacitors one at a time and attempted to see if the crystal would oscillate, in case the parasitics were in excess. That didn't work either.

I don't feel comfortable sending off a redesign until I know exactly what went wrong here, since it could be multiple things. I'm aware I should have had a keep out area and guard ring in the top layer around the crystal. Would that be the only issue though?

If possible I'd like to measure the actual load capacitance but my MM doesn't seem up to the job. The capacitance value measured across one load capacitor is either hovering around 12p or 2n.

Any ideas?

Answer 1

In line with SpehroPefhany's answer, if you want to try connecting the ground pad you can do it as follows.

1. Using a small razor knife, make four cuts (as shown) to isolate the plane area under the chip from the 3.3V top side plane.

2. Scrape off the solder mask in the area shown, especially on the ground via.

3. Apply some solder paste to the newly exposed area (including the via)

4. Resolder the part. The ground pad should now be connected.

Answer 2

You're supposed to have this footprint:

But you've omitted the pad for the EP. Can't say for sure that's the cause of your issue, but it doesn't help at all.

An RC + diode is recommended on the /RESET pin. You've just got a resistor. If you do an SPI reset it may not be necessary to have the more complex circuit.

Another possibility is that you've got the wrong value for the xtal load capacitors by a huge margin (eg. 20nF rather than 20pF). Or an open on one of the pins, it's hard to inspect leadless packages. If the package was soldered by someone who doesn't do this sort of thing regularly with proper tools, my guess would be on that as the issue.

I don't think minutia with the layout or capacitance values is the problem, you've got something else.

Answer 3

Since you are primarily suspecting the oscillator, you can remove the crystal and directly feed a 50% duty cycle clock (0 - 3.3V level) directly on OSC1 pin. Leave OSC2 open with nothing connected to it. Then, put the reset pin low manually and release it. If the device starts responding, it could indicate clock issue.

Any ideas?

The +3.3V supply also needs to be probed to see if there are any dips in the supply causing chip reset.