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I require recommendations for the layout of crystal.

I am designing a board which uses an external RTC. The Mfr. part number for RTC is PCF8523TK/1,118 and Mouser part number is 771-PCF8523TK/1118. It uses a crystal (32.768kHz, Load cap 7pF, temp range -40C +85C) with Mfr. part number ECS-.327-7-12R-TR and Mouser part number 520-.327-7-12R-T. Schematic and PCB layout are attached below.

Please share your feedback.

Schematic enter image description here

PCB layout enter image description here

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  • \$\begingroup\$ At 32 kHz, the layout isn't that critical. Your traces (wires) aren't longer than needed so my guess is that this is fine, no need to change anything. \$\endgroup\$
    – Bimpelrekkie
    Commented Jul 2, 2020 at 10:14
  • \$\begingroup\$ @Bimpelrekkie I have to disagree a bit. With 32 kHz crystal circuits, everything is extremely critical. These are far mode demanding to get working than normal crystal circuits, which usually have no issues. \$\endgroup\$
    – Justme
    Commented Jul 2, 2020 at 10:29
  • \$\begingroup\$ The PFC8523 datasheet shows it as having two 7 pF load capacitors incorporated (block diagram, p.3), and the application diagram on p.55 shows it as not using external capacitors. Is there a reason to add the external capacitors? \$\endgroup\$
    – Andrew Morton
    Commented Jul 2, 2020 at 10:34
  • \$\begingroup\$ @Justme These are far mode demanding to get working than normal crystal circuits OK, then please elaborate on that as in my experience this usually "just works". Maybe I was lucky? With crystal oscillators of 20 MHz and higher, then I agree, these are more tricky. \$\endgroup\$
    – Bimpelrekkie
    Commented Jul 2, 2020 at 12:22
  • \$\begingroup\$ @Bimpelrekkie 32kHz oscillator circuits have so high impedance, that even dust and moisture affects them. Poking the crystal with oscilloscope will stop it, so they can't be debugged with a scope. Or if it was not oscillating to begin with, it may start to oscillate. And easily oscillates with overtones. More than often, application notes for using MCUs with 32 kHz crystals are far more longer than application notes for using MCUs with standard crystals. NXP AN2606 is a good read for more info. So, consider yourself lucky (experienced?) if crystal and MCU parameters have been properly matched! \$\endgroup\$
    – Justme
    Commented Jul 2, 2020 at 12:59

2 Answers 2

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According to the datasheet the PCF8523 has integrated oscillator load capacitors, so all you need to do is add the crystal close to the OSC pins and select the capacitance you need (7 pF or 12.5 pF). It is defined by the 4th bit of the CONTROL_1 register (00h).

Datasheet

Edit: In general your crystal layout looks fine. The main thing to keep in mind is that the traces need to be kept as short as possible and the capacitors need to be of appropriate value for the trace length.

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Do not route logic_level traces near or under the XTAL/leads.

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