ATmega328P Crystal Resonator Design

Other Hardware 3rd Party Boards
1

I’m working on a custom PCB for a project of mine. I’m trying to integrate an ATmega328P-MU with a MLF32 package size. I'd like to first point out that I'm having this made using the EasyEDA website and the PCB assembly service they now offer. They have "basic" components that are much cheaper to use than other "extended" components which come with a labor fee to be placed on the board. This is influencing my design/component selection and is why I'm using the crystal I am in this design.

Having said that I'm having trouble figuring out the proper design for the crystal resonator. The crystal resonator is a Yangxing Tech X322516MLB4SI 16MHz ±10ppm 9pF 60Ω SMD, here is the datasheet.

As far as I can tell this doesn't have capacitors built in. According to the datasheet the Load Capacitance is 12pF or 20pF. So I've calculated the capacitance of each capacitor as 30pF using the formula 20(CL)=(C+10pF)/2 solving for C, is this value correct? My understanding is that the 10pF is a rule of thumb for parasitic capacitances of the IC pins, 5pF for each pin.

Below is a screenshot of my current schematic layout.

My question is rather or not this circuit will work, this is my first time designing something like this and I'm unsure whether there are other components or design considerations I may be overlooking?

I would also like to point out that in order to load the bootloader and change the fuses on this chip I'm using an ISP programmer. In the final PCB design I plan to have a 6 pin female header to allow me to quickly program the fuses and upload the code. The pins are the RST, GND, MOSI, MISO, VCC, and SCK. With that said are there any design considerations I should be aware of using this method? I've used an ISP programmer to program and upload sketches on a Pro Trinket but I've yet to do it on a standalone chip. I also don't plan on using the RESET pin for any other purpose than the ISP programming as I won't be changing the code once its uploaded. If the reset pin simply connects to the female header and nothing else will that cause any problems with the chip functioning?

I apologize for my ignorance, I've spent a lot of time reading/searching this issue and at this time I'm unsure of this design and whether it will work or not, any help with these questions would be greatly appreciated.

2

"Long" wire connected to the reset pin may cause noise and random resets. To make sure it won't happen add a pull-up resistor or a capacitor to the reset pin.

3

Smajdalf:
"Long" wire connected to the reset pin may cause noise and random resets. To make sure it won't happen add a pull-up resistor or a capacitor to the reset pin.

Would a 1K resistor be sufficient?

4

I don't know. Too weak pull-up won't help but too strong may cause problems for the programmer.
I often don't use the reset pull-up but in one project required to run continuously I noticed mysterious resets about once a month. I added a jumper into the programming connector and it is fine.

5

Smajdalf:
I don't know. Too weak pull-up won't help but too strong may cause problems for the programmer.
I often don't use the reset pull-up but in one project required to run continuously I noticed mysterious resets about once a month. I added a jumper into the programming connector and it is fine.

It seems that the general rule of thumb is 10k from what I've read. Other than the pull-up resistor, does the layout of the crystal resonator look correct to you?

6

I don't know. In general it is said the crystal should be as close to the ATMega as possible and no other signal should go under the crystal or traces to it. So far I have only used watch crystals for my own projects and load capacitors for it are included inside the ATMega. If you plan to do only a few devices specify some reasonably physically large load capacitors (0603 or 0805) to be able to replace them in the worst case. But I think slightly wrong capacitor capacity will "only" shift the frequency of the crystal slightly.
BTW you did not show layout but only a schematic.