Every Wednesday we highlight a component from the updated Dangerous Prototypes partlist. This week: clock sources. As always, there are footprints for all these parts and more in our public domain Cadsoft Eagle part library.
In our workshop the first rule of crystal selection is ‘use a surface mount HC-49/US package if it will fit’. The second rule of crystal selection is ‘make an HC-49/US fit’. These are the surface mount version of the typical “metal can” through-hole HC-49 crystal. In our experience, these are always the cheapest SMD crystals. Usually around 40 cents in 1sies at Mouser, and easy to solder too. You’ll find them on the Logic Sniffer, IR Toy, PICqueno, and many other projects.
If you must get tiny, smaller bar-style crystals are the most common. Usually they come in a package with a random size between 4mmx6mm and 5mmx8mm. There doesn’t seem to be a strict standard, which may be why they’re so expensive, but most will fit our 4x6mm footprint without an issue. These crystals run $1.20 and up in 1sies, but we were recently turned on to a new line that’s only about 60 cents each – check the partlist for links. You’ll find these smaller crystals on cramped PCBs like the Bus Blaster, Bus Pirate v4, and a few breakout boards. We avoid it when possible.
Oscillators are different than crystals, but they also provide a clock source. Oscillators are a complete circuit that outputs a precise frequency when power is applied. They come in 5volts, 3.3volts, etc., versions, but 3.3volts seems to be the most available right now. A package around 5mm x 7mm is common, though most will vary by 1mm in either dimension. We use oscillators for the FPGA on the Logic Sniffer and Dangerous DSO, and there is also an oscillator footprint on the CPLD breakout boards.
This is meant as a summary of the parts we use, not an essay on clocks. Please fill in the gaps and share your experience in the comments.