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CC2540 32.768 kHz Crystal Selection

Christopher Reggiardo
Prodigy 95 points
Other Parts Discussed in Thread: CC2540

The CC2540 reference designs use an Epson Toyocom MC-306 32.768 kHz crystal in a 4-SOJ (5.50mm pitch) package that is 8.00mm x 3.20mm (x 2.38mm high).  Other than price and having the room to spare, was there a reason not to use the similar but smaller 2-SMD crystals that are 3.20mm x 1.50mm (x 0.8mm to 0.9mm high) such as the AVX ST3215SB32768H5HPWAA (to name but one)?  Are there any concerns with assembly or humidity (e.g. no-clean flux) affecting performance for one crystal type versus another?


 

  • 0
    TI__Expert 4060 points

    Hi Christopher,

    There are a lot of different crystals that can be used, with different size/cost trade-offs. In general, the most important things to look for is accuracy (a certain number of ppm over a certain temperature range), and load capacitance. As noted in the data sheet, we recommend the load capacitance to be 12-16 pF.

    As for accuracy, the BLE spec allows up to +/- 500 ppm, but we strongly recommend using a smaller value, as the accuracy will directly affect the average current consumption, as when you are in a connection, the time spent in RX is proportional to the total crystal accuracy (master+slave).

    I am not aware of any specific assembly concerns. The crystal manufacturer should be able to provide more guidance on this.

    Best rgards,

    Karl