So far we’ve discussed choosing a crystal based on motional resistance, load capacitance, and resonant frequency. Surely that is all we care about, right? Well, not quite. Let’s say we have found two crystals, both with the same Rm, CL, and center frequency. The only difference is one is in a 2.0x1.6mm package and the other is a 3.2x2.5mm package. Does that matter to the performance, or only to the board area required? If you remember from the previous post in this series, the crystal oscillator startup time is approximately 7-15t, where t is:
Equivalently, this can be written as:
Cm is the motional capacitance of the quartz crystal. For a smaller crystal, this capacitance value tends to be smaller, and the motional inductance is higher. Therefore, as the crystal size is decreased, the startup time increases even if CL and Rm remain the same. In the CC2540 datasheet, startup time for the EM reference design is 250us. This startup time could be different if different a different crystal is used. For duty cycled applications, a slower oscillator startup time can mean higher average power consumption because the device spends less time in sleep. If the crystal is a significant contributor to the board area, there can be a tradeoff between the minimum board size and the average power consumption. In this crystal selection guide for CC253x and CC254x, crystals of four difference sizes are shown. The motional inductance and capacitance values are not always available on crystal datasheets, but can usually be obtained from the crystal vendor by request. If oscillator startup time is important to your application, you should be aware of these parameters when choosing your crystal.
Make sure to check out parts 1, 2 and 3 of this series too!
‒ Part 1: Frequency references 101
‒ Part 3: Frequency references: Startup time