Most design engineers don't look at the crystal until the rest of the components are designed in by which time it is far too late ….
Posted: 24th October 2016
So you have been tasked with designing a new piece of electronic gadgetry and know that you need to use some type of microprocessor or microcontroller to achieve your end aim. You then begin the possibly arduous task of selecting the appropriate device based on the features required plus the usual constraints of cost, cost, cost and lead-time. Eventually you reach a decision based on many factors including the architecture; will I need an 8bit, 16bit or 32bit device? What interfaces will the system need, USB, I2C, UART, SPI? What hardware will be needed, LEDs, sensors, buttons, speakers?
However critical all of the above are in the scheme of the design phase, one small and seemingly unimportant component that really needs to be given serious consideration at the front end of the design phase is the frequency controlling device that will be used to
provide the ‘clock’ frequency of the IC used.
The quartz crystal or oscillator required to provide the timing functionality may seem irrelevant in the early stage but it is surprising how many designers leave this until later in the development phase and then come across issues which hold back their design work.
Quartz based devices can be made in a multitude of frequencies, packages sizes and specifications to suit almost any application but sometimes what may have been seen as a minor specification requirement for the designer sometimes proves to hold up progress.
It is important for designers to get advice in the initial design stages when they have an idea of what particular frequency and overall specification is required as their frequency product partner can then provide guidance on what is feasible from a technical viewpoint and which device would be the most cost effective solution. This then gives the designer time to lay out a suitable PCB based on the best-fit model, have the device sampled and prove that the microprocessor operates as expected.
There are various issues that can cause designers headaches but which can possibly be avoided if a little extra time is allowed earlier in the design phase:
Example 1: Some IC manufacturers prove their designs using a large quartz crystal device such as an old type HC49 and include specific motional parameters such as low ESR for correct IC operation. Mr Engineer is then tasked with using this IC but needs his design to be as small as possible and stipulates that the crystal package must be as small as possible but still meet the parameters in the IC data sheet. However, it must be remembered that as the size of a quartz crystal reduces, its Equivalent Series Resistance (ESR) increases. The engineer is then quickly advised that the tiny package chosen cannot be made viably in volume at the low ESR required and a compromise is required. i.e. use a larger model type to achieve the low ESR or increase the ESR limit to allow a small device to be considered.
Example 2: Along the same vein as the scenario above, manufacturers sometimes use lower frequencies that are only possible in larger package sizes for quartz crystals. Again Mr Engineer is asked to shrink the design to as small as possible and has to be advised that the low frequency required is not technically feasible in such a small crystal package. Again it must be noted that as the size of a quartz crystal reduces, the minimum frequency achievable increases. Compromise options now have to be considered such as increasing the IC clock frequency, using a larger package size or moving to a small oscillator model.
Example 3: It must be noted that although all model types have a specified minimum and maximum frequency range, it is highly likely that not all frequencies within that range have been developed. Sometimes applications need non-standard frequencies that require development of the quartz blank to achieve the end specification. This process can sometimes take a number of iterations to reach the end goal and hence can impact on the lead-time.
Issues such as this can be avoided with a quick call to IQD in the early stages when our experienced Application Support team will guide you to the best solution while there is still time for consideration. It has to be remembered that each model type has its own set of limitations regards package size, frequency range, stability limits, unit costs etc. IQD and its distributors hold a large amount of standard specification devices in stock for immediate delivery but please remember that custom devices will need development which can extend their lead-time. Incidentally, if you’re interested in learning more about crystals and resonators, there’s a very good tutorial here.
It is thus important for designers to get advice at an early stage to make sure that sample parts are available when the initial equipment prototypes are built.