Crystals are used when an accurate, stable frequency is needed:
- Accurate means that the frequency is close to what it says on the package, typically within 1 part in 105.
- Stable means that does not change significantly with time or temperature.
Crystals are cut from carefully grown, high-quality quartz with specific orientations to give them high stability. Traditional crystals oscillated at frequencies of a few MHz but most small microcontrollers use low-frequency watch crystals with a frequency of 32 KHz. These are machined into complicated tuning fork shapes to give the low frequency. Adisadvantage is that their frequency is more sensitive to temperature than high-frequency crystals, but they are designed to be most stable near 25◦C. A change of 10◦C in the temperature causes the frequency to fall by about 4 parts per million (ppm). Detailed specifications are given by the manufacturers, such as Micro Crystal [58].
Stray capacitance of the PCB tracks to the crystal, which must be kept as short as possible. External capacitors are needed with many microcontrollers but they are integrated into the MSP430 for low-frequency crystals. The value is selected with the XCAPx bits in the BCSCTL3 register of the F20xx. The older Basic Clock Module (BCM, without the +) in the MSP430x1xx has a fixed capacitance of 12 pF per pin. For a highly accurate frequency, the drive current of the LFXT1 should also match the specification of the crystal. This sounds rather specialized but systems with the MSP430 may run for 10 years on a single battery and extreme accuracy and stability are needed if the clock is not to drift over this lifetime. The application note MSP430 LFXT1 Oscillator Accuracy (slaa225) describes some of the issues.
The oscillator is designed to run at low power and this renders it susceptible to electromagnetic interference. This means that the printed circuit board must be laid out carefully. There is detailed advice in the application note MSP430 32 KHz Crystal Oscillators (slaa322). Having said that, the crystal on the Olimex 1121STK is perched up in the air over the MCU itself, which seems to violate the rules but nevertheless works reliably.
It is also possible to use high-frequency crystals (above 400 KHz) with LFXT1 in most devices and some have a second oscillator XT2, which works only at high frequencies.
External capacitors must be used with high-frequency crystals and the module must be configured for a suitable range of frequencies. The module also accepts an external clock signal on XIN.
The stability of crystals is reflected electrically in their high Q factor. This means that they oscillate for a long time after being excited, like the ringing tone from a wine glass after it has been tapped. The disadvantage of this is that the oscillator takes an equally long time to reach a stable state, typically around 105 cycles. An oscillator based on a watch crystal therefore takes nearly a second to start and is almost always left running continuously. It might seem better that a 10MHz crystal starts in “only” 10 ms but the
CPU could have used around 105 cycles from the DCO in this time, which is enough to complete many tasks and return to a low-power mode. A software delay loop can be included if it is important to wait for the crystal to stabilize. This can be similar to those in the section “Automatic Control Flashing Light by Software Delay” onThe auxiliary clock ACLK can be derived only from LFXT1 in most devices so ACLK will not be available if there is no crystal. Beware if you are using a TI development kit