Clock Generators

Internal Oscillators

Digital circuits (and some analog circuits as well) frequently need a clock. This is typically provided by either an external clock source, a crystal oscillator, an RC oscillator, or a ceramic resonator. These can be had to any desired level of accuracy. But they all require external components. When space is critical, a self-contained oscillator can be the solution. It is the smallest solution and the low-cost solution. The only problem is accuracy.

A ring oscillator is easy to make, and is quite robust. But the accuracy is bad. The speed will vary with the square of the voltage, the -3/2 power of the temperature, and another +/- 20% with the wafer processing. This can result in 50% tolerances. And you will need around 1% tolerance to make a UART work. So how do you get there? Design is the answer.

Limiting Voltage Sensitivity

We cannot do anything about transistors being sensitive to voltage. But we can limit the voltage swing that the transistors see. So, the solution is to use a bandgap reference to generate a power supply for the ring oscillator. That takes care of one of the variables.

Limiting Process Sensitivity

The process sensitivity is primarily a result of normal variations in the thresholds of P and N transistors. We compensate for this with a trim. But what do we trim, and how do we do it? We have two options for trimming. The first involves adjusting the number of stages in a ring oscillator. This works best when there are a large number of stages. A better method is to adjust the ring oscillator supply voltage up or down to achieve the desired frequency. We use an op-amp to generate the supply voltage from the bandgap. The op-amp is biased with resistors that set the desired voltage. Letting the trim adjust the value of those resistors is an easy way to trim the frequency.

At one time, lasers were used to cut resistors to trim them. That is still an option, but using NVM memory is a preferred solution. We put multiple resistors in the op-amp feedback network, and use transistors to select which resistors to use. The NVM then enables the transistors.

Limiting Temperature Sensitivity

As stated earlier, transistor strengths change as a function of the temperature to the -3/2 power. We can compensate for this temperature change by increasing the ring oscillator supply voltage when temperature increases. This part is challenging, and is beyond the scope of this article. But we would be happy to discuss it in greater detail should you need it in your application.

Date

2018-05-23