Ring oscillator is an oscillator circuit made up of an even number of inverters connected in a feedback configuration. If the number of inverters was even , then the input would be always equal to the output , making the oscillation impossible. The frequency of oscillation is the reciprocal of twice the total gate delay of the inverters . For example , if there are 5 inverters with a gate delay of 100 usec each , then the oscillation frequency will be 1/(2*5*100e-6)=1 kHz . In this circuit the input inverter is replaced by a nand gate which acts as an inverter for one input if the other one is constantly high. The logical switch acts as an enable signal, which allows the oscillation to initiate. The total gate delay is 500ns which leads to an oscillation frequency of approximately 1 MHz. Feel free to change the gate delays of the inverters and the nand and see the change in oscillation frequency.
