Circuit delay monitoring apparatus and method

A method of monitoring circuit delay comprising:

• propagating a signal transition through a ring oscillator comprising a plurality of delay elements;
• sampling, under control of a reference clock signal, signal values at each of a plurality N of sampling points distributed around the ring oscillator, in order to generate an indication of a current location of the signal transition within the ring oscillator;
• connecting each of M transition counter circuits, where M is at least 2 and less than N, to an associated location within the ring oscillator, the M transition counter circuits being connected to the ring oscillator so that, at any point in time, the separation between the current location of the signal transition and the associated location of at least one of said M transition counter circuits will be greater than a predetermined amount;
• within each transition counter circuit, incrementing a count value each time the signal transition passes the associated location;
• sampling the count value under control of the reference clock signal;
• selecting, in dependence on the indication of the current location of the signal transition, one of said M transition counter circuits whose associated location is greater than said predetermined amount from the current location of the signal transition;
• and
• generating a count indication for a reference time period dependent on a sampled count value of the transition counter circuit selected by the selection circuitry, the indication of the current location of the signal transition within the ring oscillator, and reference count data relating to the start of the reference time period.

A circuit delay monitoring apparatus comprising:

• ring oscillator means comprising a plurality of delay elements, a signal transition being propagated through the delay elements of the ring oscillator means;
• a plurality N of sampling points distributed around the ring oscillator means;
• fine sampling means for sampling signal values at each of the plurality of sampling points under control of a reference clock signal, in order to generate an indication of a current location of the signal transition within the ring oscillator means;
• coarse sampling means comprising M transition counter means, where M is at least 2 and less than N, each transition counter means for connecting to an associated location within the ring oscillator means and for incrementing a count value each time the signal transition passes the associated location, the coarse sampling means further for sampling the count value under control of the reference clock signal, the M transition counter means being connected to the ring oscillator means so that, at any point in time, the separation between the current location of the signal transition and the associated location of at least one of said M transition counter means will be greater than a predetermined amount;
• selection means for selecting, in dependence on the indication of the current location of the signal transition generated by the fine sampling means, one of said M transition counter means whose associated location is greater than said predetermined amount from the current location of the signal transition;
• and
• output generation means for generating a count indication for a reference time period dependent on a sampled count value of the transition counter means selected by the selection means, the indication of the current location of the signal transition within the ring oscillator means, and reference count data relating to the start of the reference time period.