Clock skew measurement technique

A system including a plurality of latch points, said plurality of latch points including a first reference latch point and a first subject latch point, each latch point in said plurality having a clock input, a data input and a data output, each latch point in said plurality supplying to its data output in response to a given clock pulse received at the clock input of said latch point a value responsive to that data present at the data input of said latch point when an effective edge of said given clock pulse reaches the clock input of said latch point, said system further including a clock source point and clock distribution means for transmitting a clock signal from said clock source point to the clock input of each of said plurality of latch points, said system further including apparatus for measuring clock skew between said first reference latch point and said first subject latch point, said apparatus comprising:

• a test data source point;
• adjustable means for delivering strings of at least one clock pulse each to said clock source point, and for delivering to said test data source point a test data signal having a logic level transition at a presettable time relative to the time at which said strings of clock pulses are delivered to said clock source point;
• a first associated latch point which is part of said plurality of latch points, said first associated latch point having negligible clock skew with respect to said first subject latch point;
• test data distribution means for routing a test data signal from said test data source point selectably to the data input of said first reference latch point or to the data input of said first associated latch point;
• first cross-transmission means for bi-directionally transmitting to the data input of each of said first reference and first associated latch points the data signal which appears on the data input of the other of said first reference and first associated latch points;
• means for adjusting said presettable time to determine a first TA1 setting at which the path delay from said clock source point to the clock input of said first reference latch point substantially equals the path delay from said clock source point via said test data distribution means to the data input of said first reference latch point, said test data distribution means being set to route said test data signal to the data input of said first reference latch point;
• means for adjusting said presettable time to determine a first TA2 setting at which the path delay from said clock source point to the clock input of said first associated latch point substantially equals the path delay from said clock source point via said test data distribution means and said first cross-transmission means to the data input of said first associated latch point, said test data distribution means being set to route said test data signal to the data input of said first reference latch point;
• means for adjusting said presettable time to determine a first TB2 setting at which the path delay from said clock source point to the clock input of said first associated latch point substantially equals the path delay from said clock source point via said test data distribution means to the data input of said first associated latch point, said test data distribution means being set to route said test data signal to the data input of said first associated latch point;
• and
• means for adjusting said presettable time to determine a first TB1 setting at which the path delay from said clock source point to the clock input of said first reference latch point substantially equals the path delay from said clock source point via said test data distribution means and said first cross-transmission means to the data input of said first reference latch point, said test data distribution means being set to route said test data signal to the data input of said first associated latch point,
• said first TA1, TA2, TB1 and TB2 settings being usable to calculate the clock skew between said first reference latch point and said first associated latch point.