Variability resistant circuit element and signal processing method
A sequential circuit arrangement and method are provided in which a latch input signal and a latched version of the input signal are compared to derive a difference signal. This difference signal can detect when changes in the input are not propagated to the output. A second logic gate arrangement derives an error signal from the product of difference signal and a delayed version of the difference signal. This means that normal operation of the circuit is not detected as an error—only when the latched output fails to follow the input after the normally expected delay is the error signal created. The latch element output or an inverted version of the latch element output is selected in dependence on the error signal.
1. A sequential circuit arrangement, comprising:
a latch element clocked by a clock input signal;
a first logic gate arrangement for comparing a latch input signal and a latched version of the input signal to derive a difference signal;
a second logic gate arrangement for deriving an error signal from the difference signal and a delayed version of the difference signal;
a selection unit for selecting the latch element output or an inverted version of the latch element output in dependence on the error signal.
2. An arrangement as claimed in claim 1 , wherein the selection unit has a latch mode in which it holds its output at the output value at the beginning of the latch mode, the latch mode being timed based on the high or low phase of the clock input signal.
3. An arrangement as claimed in claim 1 , wherein the latch element comprises a flip flop such as a D-type flip flop.
4. An arrangement as claimed in claim 1 , comprising a clocked delay element for providing the delayed version of the difference signal.
5. An arrangement as claimed in claim 4 , wherein the clocked delay element has a delay representing at least the path delay through the latch element and through the first logic gate arrangement.
6. An arrangement as claimed in claim 4 , wherein the clocked delay element is clocked with first and second clock signals provided by the latch element, one in phase with the clock input signal and the other 180 degrees out of phase with the clock input signal.
7. An arrangement as claimed in claim 6 , wherein the clocked delay element has a latch mode in which it holds its output at the value at the beginning of the latch mode, the latch mode being timed based on the high or low phase of the clock input signal.
8. An arrangement as claimed in claim 1 , wherein the selection unit comprises a multiplexer with a clocked output buffer.
9. An arrangement as claimed in claim 8 , wherein the multiplexer is clocked with delayed versions of first and second clock signals provided by the latch element, one in phase with the clock input signal and the other 180 degrees out of phase with the clock input signal.
10. A method of processing an input signal within a sequential circuit arrangement, the method comprising:
latching the input signal timed with a clock input signal;
comparing the input signal and the latched version of the input signal to derive a difference signal;
generating a delayed version of the difference signal;
deriving an error signal from the difference signal and the delayed version of the difference signal;
selecting the latch element output or an inverted version of the latch element output in dependence on the error signal.
11. A method as claimed in claim 10 , wherein the selecting comprises operating a multiplexer with a latch mode in which the selecting is disabled and an output is provided equal to the output value at the beginning of the latch mode, the latch mode being timed based on the high or low phase of the clock input signal.
12. A method as claimed in claim 11 , comprising clocking the multiplexer with first and second clock signals provided by the latch element, one in phase with the clock input signal and the other 180 degrees out of phase with the clock input signal.
13. A method as claimed in claim 10 , wherein generating the delayed version comprises operating a clocked delay element which has a delay representing the path delay through the latch element and through the first logic gate arrangement.
14. A method as claimed in claim 10 , comprising clocking the delay element with first and second clock signals provided by the latch element, one in phase with the clock input signal and the other 180 degrees out of phase with the clock input signal.
15. A method as claimed in claim 14 , wherein generating the delayed version comprises operating the clocked delay element with latch mode in which the delay function is disabled and an output is provided equal to the output value at the beginning of the latch mode, the latch mode being timed based on the high or low phase of the clock input signal.