Programmable digital delay

1. A method of delaying successive first and second input signal edges by first and second different selectable time periods using a programmable delay line, the method comprising:

providing a control signal to each of a plurality of delay elements associated with said programmable delay line, the control signal selectively being in a first logic state or a second logic state, wherein in a first logic state the delay element selects an input from an adjacent delay element thereby to select the adjacent delay elements as part of a set of delay elements introducing said selectable time period and in a second logic state the adjacent delay element is not selected in the set;

setting the control signals for a first number of adjacent delay elements to the first logic state to introduce the first selectable time period,

wherein the control signals for the delay elements in the sequence not in the first number are set to the second logic state; and

setting the control signals of a second number of adjacent delay elements to the first logic state to introduce the second selectable time period,

wherein the control signals for the delay elements in the sequence not in the second number are set to the second logic state and wherein a reconfiguration time between outputting the delayed first input signal and receiving the second input signal is less than the maximum delay introduced by the sequence of delay elements.

2. The method according to claim 1 , wherein the second number is less than the first number and the reconfiguration time is less than the selectable time period introduced by the first number.

3. The method according to claim 1 , wherein pulses of different width each represent a data value.

4. The method according to claim 1 , wherein the first and second input signals are clock signals separated by a clock period and the reconfiguration time is less than half the clock period.

5. The method according to claim 1 , wherein said delay is provided by an outward path and a return path for said input signal through said delay element, and

wherein with the control signal in the first logic state said outward path of said adjacent delay element is selected as an input of the selecting delay element.

6. The method according to claim 5 , wherein the delay at said output equal to said selectable time period is provided by an outward path and a return path through said set of delay elements.

7. The method according to claim 1 further comprising:

inverting said first and second outputs of alternate delay elements in said sequence.

8. A programmable delay circuit comprising:

a programmable delay line arranged to delay successive first and second input signals edges by first and second different selectable time periods,

wherein said first and second successive input signal edges are received at an input of the delay line and said delay line outputs the delayed input signal edges,

wherein said delay line comprises:

a sequence of delay elements each introducing a delay, and

a selection circuit arranged to receive a control signal selectively being in a first or a second logic state,

wherein in a first logic state the delay element selects an input from an adjacent delay element thereby to select the adjacent delay element as part of a set of delay elements introducing said selectable time period and in a second logic state the adjacent delay element is not selected in the set, and

wherein the delay line resets the control signals from the first to the second logic state in a reconfiguration time between outputting the delayed first input signal edge and receiving the second input signal edge, which is less than the maximum delay introduced by the sequence of delay elements.

9. The programmable delay circuit according to claim 8 , wherein the delay at said output equal to said selectable time period is provided by an outward path and a return path through said set of delay elements.

10. The programmable delay circuit according to claim 9 , wherein the selection circuit comprises a multiplexer having a first input connected to receive an output from an adjacent delay element and a second input connected to the outward path of the same delay element.

11. The programmable delay circuit according to claim 9 , wherein each delay element comprises a second multiplexer to reset the delay line, wherein the second multiplexer includes a reset input connected to a predetermined logic value.

12. The programmable delay circuit according to claim 11 , wherein the predetermined logic value is zero.

13. The programmable delay circuit according to claim 8 , wherein the selection circuit comprises a logic gate arranged to receive said control signal.

14. The programmable delay circuit according to claim 13 , wherein said logic gate comprises at least one of:

an OR gate;

a NOR gate;

an AND gate; and

a NAND gate.

15. The programmable delay circuit according to claim 8 , wherein each delay element comprises a first input, a second input, a control input, a first output and a second output, wherein the control signal at said control input controls which signal at said first and second inputs to select for output at said second output.

16. The programmable delay circuit according to claim 15 , wherein the first and second outputs of alternate delay elements in said sequence are inverted.

17. The programmable delay circuit according to claim 15 wherein:

each delay element except the last delay element in said sequence are connected in sequence such that said first output is connected to the first input of the next delay element in the sequence;

said second input is connected to the second output of the next delay element in the sequence;

said last delay element in said sequence is connected such that its first output is connected to its first input; and

said first delay element in said sequence is connected such that its first input is connected to said input to the delay line, and its second output is connected to said output of the delay line.

18. The programmable delay circuit according to claim 8 , wherein each delay element further includes a buffer.

19. The programmable delay circuit according to claim 8 , wherein the clock edges supply the input signals.

20. A circuit to encode data as a series of pulses of differing widths, wherein each width represents a data value, the circuit comprising:

a data source supplying said data value; and

a delay line arranged to delay successive first and second input signal edges by first and second different selectable time periods,

wherein said first and second successive input signals are received at an input of the delay line and the delay line outputs the delayed input signal edges,

wherein said delay line comprises a sequence of delay elements each introducing a delay and a selection circuit arranged to receive a control signal, the control signal selectively being in a first or a second logic state,

wherein in a first logic state the delay element selects an input from an adjacent delay element thereby to select the adjacent delay element as part of a set of delay elements introducing said selectable time period and in a second logic state the adjacent delay element is not selected in the set,

wherein the delay line resets the control signals from the first to the second logic state in a reconfiguration time between outputting the delayed first input signal edge and receiving the second input signal edge, which is less than the maximum delay introduced by the sequence of delay elements, and

wherein each selectable time period represents a pulse width and the reconfiguration time is less than the longest pulse width.