Method of deskewing a differential signal and a system and circuit therefor
A high-definition multimedia interface (HDMI) receiver recovers high speed encoded data which are transmitted differentially over data channels of a lossy cable, along with a clock. Inter symbol interference, high-frequency loss, skew between the clock and data channels, and differential skew within a differential signal are compensated by analog circuits which are automatically tuned for best performance by observing the quality of the recovered analog signal. Oversampling is used to provide a 24-bit digital representation of the analog signal for determining the quality of the signal. A corresponding method of deskewing a differential signal and a system and circuit therefor are also provided.
1. An analog differential deskew circuit for reducing a time skew, deskewing, of two polarities of a differential signal, comprising:
a number of analog delay units arranged sequentially;
a means for selecting a composite delay resulting from selected analog delay units;
a means for inserting the composite delay into the polarities of the differential signal to reduce the time skew;
a means for determining a quality of the differential signal after deskewing; and
a means for controlling the means for selecting and the means for inserting in response to said quality.
2. The circuit of claim 1 , wherein the means for inserting comprises a means for inserting the composite delay into one or the other polarity of the differential signal.
3. The circuit of claim 1 , wherein each of the analog delay units has a gain which is substantially equal to 1.0.
4. The circuit of claim 1 , wherein, each of the analog delay units comprises one or more amplifiers.
5. The circuit of claim 3 , wherein each analog delay unit comprises:
first and second amplifiers having a common input, which is the input of the analog delay unit, and their outputs being summed to generate the output of the analog delay unit;
the first amplifier having a gain of (1.0-Δ), and a delay equal to a predetermined delay value; and
the second amplifier having a gain of Δ, and substantially the same delay as the first amplifier.
6. The circuit of claim 5 , wherein the first amplifier is a follower stage.
7. The circuit of claim 5 , wherein the second amplifier has a shunt capacitor for setting the gain of Δ.
8. A method of deskewing a differential signal having two polarities, comprising the steps of:
forming a variable analog delay comprising a number of analog delay units arranged sequentially;
selecting a composite delay resulting from selected analog delay units;
inserting the composite delay into the polarities of the differential signal to reduce the time skew;
determining a quality of the differential signal after deskewing; and
controlling the steps of selecting and the inserting in response to said quality.
9. The method of claim 8 , wherein the step of inserting comprises inserting the composite delay into one or the other polarity of the differential signal.
10. An analog front end (AFE) circuitry for reducing distortion of a differential data signal, comprising:
(a) an analog differential deskew circuit for adjusting a time skew of two polarities of the differential data signal, comprising:
one or more analog delay units arranged sequentially;
an analog selector, selecting a composite delay resulting from the delay units selected by the analog selector;
analog switches, inserting the composite delay into the polarities of the differential signal; and
(b) an equalizer circuit having at least two settings for adjusting a frequency response of the deskewed signal to produce an equalized signal.
11. The AFE of claim 10 , wherein the analog switches are inserting the composite delay into one or the other polarity of the differential signal.
12. An analog front end (AFE) circuitry for reducing distortion of a differential data signal, comprising:
(a) an analog differential deskew circuit for adjusting a time skew of two polarities of the differential data signal, comprising:
one or more analog delay units arranged sequentially;
an analog selector, selecting a composite delay resulting from the delay units selected by the analog selector;
analog switches, inserting the composite delay into the polarities of the differential signal;
(b) an equalizer circuit for adjusting a frequency response of the deskewed signal to produce an equalized signal; and
(c) a phase compensator for aligning a phase of the equalized signal and a phase of a clock signal.
13. The AFE circuitry of claim 12 , wherein the phase compensator comprises:
an analog phase detector generating an analog delay control signal; and
a programmable analog delay circuit in a path of the differential data signal for changing the phase of the equalized signal in response to the analog delay control signal.
14. The AFE circuitry of claim 13 , wherein the analog phase detector comprises:
a phase detector for comparing the phase of the equalized signal and the phase of the clock signal;
a window generator for detecting positive edges of the equalized signal and generating an enable signal for the phase detector; and
a circuit for converting an output of the phase detector into the analog delay control signal.
15. A system for reducing a time skew between two polarities of a differential signal representing a digital bit stream comprising:
(a) a variable analog delay for inserting a variable time delay into polarities of the differential signal for generating a deskewed signal;
(b) a multi-phase clock for generating a number M of clock phases per bit of the digital bit stream for oversampling the deskewed signal into a digital representation of the deskewed signal;
(c) a logic circuit for estimating the quality of the digital representation of the deskewed signal;
(d) a means for adjusting the variable analog delay in response to the estimated quality of the digital representation of the deskewed signal to improve said quality;
(e) an equalizer circuit for adjusting a frequency response of the deskewed signal to produce an equalized signal;
(f) an analog phase detector generating an analog delay control signal; and
(g) a programmable analog delay circuit in a path of the differential signal for changing the phase of the equalized signal in response to the analog delay control signal.
16. The system of claim 15 , wherein the variable analog delay comprises a variable analog delay for inserting a variable time delay into one or the other polarity of the differential signal.
17. The system of claim 16 , wherein the variable analog delay comprises:
a number of analog delay units arranged sequentially;
a means for selecting a composite delay resulting from selected analog delay units; and
a means for inserting the composite delay into one or the other polarity of the differential signal to reduce the time skew.
18. The system of claim 15 , further comprising a phase compensator for aligning the phase of the equalized signal and a phase of a clock signal.
19. The system of claim 15 , wherein M equals to one of the following:
M=3;
M=12;
M=3 to 12.
20. A method for reducing a time skew between two polarities of a differential signal representing a digital bit stream, comprising the steps of:
(a) generating a deskewed signal by inserting a variable analog delay into polarities of the differential signal;
(b) generating a multi-phase clock signal having at least M phases per bit of the digital bit stream;
(c) oversampling the deskewed signal with the multi-phase clock into a digital representation of the deskewed signal;
(d) estimating the quality of the digital representation of the deskewed signal; and
(c) adjusting the variable analog delay in response to the estimated quality of the digital representation of the deskewed signal to improve said quality;
(f) adjusting a frequency response of the deskewed signal to produce an equalized signal;
(g) generating an analog delay control signal; and
(h) changing the phase of the equalized signal in response to the analog delay control signal.
21. The method of claim 20 , wherein the step (a) comprises inserting a variable analog delay into one or the other polarity of the differential signal.