C-PHY receiver with self-regulated common mode servo loop
A receiving apparatus includes a terminating network for a three-wire serial bus and a feedback circuit. Each wire of the three-wire serial bus may be coupled through a resistance to a common node of the terminating network. The feedback circuit has a first amplifier circuit having an input coupled to the common node, a comparator that receives an output of the first amplifier circuit as a first input and a reference voltage as a second input, and a second amplifier circuit responsive to an output of the comparator and configured to inject a current through the common node.
1. A receiving apparatus, comprising:
a terminating network for a three-wire serial bus, each wire of the three-wire serial bus being coupled through a resistance to a common node of the terminating network; and
a feedback circuit comprising:
a first amplifier circuit having an input coupled to the common node;
a comparator that receives an output of the first amplifier circuit as a first input and a reference voltage as a second input; and
a second amplifier circuit responsive to an output of the comparator and configured to inject a current through the common node.
2. The apparatus of claim 1 , wherein the three-wire serial bus is operated in accordance with a C-PHY protocol.
3. The apparatus of claim 1 , wherein the terminating network comprises:
a capacitance configured to couple the common node to ground.
4. The apparatus of claim 1 , further comprising:
a plurality of differential receivers coupled to different pairs of wires of the three-wire serial bus, wherein the first amplifier circuit has a combination of active devices that matches a corresponding combination of active devices in an amplifier in each of the plurality of differential receivers.
5. The apparatus of claim 1 , wherein the first amplifier circuit and the second amplifier circuit comprise tunable transistors configured to set a voltage level at the common node.
6. The apparatus of claim 1 , wherein the feedback circuit is configured to regulate a voltage level at the common node based on amplitude of the current injected through the common node.
7. The apparatus of claim 1 , wherein a voltage level at the common node is calibrated during transmission of a preamble over the three-wire serial bus in accordance with a C-PHY protocol.
8. The apparatus of claim 7 , wherein the voltage level at the common node is calibrated to be less than half a nominal peak-to-peak voltage of a signal to be transmitted by a transmitter over the three-wire serial bus.
9. The apparatus of claim 1 , further comprising:
a third amplifier configured with an automatic frequency equalization circuit.
10. The apparatus of claim 9 , wherein the third amplifier comprises:
a plurality of P-type metal oxide-semiconductor input transistors; and
a source degeneration circuit.
11. An apparatus, comprising:
means for terminating a three-wire serial bus, configured to couple each wire of the three-wire serial bus through a resistance to a common node; and
means for regulating a common mode voltage level, including a feedback circuit comprising:
a first amplifier circuit having an input coupled to the common node;
a comparator that receives an output of the first amplifier circuit as a first input and a reference voltage as a second input; and
a second amplifier circuit responsive to an output of the comparator and configured to inject a current through the common node.
12. The apparatus of claim 11 , wherein the three-wire serial bus is operated in accordance with a C-PHY protocol.
13. The apparatus of claim 11 , wherein the means for terminating the three-wire serial bus comprises:
a capacitance configured to couple the common node to ground.
14. The apparatus of claim 11 , further comprising:
means for decoding data from the three-wire serial bus, including a plurality of differential receivers coupled to different pairs of wires of the three-wire serial bus, wherein the first amplifier circuit has a combination of active devices that matches a corresponding combination of active devices in an amplifier in each of the plurality of differential receivers.
15. The apparatus of claim 11 , wherein the first amplifier circuit and the second amplifier circuit comprise tunable transistors configured to set a voltage level at the common node.
16. The apparatus of claim 11 , wherein the feedback circuit is configured to regulate a voltage level at the common node based on amplitude of the current injected through the common node.
17. The apparatus of claim 11 , wherein a voltage level at the common node is calibrated during transmission of a preamble over the three-wire serial bus in accordance with a C-PHY protocol.
18. The apparatus of claim 17 , wherein the voltage level at the common node is calibrated to be less than half a nominal peak-to-peak voltage of a signal to be transmitted by a transmitter over the three-wire serial bus.
19. The apparatus of claim 11 , further comprising:
means for equalizing inputs to a differential receiver.
20. The apparatus of claim 19 , wherein the means for equalizing inputs to the differential receiver comprises:
a plurality of N-type metal-oxide-semiconductor input transistors; and
a source degeneration circuit.
21. A method for regulating a common mode voltage at a receiver, comprising:
providing a terminating network for a three-wire serial bus, each wire of the three-wire serial bus being coupled through a resistance to a common node of the terminating network; and
configuring a feedback circuit that comprises:
a first amplifier circuit having an input coupled to the common node;
a comparator that receives an output of the first amplifier circuit as a first input and a reference voltage as a second input; and
a second amplifier circuit responsive to an output of the comparator and configured to inject a current through the common node.
22. The method of claim 21 , wherein the three-wire serial bus is operated in accordance with a C-PHY protocol.
23. The method of claim 21 , wherein the terminating network comprises:
a capacitance configured to couple the common node to ground.
24. The method of claim 21 , wherein each of a plurality of differential receivers is coupled to different pairs of wires of the three-wire serial bus, and wherein the first amplifier circuit has a combination of active devices that matches a corresponding combination of active devices in an amplifier in each of the plurality of differential receivers.
25. The method of claim 21 , wherein the first amplifier circuit and the second amplifier circuit comprise tunable transistors configured to set a voltage level at the common node.
26. The method of claim 21 , wherein the feedback circuit is configured to regulate a voltage level at the common node based on amplitude of the current injected through the common node.
27. The method of claim 21 , wherein a voltage level at the common node is calibrated during transmission of a preamble over the three-wire serial bus in accordance with a C-PHY protocol.
28. The method of claim 27 , wherein the voltage level at the common node is calibrated to be less than half a nominal peak-to-peak voltage of a signal to be transmitted by a transmitter over the three-wire serial bus.
29. The method of claim 21 , further comprising:
equalizing inputs to a differential receiver.