Low power multilevel driver

1. An apparatus comprising:

a voltage termination node connected to a plurality of transmission lines via a plurality of termination impedance elements, the voltage termination node having a termination voltage corresponding to a summation of a plurality of currents flowing through the plurality of termination impedance elements, each current of the plurality of currents corresponding to a respective symbol of a plurality of symbols of a balanced codeword of a vector signaling code, each symbol of the plurality of symbols of the balanced codeword having a symbol value selected from a set of at least three symbol values;

a plurality of multi-level drivers, each multi-level driver connected to a respective transmission line of the plurality of transmission lines and to the voltage termination node via a respective termination impedance element of the plurality of termination impedance elements, each multi-level driver configured to generate a corresponding current of the plurality of currents, each multi-level driver comprising:

at least one current source connected to the respective transmission line, each current source selectively enabled to source current to the respective transmission line to drive a line voltage above the termination voltage of the termination voltage node connected to the transmission line via the respective termination impedance element of the plurality of termination impedance elements, wherein each of the at least one current sources has an output impedance different than a characteristic impedance of the transmission line; and

at least one current sink connected to the respective transmission line, each current sink selectively enabled to sink current from the transmission line to drive a line voltage below the termination voltage, each of the at least one current sinks having an output impedance different than the characteristic impedance of the transmission line.

2. The apparatus of claim 1 , wherein the termination impedance is matched to the characteristic impedance of the respective transmission line.

3. The apparatus of claim 1 , wherein each multi-level driver further comprises at least one transmit impedance element connecting at least one current source or at least one current sink to the respective transmission line.

4. The apparatus of claim 1 , wherein each current source and current sink comprises a corresponding switch configured to selectively enable sourcing or sinking of the current to the respective transmission line.

5. The apparatus of claim 4 , wherein at least one of the current sources is an active current source connected to the respective transmission line via the corresponding switch.

6. The apparatus of claim 1 , wherein at least one current sink is a resistive element connected to the respective transmission line.

7. The apparatus of claim 1 , further comprising a switching circuit connected to a single current source or a current sink, the switching circuit configured to connect the single current source or current sink to one or more transmission lines of the plurality of transmission lines.

8. The apparatus of claim 1 , wherein the voltage termination node comprises a capacitor configured to maintain the termination voltage.

9. A method comprising:

obtaining a plurality of control bits identifying a plurality of symbols of a balanced codeword of a vector signaling code, each symbol of the plurality of symbols of the balanced codeword having a symbol value selected from a set of at least three symbol values;

in response to obtaining the plurality of control bits, setting a line voltage on a plurality of transmission lines connected to a voltage termination node via a plurality of termination impedance elements, the voltage termination node having a termination voltage corresponding to a summation of a plurality of currents flowing through the plurality of termination impedance elements, each current of the plurality of currents corresponding to a respective symbol of the plurality of symbols of the balanced codeword, wherein setting the line voltage on each respective transmission line of the plurality of transmission lines comprises:

selectively sourcing current via at least one current source to the respective transmission line to drive the line voltage above the termination voltage of the termination voltage node connected to the respective transmission line via a respective termination impedance element of the plurality of termination impedance elements, wherein each of the at least one current sources has an output impedance different than a characteristic impedance of the transmission line; and

selectively sinking current via at least one current sink from the respective transmission line to drive the line voltage below the termination voltage, each of the at least one current sinks having an output impedance different than the characteristic impedance of the respective transmission line.

10. The method of claim 9 , wherein the termination impedance element is matched to the characteristic impedance of the respective transmission line.

11. The method of claim 9 , wherein at least one current source or at least one current sink is coupled to the respective transmission line via a transmit impedance.

12. The method of claim 9 , wherein selectively sourcing or sinking current comprises enabling one or more switches according to the plurality of control bits.

13. The method of claim 9 , wherein at least one of the current sources is an active current source.

14. The method of claim 9 , wherein at least one current sink is a passive current sink.

15. The method of claim 9 further comprising switching a single current source or current sink between one or more transmission lines of the plurality of transmission lines.

16. The method of claim 9 , wherein the termination voltage is maintained by charging and discharging a capacitor connected to the voltage termination node according to the plurality of currents.