Edge enhancement for signal transmitter

1. A signal transmitter circuit comprising:

an output driver circuit configured to:

transmit a signal using a multi-level pulse amplitude modulation (PAM) scheme comprising a plurality of discreet signal levels;

initiate a first transition of the signal to a first level of the multi-level PAM scheme from a second level of the multi-level PAM scheme;

initiate a second transition of the signal to the first level from a third level of the multi-level PAM scheme; and

initiate a third transition of the signal to the first level from a fourth level of the multi-level PAM scheme, wherein the fourth level has a lower voltage than the third level; and

a control circuit coupled to the output driver circuit, the control circuit configured to control a slew rate of the signal transmitter circuit to cause:

the signal to reach a threshold voltage level at a first time, the first time occurring a first duration of time after the first transition is initiated;

the signal to reach the threshold voltage level at a second time, the second time occurring the first duration of time after the second transition is initiated; and

the signal to reach the threshold voltage level at a third time, the third time occurring the first duration of time after the third transition is initiated.

2. The signal transmitter circuit of claim 1 , wherein the second level has a lower voltage than the first level, wherein the third level has a lower voltage than the second level, and wherein the threshold level has a voltage between the first level and the second level.

3. The signal transmitter circuit of claim 1 , wherein to control the slew rate of the signal transmitter circuit, the control circuit is configured to control at least one parameter of the signal transmitter circuit to adjust the slew rate during a transition of the signal.

4. The signal transmitter circuit of claim 3 , wherein to control the at least one parameter of the signal transmitter circuit, the control circuit is configured to control an output impedance of the signal transmitter circuit, wherein the slew rate increases as the output impedance decreases.

5. The signal transmitter circuit of claim 3 , wherein to control the at least one parameter of the signal transmitter circuit, the control circuit is configured to control an amount of injected current during at least one of the first transition or the second transition, wherein the slew rate increases as the amount of current increases.

6. The signal transmitter circuit of claim 5 , wherein to control the amount of injected current, the control circuit is configured to adjust a variable current driver.

7. A method comprising:

transmitting, by a signal transmitter circuit, a signal using a multi-level pulse amplitude modulation (PAM) scheme comprising a plurality of discreet signal levels;

initiating a first transition of the signal to a first level of the multi-level PAM scheme from a second level of the multi-level PAM scheme;

controlling a slew rate of the signal transmitter circuit to cause the signal to reach a threshold voltage level at a first time, the first time occurring a first duration of time after the first transition is initiated;

initiating a second transition of the signal to the first level from a third level of the multi-level PAM scheme;

controlling the slew rate of the signal transmitter circuit to cause the signal to reach the threshold voltage level at a second time, the second time occurring the first duration of time after the second transition is initiated;

initiating a third transition of the signal to the first level from a fourth level of the multi-level PAM scheme, wherein the fourth level has a lower voltage than the third level; and

controlling the slew rate of the signal during the third transition to cause the signal to reach the threshold voltage level at a third time, the third time occurring the first duration of time after the third transition is initiated.

8. The method of claim 7 , wherein the second level has a lower voltage than the first level, wherein the third level has a lower voltage than the second level, and wherein the threshold level has a voltage between the first level and the second level.

9. The method of claim 7 , wherein controlling the slew rate of the signal transmitter circuit comprises controlling at least one parameter of the signal transmitter circuit to adjust the slew rate during a transition of the signal.

10. The method of claim 9 , wherein controlling the at least one parameter of the signal transmitter circuit comprises controlling an output impedance of the signal transmitter circuit, wherein the slew rate increases as the output impedance decreases.

11. The method of claim 10 , wherein controlling the at least one parameter of the signal transmitter circuit comprises controlling an amount of current with which the signal is transmitted during at least one of the first transition or the second transition, wherein the slew rate increases as the amount of current increases.

12. The method of claim 11 , wherein controlling the amount of injected current comprises adjusting a variable current driver.

13. An integrated circuit comprising:

a signal generator circuit; and

a signal transmitter circuit coupled to the signal generator circuit, the signal transmitter circuit comprising:

an output driver circuit configured to:

transmit a signal to a memory module using a multi-level pulse amplitude modulation (PAM) scheme comprising a plurality of discreet signal levels;

initiate a first transition of the signal to a first level of the multi-level PAM scheme from a second level of the multi-level PAM scheme;

initiate a second transition of the signal to the first level from a third level of the multi-level PAM scheme; and

initiate a third transition of the signal to the first level from a fourth level of the multi-level PAM scheme, wherein the fourth level has a lower voltage than the third level; and

a control circuit coupled to the output driver circuit, the control circuit configured to control a slew rate of the signal transmitter circuit to cause:

the signal to reach a threshold voltage level at a first time, the first time occurring a first duration of time after the first transition is initiated;

the signal to reach the threshold voltage level at a second time, the second time occurring the first duration of time after the second transition is initiated; and

the signal to reach the threshold voltage level at a third time, the third time occurring the first duration of time after the third transition is initiated.

14. The integrated circuit of claim 13 , wherein the second level has a lower voltage than the first level, wherein the third level has a lower voltage than the second level, and wherein the threshold level has a voltage between the first level and the second level.

15. The integrated circuit of claim 14 , wherein to control the slew rate of the signal transmitter circuit, the control circuit is configured to control at least one parameter of the signal transmitter circuit to adjust the slew rate during a transition of the signal.

16. The integrated circuit of claim 15 , wherein to control the at least one parameter of the signal transmitter circuit, the control circuit is configured to control an output impedance of the signal transmitter circuit, wherein the slew rate increases as the output impedance decreases.

17. The integrated circuit of claim 16 , wherein to control the at least one parameter of the signal transmitter circuit, the control circuit is configured to control an amount of injected current during at least one of the first transition or the second transition, wherein the slew rate increases as the amount of current increases.

18. The integrated circuit of claim 17 , wherein to control the amount of injected current, the control circuit is configured to adjust a variable current driver.

19. A signal transmitter circuit comprising:

an output driver circuit configured to:

transmit a signal using a multi-level pulse amplitude modulation (PAM) scheme comprising a plurality of discreet signal levels;

initiate a first transition of the signal to a first level of the multi-level PAM scheme from a second level of the multi-level PAM scheme;

initiate a second transition of the signal to the first level from a third level of the multi-level PAM scheme; and

initiate a third transition of the signal to the first level from a fourth level of the multi-level PAM scheme, wherein the fourth level has a lower voltage than the third level; and

means for controlling a slew rate of the signal transmitter circuit to cause:

the signal to reach a threshold voltage level at a first time occurring a first period of time after the first transition is initiated;

the signal to reach the threshold voltage level at a second time occurring the first period of time after the second transition is initiated; and

the signal to reach the threshold voltage level at a third time, the third time occurring the first duration of time after the third transition is initiated.