On-die system and method for controlling termination impedance of memory device data bus terminals
A system for controlling the termination impedance of memory device data bus terminals is fabricated on the same die as the memory device. The system includes a termination resistor connected to each data bus terminal, which is connected in parallel with several transistors that are selectively turned on to adjust the termination impedance. The transistors are controlled by a circuit that determines the resistance of the termination resistor and turns on the correct number of transistor to properly set the termination impedance. In one example, the resistance of the termination resistor is determined by directly measuring a resistor of the same type as the termination resistor. In another example, the resistance of the termination resistor is determined indirectly by measuring parameters that affect the resistance of the termination resistor. In either case, the system can maintain the termination impedance of the data bus terminals constant despite changes in the termination resistor.
1. A termination circuit fabricated on a semiconductor die along with an electronic device having an externally accessible terminal, the termination circuit comprising:
a programmable on-die termination (“ODT”) circuit coupled to the externally accessible terminal, the programmable ODT circuit providing the externally accessible terminal with an impedance using the combination of a terminating resistor and a controllable impedance element having an impedance that is controlled by at least one programming signal received by the programmable ODT circuit; and
a programming circuit operable to provide the at least one programming signal to the ODT circuit so that the impedance provided by the combination of the terminating resistor and the controllable impedance element is substantially equal to a target impedance, the programming circuit comprising:
a first resistor having a resistance that varies in a manner that substantially matches the manner in which a resistance of the terminating resistor varies; and
a measurement circuit for measuring the impedance of the first resistor, the measurement circuit comprising:
a second resistor coupled in series with the first resistor between first and second voltages to form a voltage divider having an output node between the first resistor and the second resistor;
an analog-to-digital converter having an input coupled to the output node, the analog-to-digital converter generating a binary output signal indicative of a voltage at the output node of the voltage divider; and
a register generating a plurality of the programming signals based on the binary output signal.
2. The termination circuit of claim 1 wherein the first resistor has a resistance that varies with a temperature of the first resistor, and wherein the variation of the resistance of the first resistor with the temperature of the first resistor substantially matches the manner in which a resistance of the terminating resistor varies with the temperature of the terminating resistor.
3. The termination circuit of claim 1 wherein the first resistor has a resistance that varies with a processing parameter used to fabricate the first resistor, and wherein the variation of the resistance of the first resistor with a processing parameter used to fabricate the first resistor substantially matches the manner in which a resistance of the terminating resistor varies with a processing parameter used to fabricate the terminating resistor.
4. The termination circuit of claim 1 wherein the first resistor has a resistance that varies with one of the first and second voltages, and wherein the variation of the resistance of the first resistor with one of the first and second voltages substantially matches the manner in which a resistance of the terminating resistor varies with a voltage applied to the terminating resistor.
5. The termination circuit of claim 1 wherein the terminating resistor comprises a polysilicon resistor.
6. The termination circuit of claim 1 wherein the measurement circuit further comprises a switch coupled in series with the first resistor and the second resistor between the first and second voltages for selectively enabling the voltage divider.
7. The termination circuit of claim 1 wherein the first resistor comprises an Naa resistor.
8. The termination circuit of claim 1 wherein the programming circuit comprises:
a sensor providing an sensor output signal indicative of a condition that causes the resistance of the first resistor to vary in a manner that substantially matches the manner in which a resistance of the terminating resistor varies;
a control circuit coupled to receive the sensor output signal from the sensor, the control circuit providing an output signal based on the sensor output signal; and
a register storing a digital signal corresponding to the output signal from the control circuit, the register generating a plurality of the programming signals corresponding to the stored digital signal.
9. The termination circuit of claim 8 wherein the sensor comprises a temperature sensor providing an output signal corresponding to the temperature of a semiconductor die in which the terminating resistor is fabricated.
10. The termination circuit of claim 8 wherein the sensor comprises a voltage sensor providing an output signal corresponding to a voltage applied to a semiconductor die in which the terminating resistor is fabricated.
11. The termination circuit of claim 8 wherein the sensor comprises a processing characteristic sensor providing an output signal corresponding to a processing characteristic of a semiconductor die in which the terminating resistor is fabricated.
12. A method of terminating an externally accessible terminal of an integrated circuit using the combination of a terminating resistor and a controllable impedance element fabricated as part of the integrated circuit, the method comprising:
determining the approximate impedance of a first resistor fabricated on a semiconductor die of the integrated circuit, the first resistor having an impedance characteristic that varies in substantially the same manner as the manner in which an impedance characteristics of the terminating resistor varies, the approximate impedance of the first resistor being determined by:
coupling a second resistor in series with the first resistor between first and second voltages to form a voltage divider having an output node between the first and second resistors;
determining the amplitude of the voltage at the output node of the voltage divider; and
using the determined amplitude of the voltage to determine the impedance of the first resistor; and
adjusting the impedance of the controllable impedance element as a function of the determined impedance so that the impedance provided by the combination of the terminating resistor and the controllable impedance element is substantially equal to a target impedance.
13. The method of claim 12 , further comprising digitizing the voltage at the output node of the voltage divider before using the determined amplitude of the voltage to determine the impedance of the test resistor.
14. The method of claim 12 wherein the act of determining the approximate impedance of the terminating resistor comprises:
sensing a condition that affects the impedance of the terminating resistor; and
using the sensed condition to determine the impedance of the terminating resistor.
15. The method of claim 14 wherein the act of sensing a condition that affects the impedance of the terminating resistor comprises sensing the temperature of a semiconductor die in which the integrated circuit is fabricated.
16. The method of claim 14 wherein the act of sensing a condition that affects the impedance of the terminating resistor comprises sensing the magnitude of a supply voltage applied to the integrated circuit.
17. The method of claim 12 wherein the act of adjusting the impedance of the controllable impedance element comprises:
sensing a condition that affects the impedance of the terminating resistor; and
using the sensed condition to adjusting the impedance of the controllable impedance.
18. The method of claim 17 wherein the act of sensing a condition that affects the impedance of the terminating resistor comprises sensing the temperature of a semiconductor die in which the integrated circuit is fabricated.
19. The method of claim 17 wherein the act of sensing a condition that affects the impedance of the terminating resistor comprises sensing the magnitude of a supply voltage applied to the integrated circuit.