3.8 A 0.45-to-0.7V 1-to-6Gb/S 0.29-to-0.58pJ/b source-synchronous transceiver using automatic phase calibration in 65nm CMOS

Supply voltage (V_DD) scaling offers a means to greatly reduce power in serial link transceivers. Ideally, power efficiency at a given data rate can be improved by reducing V_DD while increasing the number of multiplexed circuits operating in parallel at lower clock frequencies [1]. Though increasing the amount of parallelism is desirable to scale V_DD, in practice, it is limited by two main factors. First, increased sensitivity to device variations (threshold voltage/dimension mismatch) at lower V_DD makes it extremely challenging to generate equally spaced multi-phase clocks needed in multiplexed transmitter and receiver. Phase calibration methods can correct phase-spacing errors [2,3], but their effectiveness at lower V_DD is limited as the calibration circuits themselves become sensitive to device variations. Second, as oscillator output swing reduces with V_DD, its phase noise degrades, making low-noise clock generation difficult to implement with low power dissipation. Phase noise can be suppressed by embedding the oscillator in a wide bandwidth analog phase-locked loop (APLL), but conventional charge-pump-based APLLs are difficult to design at low supply voltages (V_DD <;0.5V). Digital PLLs (DPLLs) can operate at low V_DD, but they suffer from conflicting noise bandwidth tradeoffs, which prevent increasing the bandwidth to suppress oscillator phase noise adequately. In this paper, we present a phase-calibration method that enables the operation of a source-synchronous transceiver down to V_DD of 0.45V. The energy efficiency and data-rate of the prototype transceiver scale from 0.29 to 0.58pJ/b and 1.3Gb/s to 6Gb/s, respectively, as V_DD is varied from 0.45 to 0.7V.