1. Digital Temperature Sensing in a Variable Supply Environment EE241 Term Project Matthew Spencer Steven Callender Spring 2009

2. Classic Temperature Measurement is Analog Accurately Measures BJT Current Big, Power-Hungry, Analog Circuit [1] M. Pertijs, K. A. Makinwa, J. H. Huijsing. A CMOS Smart Temperature Sensor With a 3σ Inaccuracy of +/-0.1°C From -55°C to 125°C. IEEE JSSC, December 2005

3. Digital Replacements Ignore Real Effects [2] Poki Chen et al. An Accurate CMOS Time-to-Digital-Converter-Based Smart Temperature Sensor. JSSC 2005 Measure Delay Pretty Well Ignore Supply and Process [3] K. Woo, D. Ham et al. Dual-DLL-Based CMOS All-Digital Temperature Sensor for Microprocessor Thermal Monitoring. ISSCC 2009.

4. Outline Motivation Current sensors: What makes them good? Comparing digital sensors and Simulation Digitally Assisted Supply Correction Supply Insensitive Biasing

5. A Worrisome Graph from a Paper … Vdd to delay constants mismatched Optimal Resolution, not supply rejection

6. “Linear With Supply” Costs a Lot of Area

7. Can the We Correct for Supply with Math? Use Digital Algorithm to Help Measurement Use Several f(C) to Guess Vdd and T

8. Digital Correction Can’t Handle Vdd Errors Good Vdd Guess, but Bad T Guess Exponentially sensitive to Vdd

9. Accounting for V dd shifts: A Better Approach Strong coupling between V dd and temperature in delay variations Very hard to isolate each effect for a given delay variation (How much is due to Vdd and how much is due to temperature?) Solution: Supply insensitive biasing Idea: “remove” any V dd shifts from the point of view of the delay cells

10. Accounting for V dd shifts: A Better Approach Once delay variations are decoupled from supply shifts, problem becomes trivial Supply insensitive biasing is the most viable approach, but there are still issues: Device Sizes Topology: New approaches may need to be explored

11. Conclusion Literature ignores real environments Correcting for supply hasn’t worked yet Delay is a BAD temperature metric … But finding something else is hard

12. Thank You Questions?