1 adaptive body bias for reducing process variations nuno alves 19 / october / 2006

2 background goal of processor design: achieve maximum operating frequency meet power density constraint process variations create differences: across a single die across multiple wafers and lots

3 differences in transistors ? so? some dies cannot be accepted because: low frequency high power consumption dies

4 solving leakage problem… leakage can be controlled to some extent using body bias. remember: non-zero body-to-source bias can modulate the threshold voltage of a transistors

5 reverse body bias (rbb) we can use rbb to reduce leakage power in standby mode by: raising the voltage of the pMOS n-wells with respect to vdd or lowering the voltage of substrate relative to gnd

6 forward body bias (fbb) Vt  by the lowering the source-body potential barrier lower Vt = higher on current hence higher performance the good the bad increase in sub-threshold and substrate-to-source leakage slows down the discharge of nodes use fbb to increase operating frequency in active mode

7 ideally Vt should be lowered for slow dies raised for leaky dies accomplished by an adaptive body bias

8 testchip 21 subsites each subsite contains: an abb generator control circuit

9 how it works? pt 1 the desired operating frequency is applied externally slows things down compare critical path with target clock period

10 how it works? pt 2 output of first ff is sampled by second ff this allows sufficient time for the body bias to stabilize

11 how it works? pt 3 PD used to clock a counter counter whose value represents the body bias to apply

12 how it works? pt 4 converting digital code to an analogical body voltage

13 how it works? pt 5 the output voltage, which biases the the pMOS transistors is a function of VREF VCCA output voltage

14 how it works? pt 6 setting the bias by modifying: VREF VCCA and setting a counter control bit

15 operation pt 1 initially frequency is lower than the target one body voltage reduces, forward biasing the pMOS transistor & increasing frequency

16 operation pt 2 phase detector changes to a permanent 1 frequency has been matched the counter is disabled, maintaining the body voltage

17 operation pt 3 once optimal voltages are determined, they can be programmed in the chip or supplies externally

18 simple adaptive body bias pt 1 optimum bias voltages are determined through measurements example: 1.a microprocessor with many circuit blocks. 2.find out the frequency of a critical path 3.a central body bias determines the body bias to apply to achieve a desired frequency. 4.apply this bias everywhere 2% total die area overhead

19 simple adaptive body bias pt 2 optimum bias is determined by applying a target clock frequency… …highest possible operating frequency for the die under the given power constraint. maximum clock frequency for this maximum frequency nMOS body bias is applied from outside pMOS body bias comes from on-chip control circuitry

20 simple adaptive body bias pt 3 repeat until we find the best combination of lowest leakage with target frequency pick target frequency manually adjust nMOS body bias pMOS body bias automatically adjusts determine leakage power

21 effects of simple body bias pt1 NBB = no body bias

22 effects of simple body bias pt2 when no body bias, only 50% dies are acceptable conclusion 1: … mostly in the low frequency bin

23 effects of simple body bias pt3 frequency variation was reduced to 1% from 4.1% more accepted dies (specially in the high frequency range) conclusion 2:

24 effects of simple body bias pt4 conclusion 3: many dies fail to meet the leakage constraint… … due to the fact that a single circuit block is used to determine the body bias for all circuit… … and there are always intra-die variations.