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The Smart Timer Unit (STU)

The main consumers in the XCXT are the oscillator circuits and counters. Thus, lowering the total power consumption requires lowering the power of these two parts of the design. From Equation 1, we see that an effective way of doing this is lowering the frequency. However, lowering the frequency also reduces time resolution. This is counter productive, since it will lead to higher guard bands than necessary. Instead, some mechanism needs to be sought that provides high temporal resolution at low power.

One solution comes in the form of a combination of a fast, and a slow clock in the same device. Figure 2 depicts this in a small block diagram. It shows that while the XCXT uses two high frequency crystals, the STU has in addition a 32kHz clock that can be used during sleep time. The trick is that just before going to sleep, the XCXT is used to calibrate the 32kHz clock, such that we can turn off the XCXT, but maintain accurate time while consuming little power during sleep. This recalibration happens independent of the MCU sleep schedule. Figure 3 visualizes this, where at the far left, and right, the MCU is active, and in-between, the XCXT is periodically turned on and off in order to compensate the 32kHz clock for changes in temperature. Note that shortly before the MCU wakes up, the XCXT is turned on in order to exploit the higher granularity of the fast crystals available to the MCU.

At first, it was thought that a slow 32kHz and a fast ($>1MHz$) clock within the XCXT itself would suffice. However, because of inherent crystal resonance properties, there are no combinations of crystals with frequencies two decades apart such that the $T$ vs $\delta f_{12}$ curve is bijective. This led us to the design choice of implementing the STU with three inexpensive crystals.

Figure 3: The Smart Timer Unit exploits the higher stability of the XCXT to compensate the 32kHz clock, and thus achieving a high precision, low power clock during sleep times.

Thomas Schmid 2008-11-14