I found some pictures of TPM modules with crystal oscillators, so those could keep time while powered on, but wouldn't be able to keep time while off. It would be possible to only count time while on, but that would be really annoying to a legitimate user that kept turning off the computer, waiting for the time limit to expire, especially if it doesn't tell the user how much time is left.
Anyway, crystal oscillators connect to one input pin and one output pin, with an internal feedback circuit that causes the crystal to resonate. It's possible to change out the crystal for a higher frequency, or even directly drive the input pin for a much higher frequency. Semiconductor manufacturers often only characterize the limit of the feedback circuit, but not the limit of the internal clock circuitry when directly driven. Considering that the logic design supports an SPI bus running up to tens of megahertz, it's totally possible that the crystal input could be driven at a similar speed, possible a thousand times faster than a normal real-time clock oscillator.
There would be several ways to mitigate such an attack, but a quick search for TPM side-channel attacks brings up multiple much simpler vulnerabilities, so it's not likely that TPM manufacturers are putting any real effort into mitigation.
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I found some pictures of TPM modules with crystal oscillators, so those could keep time while powered on, but wouldn't be able to keep time while off. It would be possible to only count time while on, but that would be really annoying to a legitimate user that kept turning off the computer, waiting for the time limit to expire, especially if it doesn't tell the user how much time is left.
Anyway, crystal oscillators connect to one input pin and one output pin, with an internal feedback circuit that causes the crystal to resonate. It's possible to change out the crystal for a higher frequency, or even directly drive the input pin for a much higher frequency. Semiconductor manufacturers often only characterize the limit of the feedback circuit, but not the limit of the internal clock circuitry when directly driven. Considering that the logic design supports an SPI bus running up to tens of megahertz, it's totally possible that the crystal input could be driven at a similar speed, possible a thousand times faster than a normal real-time clock oscillator.
There would be several ways to mitigate such an attack, but a quick search for TPM side-channel attacks brings up multiple much simpler vulnerabilities, so it's not likely that TPM manufacturers are putting any real effort into mitigation.