UK processor firm ARM has built a digital storage oscilloscope on a chip, and a programmable load, to investigate voltage rail behaviour amongst multiple processor cores.
The transients are nanoseconds in duration, millivolts in height, and when they occur is dependent on the instantaneous interaction of application software, operating system, multiple cores and memory – so it is helpful to have a scope on the spot.
Alongside the scope and programmable load are dual Cortex-A57s, quad Cortex-A53s, and a T624 graphics processor – all in 28nm CMOS.
The scope is all-digital and monitors the supply to the dual A57s and their cache.
Its sensor is a 31 stage ring oscillator powered by the supply rail whose phase measured digitally. The scope digitally differentiates phase to derive frequency in number form, which is linear with respect to the VCO supply voltage to first order, according to ARM.
Phase measurement is actually in three parts: two measurements of the phase of the whole rotations around the ring oscillator, and one of individual stages, which are separately differentiated and then combined.
Scope like facilities include threshold and gradient triggers that can initiate waveform capture of up to 2k points into an internal SRAM trace buffer, and a decimator which allows bandwidth/sample-rate trade-off to measure low-frequency transients.
The logic needed for trigger, waveform capture, data logging, and correlating all the information is powered separately from the measured rail.
Calibration loading comes from 512 parallel nine-stage ring-oscillators which are enabled by a local controller to produce load waveforms including square wave, step and impulse.
Automatic calibration is provided by the local controller, which can also set-up the off-chip dc-dc converters that supply the cores.
At 800Msample/s, resolution is 3.3mV, and INLmax is -0.9/+0.7 LSB (after correction) over the 400mV input voltage range.
Up to 2.24Gsample/s is available with 10.8mV resolution, and 1.6mV resolution can be had at 400Msample/s.
ARM’s work on the scope is covered by ISSCC 2015 paper 14.6.
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