Following is a rough overview of the part we want the driver for.
Any input on this will be appreciated.
thanks,
Chris
=== Introduction
Following the recent focus on low-power systems, Microchip has
designed a companion chip series capable of measuring the
electrical energy flow of an electrical system. The series in
question, EM Chip is capable of measuring the energy flow (in or
out) of an electrical system by monitoring one or more power
rails. The 1st chip of the series, EM Chip is able to measure the
net energy flow over 4 different power rails. The chip itself is a
small QFN16 (4mm x 4mm x 0.5mm) or WLCSP16 (2.25mm x 2.2mm) package.
=== Theory of Operation
In order to measure the amount of energy entering/exiting a system,
a small shunt resistor is interleaved on a power rail. By measuring
the small voltage drop across the shunt resistor (a known value),
the electrical current is measured.
I = U/R
Depending on whether the system is consuming or producing energy,
the measured current value can be either a positive or negative
number.
To get the instantaneous power figure we will have to measure the
power rail’s voltage and multiply it with the measured value for
the current
P = V * I = V * U_shunt/R
Knowing the instantaneous power and by making a high enough sample
rate (for measuring the current and the power rail voltage), we can
assume the measured value for power is equal to the average power
figure for the amount of time between 2 sampling moments.
Now that we also know the average power for a given time interval
(dt_x = time between 2 sampling moments; sampling speed is known),
we can measure the energy amount entering or exiting the system
between 2 sampling moments
E_partial_x = P * dt_x
Having the energy information available, we can continue to add the
subsequent energy values for as long as the system is active. The
amount of energy is the sum of all the partial energy values
measured for each time interval E = Sum (E_partial_x), where x can
take values from 0 till infinite.
=== Chip Operation
The EM Chip chip uses the same principles of operation as presented
in the “Theory of Operation” and it does so for a number of 4
channels. 4 independent power rails can be energy monitored in the
same time. The chip is controlled over I2C/SMBus by an I2C/SMBus
master.
In order to reduce the amount of information traffic between the EM
Chip and the entity asking for its information, the chip features
large accumulator energy registers for the accumulated energy
values. It can accumulate up to 2^24 power values. Depending on the
selected chip’s sampling speed, such a number of samples are
produced in about 4.5 hours (fastest sampling speed) up to 582.5
hours or more than 24 days (lowest sampling speed).
EM Chip measures the power as a 28-bits number. The 28-bits number
is the multiplication result of the 16-bits number used to measure
the power rail voltage and the 12-bits number used for measuring
the voltage drop across the power rail shunt resistor.
If bidirectional energy flow is required, the power is measured as a
27-bits number and 1 bit for the sign. When no bidirectional flow is
needed, the power value is measured as an unsigned 28-bits number.
Due to the relatively large size of the accumulated energy
registers inside the chip (48 bits), in the worst case scenario
(power values are full scale numbers), a number of 2^20 full-scale
power values can be measured before energy register’s overflow.
Using the fastest sampling speed, the accumulated energy registers
overflows only after a bit over 17 minutes, while at the lowest
sampling speed, it would overflow in over 36 hours.
Thus, the chip requires infrequent reads of the accumulated energy
registers. Even in the worst case scenario, the time between 2
consecutive energy accumulator reads can be over 17 minutes !
In order to keep a longer history of energy measurements, an
I2C/SMBus master (e.g. SoC) would read the accumulated energy
register values and then use larger “soft” accumulated energy
registers to extend the maximum overflow period.
The EM Chip chip can monitor rails up to 32V. It can monitor the
energy amounts used by various sub-components of a system (e.g.
CPU, GPU, memory, hard-drives, USB ports, backlight, wireless
adapters, cameras, displays, …)
EM Chip is able to start operating immediately after power-up with
no CPU intervention at all. Such a feature is useful, because it
can show the amount of energy consumed by a system before the
latter finished booting its operating system.
The chip’s own current draw is around 20uA (in low-power mode,
lowest sampling speed - 8 samples/sec) to 675uA (for the highest
sampling speed - 1024 samples/sec)
When operated in low-power mode, it can be used to monitor the
stand-by energy draw of the system. As an example, such a mode is
useful when a system is suspended to RAM or to measure the energy
usage from the power on till the booting process is finished.
=== Linux Driver
While the chip is due to be publicly released in Q3 2017, a
selected number of PC OEM manufacturers will include one or more EM
Chip chips on their systems. We would like to include a driver for
EM Chip chip and its follow-up products, such that Linux Kernel
will be able to provide the energy information as soon as computing
systems using this series of chips will become available.
By providing accurate energy measurements, the computing systems
along with their operating systems will be able to run more
efficiently.
