Power to the Pii-ple

The third revision of the Raspberry Pi can best be summed up by the old adage of more of the same.

 
The third revision of the Raspberry Pi can best be summed up by the old adage of more of the same. A faster processor and Power over Ethernet capability were advertised – OEMsecrets tells you what you need to know.

Raspberry Pi’s are always sold via the ecosystem. This is a promise which the foundation, by and large, manages to keep: if you use a sufficiently recent version of RaspBian so that the new SOC is supported, the same memory card can also be used in older versions of the process computer. When looking at the thing from the top, not many differences can be seen. The most important change is the addition of the four pin header for the Power over Ethernet hat: it might cause problems with some cases. Other than that, the physical dimensions remain the same.

One interesting aspect is the heat spreader above the SoC. Eben Upton’s engineers used this opportunity to redesign the PCBs stack, thereby improving heat dissipation into the ground plane. This has two effects; first of all, the nominal maximum frequency has now increased to 1400 MHz from 1200 MHz. An important side effect is the ability of the process computer to sustain performance for longer runs – this can be shown in the figure which plots the temperature against the time when the process computer’s SoC is under heavy load.

The actual increase in processing power is rather modest. A short run of SysBench reveals the following results – the amount of RAM was not increased from the predecessor:

pi@newraspberrypi:~ $ time sysbench –test=cpu –cpu-max-prime=20000 –num-threads=4 run
real 1m22.757s
pi@oldraspberrypi:~ $ time sysbench –test=cpu –cpu-max-prime=20000 –num-threads=4 run
real 1m35.085s

The Raspberry Pi foundation, of course, stays true to the MicroUSB port used for power supply. Some pundits claim that the plug is over strained – recently, MicroUSB plugs capable of carrying 3A have been provided by companies such as Wurth’s WR-Com series.

As for the actual power consumption, it increased a small bit. Our table contains some values which were obtained from a HP 6624A system power supply.

Measurement RPi IIIB RPi IIIB+
Desktop showing 0.26A 0.39A
Full SysBench load 0.73A 0.96A

With that, it is time to sum up the first article on the new Raspberry Pi 3B+. Tune in soon for a second part looking at the network engine in more detail.

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ST Distance

Measure distance with ST’s VL53L1X distance sensor

 

Creating “color” images is not new. Microsoft’s Kinect sensor introduced developers to getting spatial information: a trick which STM now make manageable via their laser-based distance sensor monoliths which are ideally suited to drone landing sensors, distance-sensitive ignitors and other trinkets.

All in one…

Unlike the sensor used in the Kinect, ST’s sensor is “one-dimensional”. The optical window at the top of the chip acts as viewfinder and emitter, while distance to “objects” in front of the detector is returned as a one-dimensional value (think: something is 50cm away from me). Furthermore, the use of LASER technology makes the system almost completely independent of the target material – the color-related problems known from classic IR components should not occur here.Host-Sensor connectivity is accomplished via the I2C bus: sadly, ST does not expose an address selector, thereby limiting you to one sensor per bus controller. The actual communication protocol is described in a seperate document: as ST loves to change its URL schemes, simply google for UM2356 to find it. Furthermore, a driver written in C is made available to aid implementors.

While current consumption is moderate in the range of less than 20mA when active, users must be aware of the maximum supply voltage of 3V5. This makes integrating the system into 5V I2C busses difficult – a level shifter made up of two FETs is a workaround (see directionalLevelShifter and application-note).

The inclusion of gigabit ethernet raised quite a few eyebrows: after all, the actual SOC is not able to provide more than about 480 Mbits of total bandwidth to all of the peripheral devices. In practice, this limitation shows up doing iperf runs:

 

Calibrate me!

Most sensors require modifications to case design: without a cutout “to look out from”, all kinds of strange problems occur. ST simplifies this by a dedicated calibration routine, which should be run on every unit in the factory. This also weeds out individual optical differences in the “window”, and accounts for any changes caused by reflowing.

Another nice aspect is the ability to set the “region of interest”. While the field of view, by default, is quite wide, the optical array can be tuned to limit it. In an eery similarity to the above-mentioned Kinect, detection range also is specified in “classes”.

The only disadvantage of the part is the insanely small footprint. ST uses a non-leaded case (Optical LGA) which is but 4.9×2.5×1.56 mm small: while the package can be reflowed with ease, fitting it to a prototype using a soldering station is borderline impossible.

As with all new chips, availability is an issue. On OEMsecrets, prices range from 2.8€ to 5€ in small quantities – as always, a price comparison is your wallet’s friend.