RoadTest: Raspberry Pi 2: We're Giving Away 50 Units!!!
Author: k12h
Creation date:
Evaluation Type: Independent Products
Did you receive all parts the manufacturer stated would be included in the package?: True
What other parts do you consider comparable to this product?: Other parts: Power supply, power supply microUSB cable, Ethernet cable, desktop PC. Comparable products: Raspberry Pi model B
What were the biggest problems encountered?: Providing adequate power supply. In particular, proper microUSB cable is not trivial to choose.
Detailed Review:
I like playing with embedded devices, but at the same time I do not have much time for it. I also like applications that contribute to the community. Two cases of such applications are contributing computing power to BOINC scientific computations and contributing network bandwidth to Tor. While So while I am not using a device, why not make it crunch numbers for a RPi-compatible BOINC project such as Einstein@home and run a Tor relay?
I have already been running a Raspberry Pi model B for 2.5 years (well, two of them, one at home and one in a colocation) as a Tor relay, and tried it briefly for BOINC computations. It's limitations were that the Tor relay was often crashing because of lack of memory, and BOINC computations were competing for clock cycles with the relay on its single core. Pi 2 has 4 cores and twice the memory, so I am curious to see if how it performs in this applications. The obvious expectation is that more performance (at the same price) makes it better.
The Pi 2 I am testing runs without any peripherals connected, Raspbian, console-only through a 3.3V USB-to-serial cable (no X running), Ethernet connected.
I have spent more time looking into the power issues than into the initial applications.
Nothing really new here, there are enough materials on the internet on all the points below.
Here are links to additional materials that I find useful.
1. Power and stability issues
2. Einstein@home
3. Tor relay
The device needs adequate current and voltage to work. When they are not enough, the device works unstable. In particular, the CPU is automatically throttled to 600 MHz and peripherals start failing. In my case this was the onboard network adapter (which is a USB peripheral), so the device lost network connection. The onboard red power LED is connected to a voltage supervisor; the LED blinks when the voltage drops below about 4.67 volts (R-Pi Troubleshooting - eLinux.org). The more current the device uses, the higher the voltage drop on the power cable, the lower the voltage at the device.
Though the article Raspberry PI:- USB power cables, crashing and other problems is in depth and conclusive, I did some extensive testing to illustrate the issue more. I was just curious how different power supply configurations compare.
I was measuring voltage between the PP2 and PP5 test points with a multimeter when the device was under full load (all 4 cores were running Einstein@home tasks). The measurements are an average over about 10 seconds (using a feature of the multimeter). A consumer power meter was used with the socket power adapters to provide a rough indication of the power consumption. The figure below shows my quick "test bench".
The tables below show measurement results. For two power supplies I tried all the cables I had. If the specs are printed on the cable, they are given in the table.
| Cable | Samsung 5.3V 2.0A | ModMyPi 5.25V 2A |
|---|---|---|
| from Samsung Galaxy S4 mini (23AWGx2c + 30AWGx2c), 1m | 5.06 | 5.31 |
| from GIGABYTE GZ-G90B0 portable power storage, 1m | 5.18 | 5.38 |
| "USB 2.0 high speed mini cable", 1m | 5.03 | 5.20 |
| "USB 2.0 SHUNXINYUAN" (28AWG/1p 28AWG/2c), 1m | 5.09 | 5.28 |
| unidentified cable, 1m | 5.05 | 5.26 |
| from ModMyPi black (28AWG/1P+28AWG/2C), 0.5m | 5.12 | 5.32 |
| from modMyPi black (28AWG/1PR+26AWG/2C), 1.8m | 5.07 | 5.25 |
| from ModMyPi flat ribbon, 2m | 5.08 | 5.28 |
Conclusions: if the cable has no specs, consider it as a poor one, unless it comes from a power storage (in which case there is more reason for it to be good). If the cable has specs, you know what to expect.
Then I continued only with the best cable (i.e. the one with lowest resistance), from GIGABYTE portable power storage, to compare the rest of the power supplies.
| Power supply | Voltage at RPi, V | Power at the socket, W | Power at the socket with RPi disconnected, W | Notes |
|---|---|---|---|---|
| Samsung 5.3V 2.0A | 5.18 | 3.4-3.5 | 0 | |
| ModMyPi 5.25V 2A | 5.38 | 3.8-3.9 | 0.3 | >5.25 V at device, not good |
| Samsung 5.0V 1.0A | 4.96 | 3.3-3.5 | 0 | |
| Garmin-Asus 5V 1.0A | 4.98 | 3.6-3.7 | 0.1 | |
| Apple 5.1V 2.1A | 4.98 | 3.2-3.4 | 0 | |
| USB port on a PowerCube extender | 5.11 | - | - | |
| USB 3.0 PC (with AsRock AppCharger driver) | 4.83 | - | - | |
| USB 2.0 PC | 4.78 | - | - | RPi power LED blinking |
| USB 2.0 HG655D modem | 4.82 | - | - |
These results confirm the conclusions from Raspberry PI:- USB power cables, crashing and other problems. A good USB cable is essential, a over-voltage power supply is not needed. It appears that a power supply from ModMyPi leads to more than 5.25 V at device even with some poor cables (while 5.25 V is the upper threshold in the USB standard).
Moreover, with a good USB cable, powering from a PC USB port that is capable of providing more than the standard 500 mA (as in my case of an AsRock motherboard with the AppCharger driver installed in Windows 7) makes it possible to run applications on RPi that fully load the CPU. In this case, my rough measurements lead to an estimate of about 700 mA.
