| Stargate Power Modifications |
Last Modified: 2006-08-21
The Stargate processor board and daughter card combination uses the PXA255 XScale processor by Intel which and has onboard power regulation for USB peripherals, 10/100 base-T network interfaces, PCMCIA and CF cards, and the multiple power requirements for the XScale processor itself. There are several ways to power the Stargate board. This HOWTO attempts to address the issues around these options and give you a sense of how to best power the board for your particular circumstance.
Stargate Main Board Regulation
The Stargate main board has a power connector and regulation for the processor only. The processor is fed 3.3V via a Micrel LDO (371013.0-BM) regulator that usually starts with a clean 5V supply and thermally regulates this down to the 3.3 required by the XScale and peripheral chips. This regulator is a 1A peak regulator so it can handle the basic requirements of the XScale's family of components as well as the 3.3V supply for the PCMCIA and CF slots (when jumpered to 3.3V). If you have a Lithium battery connected directly to the Stargate board (the folks at Intel in the Ubiquitous Computing Group have this), this regulation is all that is needed to run the board.
Daughter Board Regulation
The daughter card for the Stargate includes an additional 5V LDO ( Micrel 371015.0-BM) regulator onboard which supplies the USB host connector on the card as well as the ethernet port. This is also a 1A peak regulator. When using the daughter card, the 5V from the daughter card's regulator is fed to the Stargate's 3.3V regulator. The daughter card offers parallel connections for the wall supply jack and a convenient 0.1" 2-conductor header for alternative supply.
Alternative Power Supplies
Since the Stargate is such a great mobile compute device, it is common to want to power this from some other power source. At Acroname, we use the Stargate in robotics applications so we typically want to utilize power we already have in place on the robots to supply the Stargates. The 37101 family of regulators have a maximum supply rating of around 16V but keep in mind that the power is regulated through the dissipation of heat. To drop 500 mA of power from 10V to 5V requires the regulator to shunt 2.5 Watts of heat. That is a bunch of heat for a small SO-8 package. Due to real-estate and design considerations, these chips are mounted on the board with minimal exposed copper which can help dissipate the heat. That means if you run the board off a large voltage supply (say 12V) and hook up a bunch of peripherals, the regulator will thermally fail.
At Acroname, when we insall a Stargate on the Garcia robot as an option the user selects, we add a small heatsink directly to the chip on the daughter card (U7) to help in the dissipation of heat. When running in this configuration with a small CF 802.11b card and the main processor, this sink gets warm to the touch but not too hot (with a 7.2V NiMH battery pack).
The heatsink fins can easily be picked out on the modified Stargate for the Garcia robot.
We did some testing at Acroname for the Garcia robot using a Stargate on the Garcia. The Garcia uses a high-capacity 3000mAh NiMH battery pack at 7.2V (6-cells). This pack can power the robot alone with no peripherals for 9 hours of continuous running on smooth floors. We added to this base configuration, a Stargate processor board and daughter card. In addition, we configured this board with a AmbiCom CF wirless 802.11b card. The Stargate was then booted and the wireless configured.
When idle, the processor, daughter card, and wireless combination consumed around 290 mA of power. This is pretty amazing for a completely funcitonal Linux computer with an 802.11b connection up and running. We then did a pervasive directory traversal and output to an ssh session that cranked up the speed-stepping processor and also spewed TCP/IP traffic over the 802.11b network. We feel this represents a maximal-use scenario and this brought the current consumption up to just shy of 500 mA of power consumption, well within the range of the onboard 5V LDO regulator of the daughter card. Running this way, the heatsink (likely required at these currents) was quite warm not painful to the touch.
USB Power considerations
USB can consume maximally 500mA for each connected periperhal, depending on the power needs of these devices. Since the Stargate has one USB port on the daughter card, a low-power USB device could directly plug in. If you are regulating from a higher voltage than 5V on the daughter card, great care must be taken to not exceed the capacity of the onboard LDO regulator which in practice is near its effective limits in the scenario described above in our power consumption tests.
One very easy way to extend the power capabilties of your USB is to use a powered USB hub. These are very innexpensive and readily available in 4-port configurations and more. These hubs typically take 5V power and the wall supplies included with them offer around 2.4A of power (that allows 4x500mA for each port and some overhead). Adding one of these to the Stargate on a mobile robot with a seperate 2A voltage regulator from the robot's battery would offer great expansion and allow scaling the power needs without sacrificing too much in terms of efficiency. It would also place the peripheral power consumption burden on the USB hub's source, not the Stargate's.
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