Thread: 770 USB Host Mode - Knowledge base
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Simon is offline Simon
2006-01-12 , 17:06
Posts: 191 | Thanked: 9 times | Joined on Nov 2005
#38
Originally Posted by bhima
Did you build in any regulation or not? I'm planning to build a battery powered one to complement my current externally powered unit, and would obviously like to avoid the crashing problem. I'm hoping that a regulator which simply cuts out totally rather than slowly drops the voltage will help fix that.

Another issue to be aware of is that you are violating the USB spec if you provide live data to a device without providing power first - USB cables are designed to ensure you get power first. I don't know if this will cause damage, but some semiconductors are unhappy if you do that.
Given what you say I might remove the power switch from mine. It is unessary anyway. If I can I might put in a low current LED that switches off when my battery voltage drops too low. Maybe using a zener somehow? Tricky with NiMH since when considered flat they are still giving out about 1.1 volts I think? I tbelieve most regulators like 7805s and so on just stop regulating when the input voltage drops too low rather than switch off. They need so many volts above the regulated output to keep regulating. When the input voltage drops below their cut off point you just get out what goes in. I used a low drop out type (2940 from memory) which regulates down to .5 a volt above rated. So my 5 cell NiMH battery will give say 6 volts fully charged (actually more) and 5.5 volts flat which is where the regulator stops regulating anyway. I need my LED to turn off then so I know to recharge it.

Be aware I an not an EE! I am just a tinkerer Things I say or do probably make real EEs cringe! I am actually a low drop out EE. I did the first year of the EE degree then changed to computer science. Much less maths!

Mike, there are instructions on how to enable and disable host mode from within a terminal with a Linux command but you need to have root access to do so. I am a little rushed now or I would post the links for you but if you search the forum for the text 'echo host >' you should find the relevent posts.