Forum : ST7/STM8
Original Post
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October 5, 2009 - 9:09pm
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We would like to be able to connect the R-Link to our STM8 application without connecting external power. Is this possible? I see that the printed "STM8/ST7 notes" that were included said, "Note: The RLink does NOT power the target board." However, there is a jumper (PW-5V) that looks like it can be used for this purpose. Where can I find the maximum current or power specifications? I was able to find the following schematic on your site |
Hi,
The VCC5 can indeed be connected to the VCC_TARG using the PW_5V jumper, so that the RLink powers the target board.
The VCC5 is the USB 5V through a diode.
This diode is either 10BQ015 or STPS1L30A, depending on the version of RLink.
That makes the actual value around 4.75V.
That means it can only work on target boards that will accept this voltage.
Now, the power available there depends on your USB hub, but it cannot exceed 500mA in the best cases.
We recommend to use this power scheme only if you have components that don't draw to much current.
Best Regards,
Vincent
Thank you, for your help! That is exactly what I wanted to know. This application should never draw more than 300mA, so the USB 500mA limit should not be a problem.
Vincent, I'm guessing from your post:
a) The RLink is enumerated on USB requesting 500mA.
b) The USB VDD power is shared between the RLink circuitry and what I might use at jumper PW-5V.
c) The RLink probably uses up to a few tens of milliamps, so most, but not all of the 500mA is available at PW-5V.
Can you confirm and/or enlighten further?
Thanks,
Mike Layton
Hi,
Yes all this is correct.
We never performed any precise measurements, and it can also depend on your USB HUB and the other USB devices that are connected, and on the operations that you perform (it will draw more during Flash programming of ST7s, for example) so we don't give any guarantee on this aspect.
However, our REva kits and Primers are powered this way and some of them include quite big components.
You can take a look at the schematics of the REva boards and Primers.
If you make any measurements, please publish them here, for the benefit of other users.
Best Regards,
Vincent
Sounds good. Thanks.
The only worrisome thing might be if the RLink used most of the 500mA, and you have confirmed that it doesn't.
As for measurements, I think the thing we could measure would mostly be how well the hub enforces its limits. In my experience, this is either not very well or not at all. (Not to mention that OS's I have used give no help on power consumption of the USB and devices.)
But I'm quite happy being able to draw a few tens of mA.
I am very happy finding this topic on forum. But I have a little doubt.
I am about to finish my isolated interface for RLink. For simplicity I supply it from RLink, from it's pin number 2 (of 24pin connector) named “host vcc5”. It will draw just a few mA.
But before I order the pcb and components I prefer to consult a problem that worries me a little.
As I remember (but not sure – read long time ago) USB has a state when you must not draw the full current from it. Not sure: enumeration? suspend? 500uA? 2.5mA? The USB controller (like FTDI) issues a signal then to cut the load off from USB supply. But I do not see such a signal in RLink 24 pin connector. So my interface will draw a current continuously. Isn't it the problem? Will they cooperate well?
Of course I could read more about USB but probably you just know it. And (more important) I can't guess what is inside the RLink. Maybe it contains a switch that cuts the supply off from the connector24 when necessary?
Thanks for any help
Robert
Hi,
No the RLink contains no system to cut the power during some operations.
There is just a protection diode, which is why VCC5 is around 4.8V and not 5V.
However, keep in mind that what you plan to do is exactly what we do in most of our demo boards (REva, Primer, etc.) which contain a RLink that powers the whole board. Some of them include very big components (STR9, LCDs, regulators, battery chargers, etc.) and we never observed any problems like those you describe. If your application only draws "a few mA", then there should be no problem.
The simple test for you is to put a resistor between pins 2 (VCC5) and 4 (GND) of the RLink, with a value that will make it draw a little more current than the maximum that you plan your application will draw. (100mA) Then plug the RLink and see if the resistor prevents it from enumerating. If you can read the RLink Serial Number, then it's fine...
I hope it helps.
Best Regards,
Vincent