Post by spiff » Sat Mar 21, 2009 11:21 am
First, thanks for your reply. I'm not trying to get into an argument here, but simply understanding where the limitations are. Since the electrical performance and error correction is not up to spec anyway with AVR-USB, the device will never be fully compliant anyways, but it is always nice to take conscious decisions about where the design is not up to spec.
Grendel wrote:Well, first, the 500uA are really only 300uA -- a host has 15k pull-downs on D- & D+, you get a 16.5k base load to Vterm; 200uA.
That is new to me. What I don't understand is why they specify 500uA for the device if the host consumes some of that.
Grendel wrote:If you want timely wakeup you should only use Standby (note that INT0 can only be used as a level ISR) -- ~150uA if you also shut everything else down (BOD, AC, ADC ...)
I agree there is not much fun in shutting down the uController in a way where it cannot be woken up again
Grendel wrote:An LDO is not only expensive (~$7, at least the one I looked at @ DigiKey), but also has a GND current of up to 200uA, 550uA already. Plus, you can't issue a remote wakeup to the host since you would have to get out of standby for that.
There are plenty of cheap LDOs (<$2.5 on DigiKey), and also with a much lower quiescent current. Usually the GND current is with a much higher load than what we are looking at here. The parameters change a lot if you don't require too much power in the operational state. For instance if the circuit operates at <50mA, this could be used:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM2936Z-3.3-NDGrendel wrote:IMHO it's way too much a hassle to meet the specs. But if you do, let us know

As stated earlier, I just want to have considered it, and actively decided to let it slide. There is a high probability that I will end up with the same conclusion: "It's way too much a hassle to meet the specs."
But I'm getting sidetracked here. Can someone tell me what the requirements for the pull-up on D- is. As I see it, a pull-up to 5V is not the same as a pull-up to 3.3V. Perhaps this is the reason a 2.2K is fine when it pulls up to 5V?
First, thanks for your reply. I'm not trying to get into an argument here, but simply understanding where the limitations are. Since the electrical performance and error correction is not up to spec anyway with AVR-USB, the device will never be fully compliant anyways, but it is always nice to take conscious decisions about where the design is not up to spec.
[quote="Grendel"]Well, first, the 500uA are really only 300uA -- a host has 15k pull-downs on D- & D+, you get a 16.5k base load to Vterm; 200uA.[/quote]
That is new to me. What I don't understand is why they specify 500uA for the device if the host consumes some of that.
[quote="Grendel"]If you want timely wakeup you should only use Standby (note that INT0 can only be used as a level ISR) -- ~150uA if you also shut everything else down (BOD, AC, ADC ...)[/quote]
I agree there is not much fun in shutting down the uController in a way where it cannot be woken up again :wink:
[quote="Grendel"]An LDO is not only expensive (~$7, at least the one I looked at @ DigiKey), but also has a GND current of up to 200uA, 550uA already. Plus, you can't issue a remote wakeup to the host since you would have to get out of standby for that.[/quote]
There are plenty of cheap LDOs (<$2.5 on DigiKey), and also with a much lower quiescent current. Usually the GND current is with a much higher load than what we are looking at here. The parameters change a lot if you don't require too much power in the operational state. For instance if the circuit operates at <50mA, this could be used:
[url]http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=LM2936Z-3.3-ND[/url]
[quote="Grendel"]IMHO it's way too much a hassle to meet the specs. But if you do, let us know :)[/quote]
As stated earlier, I just want to have considered it, and actively decided to let it slide. There is a high probability that I will end up with the same conclusion: "It's way too much a hassle to meet the specs."
But I'm getting sidetracked here. Can someone tell me what the requirements for the pull-up on D- is. As I see it, a pull-up to 5V is not the same as a pull-up to 3.3V. Perhaps this is the reason a 2.2K is fine when it pulls up to 5V?