Hi,
I have a doubt regarding USB voltages and the Primer2. According to the "Universal Serial Bus Specification Revision 1.1" document, the line D+ must be pulled up with a 1.5K resistor connected to Vterm. This Vterm is said to be in the 3.0V (min) to 3.6V (max) range.
In the Primer2 schematic, this 1.5k resistor (R38) is pulled up to 2.8V, and having into account the BC847 saturation voltage (0.2V max), it must be about 2.6V worst case, which is 0.4V lower than the 3.0V required by the specs.
At the specs it is also said that the pull up resistor should pull up the D+ line up to a voltage higher than Vih (min), which is 2V. If we also have into account the voltage divider formed by the internal pull down of 15k of the USB hub, it seems to be accomplished by the Primer2, since 2.6V/(15k+1.5k)*15k = 2.36V > 2V. But then I wonder 2 things:
1) Why 1V of margin between Vih (min) and Vterm (min)? I think it is excessive, having into account most modern devices work at low voltages...
2) Since the Primer2 works, it means it is OK to use the Vterm=2.8V. But then, could you say your device is USB compliant (even if it doesn't accomplish this single spec)? I'm not asking about Primer2 anymore, just any device using Vterm < 3.0V.
And there is something else. At the STM32F103 High densisty family datasheet, it is said the USB minimum operating voltage is 3.0V -> "The STM32F103xx USB functionality is ensured down to 2.7V but not the full USB electrical characteristics which are degraded in the 2.7-to-3.0V VDD voltage range". What does "degraded" means? Does it means the Primer2 USB may have communication problems?
Thank you!
Last edited by brothe_r (2011-03-20 16:49:58)
