a month ago •
I'm guessing that very few of you know that USB certifications impose a very strict limit on a USB connected device's inrush current.
What's the inrush current?
It is basically the current that a device receives when it is connected to a USB port. It is usually high given the numerous decoupling capacitors on a board.
How to measure it?
One of the solutions is to do like I did: put a shunt resistor between your device and power supply, measure the voltage across it. As I'm sure you already know, it will represent the current going through it.
What should I see above?
This is the voltage across the shunt resistor when the device is first plugged to my computer. The USB certification specifies a maximum 50uC charge (Q = I*dt integrated) when the current is above 100mA.
Looking at the oscilloscope you can see a 500mV vertical axis, corresponding to I = V / R = 500/1.5 = 333mA. The horizontal axis is 20us.
What we are going to do simply count the number of "blocks" for given time bins (red zone). Let's say we have 5 time bins here:
- first time bin has 4 blocks (1.333A)
- second has 3 blocks (1A)
- third has 2 (0.666A)
- 4th and 5th have 1 (0.333A)
Yes, this is a very approximate measurement ;). Anyway, this gives us: (1.33+1+0.66+0.33+0.33)*20us = 53uC!
This is therefore very close to the norm.... we should reduce one of our capacitors' value then!
a month ago •
Dear Mooltipass enthusiasts,
I know I promised you guys more frequent updates but as you can guess things have been quite crazy during these last weeks ;).
Here is our newsletter #4:
- As you may have seen in the latest Mooltipass article our current hardware is considered as final. This is a major step, meaning that we can concentrate all of our resources on the firmware development process and proceed with the beta testers program.
- Speaking of the latter I'm currently busy asking quotes to different assemblers... once I get them a final contribution price will be agreed on and selected beta testers will be contacted
- Missed the beta testers call? Here is a quick form you can still fill. Please note that very few will be selected from there as we want to privilege people that have been supporting us from the start
- All the low level libraries (except the node management code) are finished!
- A "new" contributor is currently designing our user interface. We especially asked a non-geek person as we want our GUI to be as simple to use as possible. Here is a draft, what do you think?
- The Mooltipass has been tested on Linux / Mac / Windows / Android and we are extremely happy to inform you that it doesn't require any driver to function!
- A chrome plugin (using the chrome.hid library) has been created and tested together with the Mooltipass.
As you may have understood we're very close to getting the Mooltipass in our supporters' hands. Words of encouragement are strongly welcome as they keep us going during this long process :).
2 months ago •
Latest smart cards, cases and front panels!