[Cryptech Tech] STM32F4 <-> Novena Bridge

[Fredrik Thulin]

On Thursday, June 11, 2015 09:50:26 PM Peter Stuge wrote:
> Fredrik Thulin wrote:
> > > I've started this task, see the attached picture for tentative board
> > > size
> > > (10x5 cm) and placements.
> > 
> > Finished routing of the FMC signals. New picture attached.
> 
> Sorry that this comes only after they've been routed - is there a
> reason for the STM32 to be on top instead on bottom?

I suspect I will be assembling these bridge dev boards myself, so reflow 
soldering will work best with as many components as possible on the top side. 

The FXP connector on the bottom side I can solder by hand.

Other than that, no real reason.

> It looks all right, but moving vias around a little so that a trace
> which goes straight on top and wants to go left on bottom does not
> first go diagonally right to a via on top and then straight left on
> bottom, but instead goes diagonally left to a via on top, and then
> continues left on bottom. Does that make sense? It's a minor
> optimization to reduce reflections and make things neater.

No sorry I can't really make sense of it. Please make an annotated image 
showing a suggested move, like Pavel and Jacob did while reviewing the Novena 
noise board a couple of weeks ago. Those were easy to understand.

Another consideration: except from the right hand side, I tried to not route 
things inwards underneath the STM32, in order to leave room for all the  
necessary VDD/VSS vias that I will add next.

I also expect to add an FT232H USB-UART chip, and to try and route a couple of 
SPI, I2C and GPIO interfaces to headers on the edges of the board. And some 
LEDs.

...
> > (Pavel) helped me do some back-of-the-envelope calculations in
> > Stockholm that said the traces should not differ by more than 150
> > mm (!) for IIRC 100 MHz signals so maybe it's not worth any more
> > effort at all?
> 
> --8<-- Chapter 10: Time and Distance, Right The First Time
> Electrical signals travel as electromagnetic energy. This EM energy
> travels at the speed of light ... roughly one foot or 30 centimeters
> in a billionth of a second or a nanosecond (10 -9 second). When this
> electromagnetic energy must travel in a dielectric or insulator, such
> as in a PCB, it is slowed down by the dielectric. In a PCB, the
> dielectric slows the signal down to roughly half the speed of light
> in a vacuum. As a result, a signal will travel roughly 6” or 15
> centimeters in one nanosecond along a trace in a PCB.
> -->8--
> 
> When trace lengths vary less than 150 mm then signals can be said
> to be synchronized to within less than 1 ns. 100 MHz has a 10 ns
> period and 10:1 is a safe margin.

Thank you for providing the information describing the calculations we made, 
good stuff.

/Fredrik