[Cryptech Tech] Novena dev-bridge board status

[Peter Stuge]

Jacob wrote:
> - R9 is 100 Ohm, meaning 50 mA max power to the design. The consumers on 
> the board, especially during switching, require more than that. The board 
> is probably starved.

Good point but not critical - very easy to rework.

I took a closer look at the power supply inputs - it seems that the
Micro-USB VBUS is connected to VCC, meaning 3V3, which isn't right.

Suggest add schottky diodes, e.g. one BAT54C if 200mA is enough for
the board, between 5V/VBUS and U1:

5V ---|>|-+
          +-- U1 VIN
VBUS -|>|-+


> - I don't see the Ref Des of the decoupling caps of the STM32F in the PDF, 
> so it is hard for me to verify association to the layout
> ( I suspect that their placement on board is not optimal).

Recommend the "Right The First Time" book that was posted already.
Quoting chapter 35 Power Subsystem Inductance:
--8<--
Capacitor Placement

Because of the low inductance of the power plane pair, the placement
of the capacitors is not as critical as the consideration of via
length and using the power plane close to the IC package side of the
PC board.

To show the effect of capacitor placement, a single 0.1 µF 0603
capacitor was meassured on four different locations on the PCB, as
depicted in Figure 35.9. The two coax measurement leads were on one
of the narrow ends of the board. [10.3" x 4.0" in size] The capacitor
locations were at approximately 2.5" intervals down the long
dimension of the board. Figure 35.10 shows a magnified view of the
impedance vs. capacitor placement.

Notice that the variation of the placement of the capacitor has only
an 8% change in the series resonance frequency and almost no change
in the parallel resonance frequency. The effects of the copper plane
resistance seem to be larger than the inductive effects.
-->8--

The book recommends designing power planes to function as capacitance,
rather than to rely on discrete capacitors, making for a much more
robust design.

Shorter traces are of course still always better.


//Peter