[Cryptech Tech] Noise board on Novena

[Bernd Paysan]

Am Dienstag, 23. September 2014, 08:54:16 schrieb Fredrik Thulin:
> On Tuesday, September 23, 2014 06:04:58 AM Benedikt Stockebrand wrote:
> > Hi Joachim and list,
> > 
> > Joachim Strömbergson <joachim at secworks.se> writes:
> > > [Why SPI is a pain]
> > > 
> > > That (and a few other details) is what i mean with "don't like". ;-)
> > > 
> > > I'm spending a lot of time in another project with SPI. The positive
> > > thing with SPI is that it is commonly supported by many devices.
> > 
> > that's the point: I just wanted to make sure this didn't look like an
> > unjustified personal dislike, but like a justified one:-)
> 
> I agree - SPI has it's downsides. I'll be easily swayed in the matter if you
> can argue why some other alternative is better.
> 
> As I see it, the alternatives on this levels for the speeds we are talking
> about are SPI, I2C, UART, and bit-banging.
> 
> The appealing thing about SPI, I2C and UART in my mind is that they are well
> supported (often in hardware) among MCUs, and other people could take a
> entropy module speaking one of those, connect it to a Raspberry Pi (for
> example) and start interacting with it in about 10 lines of Python code.
> 
> This makes it easier for others to test, develop and audit entropy sources
> for Cryptech HSMs. The entropy sources would also be easier to reuse in
> other applications.
> 
> Bit-banging, although preferably with less than 32 parallel data lines,
> would probably be easier if we didn't have to care about other people, but
> I think we should care.
> 
> Let me hear your arguments for something else than SPI =)

I've used SPI, I²C and UARTs in different projects, and depending on the 
environment, I prefer different things. SPI has the advantage of an explicit 
clock and all signals are push-pull (whereas I²C is pullup+pull, which limits 
the speed).  The four modes are a disadvantage; usually I just commit to the 
one mode that makes sense (the others don't really ;-).

For things that connect to a PC, however, I prefer UART, because USB to serial 
is very cheap and easy to get (and supported out of the box by the OS), 
whereas USB to SPI requires a specific FTDI device and a special library.  
When I use UART protocols, I make sure that the first byte you will send over 
the UART is odd, so the device can do automatic baud rate adjustment (like 
"AT" modem commands - 'A' is odd: 65).  UART has the advantage of needing just 
two lines (RX and TX; when you talk to hardware, you don't need flow control, 
the hardware will always be fast enough to respond), and there's no mode 
inflation (when you use 8N1).

-- 
Bernd Paysan
"If you want it done right, you have to do it yourself"
http://bernd-paysan.de/
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