Monday, 27 October 2014

Part One: Time for custom OTP generation hardware ?

I have been looking at how to generate good random numbers and also at the Raspberry Pi's maximum data throughputs rates using wifi (9.9MiB/s), ethernet (11.2MiB/s) and disk (22MiB/s). The Raspberry Pi is good at some things, but it is not very good at moving large amounts of data about really fast. If it was I'd probably look at some way of generating good random numbers and transferring in the data in via the GPIO pins. But even doing this, the GPIO pins have a limit of about 10MHz maybe, some testing would need to be done to see if this could work or if the data rate could be pushed higher (use more pins to transfer more bits in parallel).

I looked into using the BeagleBone Black, but it also only has 10/100 Ethernet (circa 11.2MiB/s). And if I made a custom board with the 324 BGA single core 720MHz-1GHz  Cortex-A8 CPU it probably would not have enough oomph to fully saturate the gigabit built into the TI AM3359 CPU. OK, even if it could generate OTP data at a gigabit per second rates, I could not easily push it to a disk at that rate, so maybe the chip is good enough. It does include a RNG Crypto Hardware Accelerator (/dev/hwrng), but I trust that about as much as I would trust the similar RNG in the Raspberry Pi. The AM3359 does have a 12-bit ADC 200KSPS inbuilt but that would be too slow to sample a noise source for random numbers to generate random numbers at any decent rate. And the chip are a little bit on the expensive side as well:
AM3359ZCZD72  1+ @ €31.47; 15+ @ €29.11; 50+ @ €27.14; 100+ @ €26.45 (April 2014).
AM3359BZCZA80 1+ @ €47.48; 10+ @ €45.20; 25+ @ €42.53; 50+ @ €40.11 (April 2014).
AM3358BZCZ672 1+ @ €39.60; 10+ @ €37.08; 25+ @ €34.98; 50+ @ €31.06 (April 2014).
AM3358BZCZ100 1+ @ €46.84; 10+ @ €44.61; 25+ @ €41.97; 50+ @ €39.59 (April 2014).
AM3357BZCZA80 1+ @ €41.97; 10+ @ €39.97; 25+ @ €37.60; 50+ @ €35.48 (April 2014).

Refer to "8.1.2 Device Nomenclature" in the AM335x datasheet for the exact meaning of the Suffix's.

The Airspy project is using a  LPC4370 ARM CPU which is slow at only 204MHz in comparison to a TI AM3359, but it does have one very nice feature a 12-bit 80MSPS ADC. It also has three ARM  cores a Cortex-M4 and two Cortex-M0's.There is a 100 BGA chip with 3 multiplexed ADC channels or a 256 BGA chip with 6 multiplexed ADC channels.
LPC4370FET100 1+ @ €10.77; 10+ @  €9.46; 25+ @ €8.43 ; 50+ @ €7.19 (April 2014)
LPC4370FET256 1+ @ €14.85; 10+ @ €12.97; 25+ @ €11.58; 50+ @ €9.86 (April 2014)

The airspy project was spawned into existence by the HackRF project which originally used a LPC4330,  well now a LPC4320 (no need to 10/100 Ethernet). This chip has the advantage of not being a BGA device, it is a 144 pin QFP, but the data, in the case of the HackRF One,  is transferred from a separate ADC to the ARM using SGPIO.

LPC4320FBD144,551 1+ @ €8.78; 10+ @ €7.55; 50+ @ €6.19; 100+ @ €4.20 (April 2014)
LPC4330FBD144,551 1+ @ €9.89; 10+ @ €8.50; 50+ @ €6.98; 100+ @ €4.43 (April 2014)


The complexity of using SGPIO and the cost of a separate high speed ADC using a LPC4320, makes the  LPC4370 with its inbuilt 80MSPS ADC look like the best overall cheapest solution for what I want to do.

The fact that the Airspy has a 80dB Spurious-free dynamic range that I could use this to amplify the noise signal from a reverse biased zanier diode or a npn transistor acting as a  germanium diode is nice. Because the noise signal needs about 40dB of gain to be usable. One problem with using the Airspy is that it is only sampling at 20MSPS and not 80MSPS. But as a open software test platform, that could be used to see if making a dedicated board with the LPC4370 BGA chip is feasible or not and it is a good easy first step. The Airspy will have two high speed ADC inputs (using a u.FL connector) for the two spare multiplexed ADC channels on the 100 BGA chip. After the initial tests these ADC ports could be used with a custom firmware to test sampling at 80MSPS, 40MSPS per channel from two shielded secure noise sources.

The main advantage of doing some testing using an Airspy would be not having to design and build a fully custom development board with this 100-256 BGA chip. Ok in the short term it may cost a bit more, but if it does not work I would still have a very useful SDR receiver. Unfortunately there is no way to push more then about 40MiB/sec through USB 2.0. So even though I could be sampling 80MSPS at 12bits (120MiB/sec), I would need to concentrate this entropy by a factor of three. And that is provisional on there being enough spare CPU headroom available to do so before transferring the sampled noise to a PC. The fact that the chip supports SIMD (ARM NEON) may make this just about possible with a good pinch of lateral thinking.


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