Role for FPGA or CPLD with Raspberry Pi

@ John Beetem,

The Spartan chip on the Special Computing board is £8.60 one at a time from Farnell.

Let's assume a pcb 50mm x 75mm, FPGA image stored in RPi, a little processor to manage FPGA boot up (might not be needed), 3.3V regulator, 1.2V regulator,

To keep it cheap we'll clock the FPGA from the uP's built in oscillator.

If this is for playing with we need some buffering and 5V or better drive - so we'll stick on a couple of ULN2003A which are dead cheap and can switch a bit of power.

These chips are at mega risk so we'll use DIL parts in sockets

And we had better have some buffered inputs to protect the FPGA - so we'll have 4 x dual comparators with resistor input networks to give us some toughened inputs.

All the component prices are from Farnell's website today and for one off. The pcb prices is 50 off from pcbtrain.

It should be quite easy to do better than these prices so all it needs is for someone to finance the board.

I still think the Lattice XP2 is a better choice than the Xilinx part (on chip flash, easier to use (IMO) free tools and I always feel that because Lattice are the PGA underdog they try harder.

Uisng a 100 pin XP2 might push the price up a tiny bit.

Anyway -the point of all this - if you can use a soldering iron an FPGA add on board that you could use with an RPi or lots of other things should cost you <£25.

If you want it built and tested it's a bit harder to guess - all depends on volume but 50 isn't enough to be viable (take too long to explain why here) - you need to think in terms of 1000 at which point the BOM cost will be down to about £13 - so think of it retailing at £40 + VAT.

.

If there is any interest I can take it further .... I'm quite happy to do the hardware design (and probably the ARM code)  as an open source project - with all this talk in the UK of encouraging engineering can anyone get a grant to apy for some boards - I don't mind spending time but I can't put actual cash into it.

Michael Kellett

@ John Beetem,

The Spartan chip on the Special Computing board is £8.60 one at a time from Farnell.

Let's assume a pcb 50mm x 75mm, FPGA image stored in RPi, a little processor to manage FPGA boot up (might not be needed), 3.3V regulator, 1.2V regulator,

To keep it cheap we'll clock the FPGA from the uP's built in oscillator.

If this is for playing with we need some buffering and 5V or better drive - so we'll stick on a couple of ULN2003A which are dead cheap and can switch a bit of power.

These chips are at mega risk so we'll use DIL parts in sockets

And we had better have some buffered inputs to protect the FPGA - so we'll have 4 x dual comparators with resistor input networks to give us some toughened inputs.

All the component prices are from Farnell's website today and for one off. The pcb prices is 50 off from pcbtrain.

It should be quite easy to do better than these prices so all it needs is for someone to finance the board.

I still think the Lattice XP2 is a better choice than the Xilinx part (on chip flash, easier to use (IMO) free tools and I always feel that because Lattice are the PGA underdog they try harder.

Uisng a 100 pin XP2 might push the price up a tiny bit.

Anyway -the point of all this - if you can use a soldering iron an FPGA add on board that you could use with an RPi or lots of other things should cost you <£25.

If you want it built and tested it's a bit harder to guess - all depends on volume but 50 isn't enough to be viable (take too long to explain why here) - you need to think in terms of 1000 at which point the BOM cost will be down to about £13 - so think of it retailing at £40 + VAT.

.

If there is any interest I can take it further .... I'm quite happy to do the hardware design (and probably the ARM code)  as an open source project - with all this talk in the UK of encouraging engineering can anyone get a grant to apy for some boards - I don't mind spending time but I can't put actual cash into it.

Michael Kellett

arg - the table looked OK in the add reply box - it's useable as it is so I'll leave it.

Michael Kellett

Terrific BOM cost estimate, thank you Michael!  And using DIL packs for the buffers is very much "designing for reality", given the target user in mind.

Chasing around Lattice XP2 links, I see that the Brevia dev kit http://www.latticesemi.com/products/developmenthardware/developmentkits/xp2brevia2developmentkit.cfm originally launched with a promotional price of $29 !!!  Damn, wish I'd known at the time.   It's still not too bad today at $49, although I can't seem to find a UK supplier.

Lattice XP2 device prices seem to be about double those for Xilinx XC3S200A, UKP 14.91 for 5K LUTS from Farnell for 144TQFP -- http://uk.farnell.com/lattice-semiconductor/lfxp2-5e-5tn144c/fpga-5k-luts-100-i-o-dsp-144tqfp/dp/1571999 and UKP 16.14 from RS for 100VQFP -- http://uk.rs-online.com/web/p/fpga/7030840/ .

For personal use where BOM cost is not very critical, the higher device price of XP2 versus Spartan-3A is probably not a hurdle, if there are good reasons for choosing Lattice as Michael mentioned.

Lattice don't do a part in the 4000 LUT range in a 100 pin package so that hurts a bit on price.

Another nice Lattice candidiate is the MachXO2  - a 144 pin 4000LUT part is just under $15 one off from their website.

It's an FPGA/CPLD crossover but has hard logic for SPI, I2C and some other goodies. No multipliers.

I'll go thinking but must rush now having just had word of a six person canoe going cheep !

Michael Kellett