Thursday, March 31, 2016

The demise of the BSS83

I learned yesterday that the NXP BSS83 has been discontinued  as "non-manufacturable" and is no longer available. The last order date was December 2014 and the last delivery date December 2015. When I bought components for this project my intention was to order more than enough for the complete board set, but now I'm going to have to make sure what I have on-hand is enough. Fortunately 4-terminal FETs are only required in the pass-gate configuration and a few other critical spots.

I heard this from someone who is also interested in building a discrete component version of the 4004. Welcome to my insanity, Aston!

Over the years I've come across various FETs that could be used for such a project. The BSS83 RF FET was a good choice because it has a reasonable threshold voltage and a low gate capacitance (1.5pF), though it was N-channel rather than the 4004's P-channel construction. Others I identified included the N-channel Calogic SST215 (which might be a drop-in replacement for the BSS83) and the P-channel Micrel MIC94050. Tim McNerney suggested using the P-channel dual-FET BSS84 with the two transistors wired in series source-to-source (or drain-to-drain).

Aston has started experimenting with the MIC94050, which is now manufactured by Microchip. He asked whether it would work in the bootstrap load circuit given its low on-state resistance. My answer is it depends on what you're trying to do with it. My attempt to recreate the bootstrap load circuit was only to understand why this circuit was needed and how it worked.  My test circuit added several other components including current-limiting resistors, and I was able to recreate the circuit's operation well enough to understand it.

Tim McNerney, who is far more focused on recreating the exact design than I am, told me that Federico Faggin told him not to try to recreate the bootstrap load in a discrete component implementation because he'd never be able to match the characteristics with off-the-shelf components. I concur. You  end up with a complicated circuit where a simple resistor will do the job.

Fortunately bootstrap loads aren't necessary if you use true resistors as loads, as resistors don't turn off when the voltage across them drops below a FET's turn-on threshold the way FET loads do.

What other challenges might a hobbyist face when using the MIC94050 rather than a BSS83? I'd guess speed and power consumption will be the biggest factors. The MIC94050 has an input capacitance of 600pF, compared to the BSS83's 1.5pF and the FDV301's 9.5pF. For a given load resistor value, this will greatly increase propagation delays. To put some numbers to it, let's look at the rise-times with a 4.7K load pulling up the gate of a single FET. Trc with a BSS83 is about 7ns, and with an FDV301 about 45ns. With the MIC94050 it's 2.8us, which is more than double the 1.35us cycle time of the 4004.

To achieve a 45ns rise time we'd need to use 75 ohm load resistors, which would draw 67mA when pulled low. Assuming half of the 473 resistors are being pulled low at any time that's 14 amps at 5 volts, or about 72 watts. That'd keep your hands warm in the winter!

Worse, I haven't yet identified the longest combinational path through the 4004 logic. My recreated Instruction Pointer board appears to work quite nicely with a 2.0us cycle time, with the rest of the CPU emulated by an FPGA, but I think the critical path is in the ALU circuits. Thus I'm not sure even 45ns is fast enough to achieve a 1.35us cycle time.

Also consider the drive requirements for the CLK1 and CLK2 paths. CLK1 has to drive 28 FETs in parallel, and CLK2 has to drive 51. A little math says that's 17,000pF and 31,000pF respectively. The TC4427A Power MOSFET driver I used on my FPGA interface board is specified to drive 1,000pF at 5V in about 50ns, and has performance curves that show it driving 2,200pF in about 90ns, but we're still off by a factor of about x14.

All is not lost. The leakage current in modern discrete components is a lot lower than it was early in the development of integrated circuit technology. My DRAM test circuit appeared to hold its state for several seconds, while the 4004 refreshed its IP and Scratchpad registers every 500us or so. Since running a recreated 4004 built with MIC94050s at 741KHz isn't going to be an option, take advantage of the low leakage and run the system at a (much) lower speed. That will make everything easier to work with too (not everyone can splurge on a 1GHz digital oscilloscope).

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