[HN Gopher] An unusual 7400-series chip implemented with a gate ...
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An unusual 7400-series chip implemented with a gate array
Author : codezero
Score : 141 points
Date : 2024-03-30 17:33 UTC (1 days ago)
(HTM) web link (www.righto.com)
(TXT) w3m dump (www.righto.com)
| formerly_proven wrote:
| Sort of the opposite of a "we have 7400 series at home" joke.
| ngcc_hk wrote:
| Copied from above. One does have :
| https://hackaday.io/project/185131-the-hack-computer-from-na...
| kens wrote:
| Author here if there are any questions...
| cosmolev wrote:
| Why have you wasted 80% of the die?
| kens wrote:
| I'm not sure what you're asking. IDT used the same gate array
| for numerous 7400-series products. As a result, simple chips
| would waste most of the die, while complex chips would use
| most of it. The tradeoff is that using a gate array saves
| design costs, although each chip is more expensive to
| manufacture due to the wasted silicon. Since IDT was selling
| into low volume, price-insensitive markets (military), the
| tradeoff was worthwhile.
| ekimehtor wrote:
| I may be dating myself here but I seem to recall Intel
| offering a hobbled 486 processor in which the math
| processor was disconnected for no other reason than
| marketing. Regarding the Gate array Technology it seems
| we've come full circle with the new risk V processors being
| offered up at incredibly cheap prices.
| cdcarter wrote:
| kens is the author of the write up, not the designer of the
| chip.
| wildzzz wrote:
| Why use a $20 Arduino when you could have just gotten a $1
| custom PCB, $0.50 microcontroller, and a handful of jellybean
| parts to do the same job? Because one costs less in Non-
| Recurring Engineering (NRE) than the other, you'd have to
| design the schematic and layout and then select parts that
| are affordable and available. Sometimes its just makes more
| sense to use a reusable component than doing something
| custom, especially when you don't plan to sell a lot of them.
| Someone needed a 1-to-4 decoder and for some reason, couldn't
| find one on the market that fit the application. The Gate
| Array already existed and was cheap to configure so that's
| what they went with. It's overkill but so is getting a brand
| new die made for such a throwaway part of the design.
| francescovv wrote:
| > transistors in an orderly matrix (...) forming scattered
| circuits connected by thin metal wires.
|
| That's [ULA], isn't it? This tech was also known as "Gate Array",
| before FPGA came along.
|
| [ULA] https://en.wikipedia.org/wiki/Uncommitted_logic_array
| kens wrote:
| Yes, a ULA is another name for a gate array. A while ago I
| bought an 8086 chip on eBay that turned out to be a random ULA
| chip that was re-labeled:
| https://www.righto.com/2020/08/inside-counterfeit-8086-proce...
| hyperman1 wrote:
| From that article: The book The ZX Spectrum
| ULA: How to design a microcomputer discusses [...]
|
| I had no idea such a book even existed. Now I am really
| curious about what other hidden gems exist on your bookshelf.
| WalterBright wrote:
| I was intrigued, thinking that someone had managed to clone
| an 8086 with a gate array. What a disappointment.
| RetroTechie wrote:
| I've read that some fake sound chips from eBay actually
| used a gate array. Not original parts, not a 100%
| equivalent drop-in, but close enough to work & convince
| most buyers. Don't know if true but sounds plausible. And
| if so, a (relatively) simple cpu wouldn't be much of a
| stretch.
|
| Ken's article doesn't mention it, but there's another
| reason to use a gate array: pre-fabrication, and through
| that, easier stock-keeping.
|
| Say you have ~100 different IC's like the one described.
| One could then do all the manufacturing steps to produce
| eg. 100k of those gate arrays, _except_ the last
| fabrication step. And possibly keep a large stock of those
| 'blank' gate arrays.
|
| Then (when it's known which of those 100 IC's is needed),
| apply only the last production step to those 'blanks', and
| presto: selected IC ready - in large volume if needed. Or
| produce IC's where original part has become hard-to-find.
|
| For a mil-spec part, that flexibility might be among
| reasons to go for a gate array.
|
| These days, something like a fuse-based FPGA might be used
| instead?
| kens wrote:
| > Ken's article doesn't mention it
|
| See footnote #5 :-)
| RetroTechie wrote:
| Ah, righto! Also I see you elaborated more on this aspect
| in article on that fake 8086. For anyone interested:
|
| https://www.righto.com/2020/08/inside-
| counterfeit-8086-proce...
|
| As always: great stuff!
| phibz wrote:
| Ken's footnotes are usually my favorite part of his
| articles!
| Taniwha wrote:
| Yes it's a primitive form of gate array.
|
| The thing is gate arrays are sort of something between a full
| custom part and an FPGA, they cost less per chip than a gate
| array, but more per chip than full custom. On the other hand
| NRE (up front 1 time cost)) is a lot lower than for full custom
| (I've built both).
|
| These parts are mil-spec which means that their volumes will be
| lower, it may have made sense to build a mil-spec pad ring then
| spin out a range of low volume mil-spec 7400 parts from it
| pclmulqdq wrote:
| I have heard of this structure also referred to as a PLA -
| programmable logic array, and I think a ULA is specifically for
| a PLA that is embedded in the empty space of a bigger design
| (so that in case something is broken, a debug is one mask, not
| a full respin).
|
| I am not sure that Wikipedia is out-of-date enough to have the
| precise terminology, but I also may be wrong.
| kens wrote:
| The terminology is a bit of a mess, but usually a PLA is
| highly structured with an AND plane and an OR plane, so it
| implements sum-of-products logic. A gate array is more
| general, with arbitrary connections.
| hyperman1 wrote:
| Funny to see this appear online today. I was discussing the real
| nand2tetris computer built from 74 ICs yesterday with my 8 year
| old son yesterday:
|
| https://hackaday.io/project/185131-the-hack-computer-from-na...
|
| and now this pops up.
|
| I'm thinking of buying a bunch of 74xx ICs and buttons and
| led+resistors somewhere and let him mess with them. It's stone
| age digital tech according to current norms, but I'd rather see
| him do it with physical components than behind a screen.
| pulvinar wrote:
| That's what my dad did. I remember it as confusing, the parts
| being marked like SN7424 7411 with one of those numbers being
| the date code.
| Spastche wrote:
| do it. you can use them all for basic synth building too.
| sequencers and square waves are stupidly easy to build
| gumby wrote:
| That's how I got started -- 7400 TTL chips are very forgiving
| to a young person's hand.
|
| I was amazed you could just write away to National and the
| day's send back a databook! I would pour over them and think of
| ways to use various chips.
| colanderman wrote:
| Vintage X-in-1 kits. Particularly the 300-in-1 kit had a
| breadboard and a bunch of logic chips. 500-in-1 similar but
| even has a microprocessor.
| hyperman1 wrote:
| I owned a 200 in 1 as a kid, and spent a lof of time playing
| it. I've never really looked at the others.
|
| The 300 in 1 is indeed a level up. I'll think about it.
| WalterBright wrote:
| A EE in my dorm in the 70's designed and built his own CPU out
| of 74xx chips when he was in high school. Even the other
| students were in awe of that.
| cjk2 wrote:
| I did that in the early 90s. Was only a 4-bit job but it
| mostly worked! Edit: it only ever ran one program which was
| to count to 99.
|
| There is a whole community of these machines:
| https://www.homebrewcpuring.org
| userbinator wrote:
| I couldn't find pictures of a '139 implemented the "normal" way,
| but here's a 1-of-8 decoder that is: https://www.richis-
| lab.de/logic20.htm
|
| The dual-die+decoder EEPROMs reminds me of these quad-die EPROMs:
| https://www.eevblog.com/forum/chat/identify-these-weird-epro...
|
| Are you sure there's not another two 28HC64s on the other side
| too?
| kens wrote:
| Yes, I'm sure there are just two EEPROMs. Someone on Twitter
| found the datasheet for the module (DPE8M628), which has
| details:
| https://www.datasheetarchive.com/datasheet?id=a3c8d65ff60242...
| ekimehtor wrote:
| I know it's not adding very much to the conversation but I just
| have to give kudos to the author of this article. It's always
| nice to see what the magic smoke inside those chips look like
| before I let it out! So great job I look forward to reading more
| of your articles!
| kens wrote:
| Thanks! I'm glad you enjoyed the article.
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