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cjs
PostPosted: Tue Nov 24, 2020 9:54 am 
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BigEd wrote:
And I found this Q&A: What is the difference between Flash memory and EEPROM?

Ah, good find, and that reinforces my current impression that the modern usage is that EEPROM offers byte-wise erase. (I'd actually been to that page and several of the linked ones, and upvoted the answers, but entirely forgotten them.)

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EEPROM(2) - EPROM which can be erased electrically and also rewritten byte-wise.

That's not quite accurate; all the EPROMs (and even PROMs) I've met can be "written" bytewise, so long as what you're writing doesn't try to change any 0 bits to 1 bits; it's been able to erase byte-wise (i.e., set a single byte to $FF) that seems to be the discriminator here (and would make your "EEPROM(1)" not EEPROM in that use of the terminology.

(BTW, if there's a mod about, and the software supports it, this message and the six preceeding it may want to be moved to a new thread. I hadn't realized this would be so involved....)

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BigDumbDinosaur
PostPosted: Tue Nov 24, 2020 10:00 am 
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One thing I've noticed about the OTP EPROMs is they typically have faster access times than their UV-erasable equivalents. For example, I have some Atmel (now Microchip) UV-erasable EPROMs rated at 55ns. I also have the OTP versions of those parts, which are rated at 45ns. I'm wondering if there is more to an OTP part that just not having the erasure window.

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GARTHWILSON
PostPosted: Sat Oct 30, 2021 1:48 am 
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cjs wrote:
I had also considered building some diode ROM boards with DIP switches, but there again expense put me off; even 32 bytes starts to get pretty awkward.

The recent topic "Design challenge: 6502-based EPROM programmer made me think of this topic again, and the topic on dead-start-panel computers at viewtopic.php?f=4&t=4341, but my comment might fit in better here. The DIP array would need diodes, right? So how about omitting the DIPs and just soldering diodes in the appropriate places in a matrix. It would be more compact (standing the diodes up), and cheaper too, as you can get a thousand 1N4148 diodes for $19 ($.019 each for that quantity), enough for approximately 256 bytes, at Mouser, here.

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BigDumbDinosaur
PostPosted: Sat Oct 30, 2021 3:56 am 
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GARTHWILSON wrote:
The recent topic "Design challenge: 6502-based EPROM programmer made me think of this topic again, and the topic on dead-start-panel computers at viewtopic.php?f=4&t=4341, but my comment might fit in better here. The DIP array would need diodes, right? So how about omitting the DIPs and just soldering diodes in the appropriate places in a matrix. It would be more compact (standing the diodes up), and cheaper too, as you can get a thousand 1N4148 diodes for $19 ($.019 each for that quantity), enough for approximately 256 bytes, at Mouser, here.

The 1N4148 has a reverse recovery time of 4ns, which means a "ROM" built from such an arraye of diodes would be faster than any real ROM. You'd need a buffer to gate the ROM in and out of address space, but even that wouldn't add a lot of prop time.

If I were going to do something like this, I'd use a small-signal Schottky to get the forward drop as low as possible. That said, I shudder thinking about installing and soldering enough diodes to produce a usable ROM.

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GARTHWILSON
PostPosted: Sat Oct 30, 2021 4:04 am 
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There will be pull-up resistors on the internal data bus though, meaning that going up will be slow, so you'd want to make sure the diodes are usually off, and then when an address is selected, you only have to do the pull-down through diodes to get valid data (at least the way I'm envisioning it). Then nearly all the delay from a dormant state will be in the address decoders and the output buffer.

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BigDumbDinosaur
PostPosted: Sat Oct 30, 2021 4:51 am 
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GARTHWILSON wrote:
There will be pull-up resistors on the internal data bus though, meaning that going up will be slow, so you'd want to make sure the diodes are usually off, and then when an address is selected, you only have to do the pull-down through diodes to get valid data (at least the way I'm envisioning it). Then nearly all the delay from a dormant state will be in the address decoders and the output buffer.

The resistors could be a low value to hasten the pullup, but there's still going be some palpable delay.

Any way you look at it, it's very much like using a stone axe to carve out a fine piece of furniture. Just be glad we have EPROMs and the means to program them.

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