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Comment by analog31 | Hacker News Reader

Back in the days of hand made printed circuits, I randomly assigned both data and address pins on a microprocessor circuit, and got everything onto a single side with just one or two jumpers.

I felt so clever. Then I remembered that the program in the ROM assumed a particular bit numbering, literally while my board was bubbling away in the ferric chloride. Oops.

Rather than re-design the board, I thought about writing a program to rearrange my binaries, or make a socket adapter for the EEPROM programmer. The socket adapter won out.

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That's kind of how the TMS1000 microcontroller (used in the Speak n Spell) works. Instead of incrementing the program counter on each instruction (like every normal processor), they saved a few gates by using a linear feedback shift register. The result is the program counter goes through a pseudo-random but predictable sequence. So they just program the code into the ROM in the same sequence and everything works just fine. (Some day I'll write a blog post about this, since it's interesting to look at the silicon that does this.)

The CIC (copy-protection chip) in the NES used a tiny 4-bit MCU that also had an LFSR for a program counter:

http://hackmii.com/2010/01/the-weird-and-wonderful-cic/

That sounds killer. I can't wait to see what you post. One of the things I find fascinating is the limitations that each generation of engineers deals with in order to get the next problem solved, or the next product out the door. Those were the days when bits and megahertz were expensive.

This is a very weird and specific question I know, but I figured I'd ask anyway because you seem to have experience with this chip.

In 'Halt and Catch Fire,'[0] one of the characters loads his children's names onto a Speak n Spell's memory. He's portrayed as a very talented engineer, and a lot of the show seems to be pretty true to the tech.

My question is, would this be possible for someone with a lot of patience, experience, and a home workshop at that point, or is it an apocryphal story?

[0] A last TV season about an early PC startup in the 80s

I'd say its possible but unlikely that someone reprogrammed their Speak n Spell. The first tricky thing is the TMS5100 voice synthesis chip uses a complex LPC-10 encoding to encode the sound with a very low bit rate (1100 bits/sec). Basically it's modeling the filter characteristics of the vocal tract. So the first problem is you have to convert your audio signal into this representation, which is going to be really, really hard unless you have access to the TI system that does this conversion.

The second problem is the speech data is stored in a TMS6100 ROM which is kind of a strange chip: the 14-bit address is loaded 4 bits at a time, and then the ROM steps sequentially through memory from there. The point is that you can't reprogram this chip (since it's a ROM), and emulating it with a standard EPROM would be a big pain.

I should point out that I don't have firsthand experience with these chips (apart from using a Speak n Spell years ago). But I happen to have been studying them in detail a couple weeks ago for random reasons.

For more information on this chipset, the datasheets are at http://www.datasheet-pdf.com/datasheet-html/T/M/S/TMS5100_Te... and http://www.ti99.com/exelvision/website/telechargement/tms610...

I just did a search and found someone who hacked new words into a Speak n Spell a couple years ago. But he needed to use a CPLD (like a FPGA) to simulate the ROM, and a Windows LPC encoding program, so this wouldn't have been possible in the 80s. http://furrtek.free.fr/index.php?a=speakandspell&ss=1&i=2

Looked it up and that show is set in 1983 so it does seem rather implausible but not impossible of course. :) Here's a good run through using the Windows LPC encoding program. http://www.youtube.com/watch?gl=GB&v=wVDE-6TtmFQ

Hmm... his wife worked at TI as an engineer IIRC. So it's a tenuous, unlikely, made-for-tv kind of possibility indeed.

It's gotten me interested in circuit bending again, however.

In the show, one of the characters works for TI, so has access to all kinds of internal tools for the task. Its not unrealistic to imagine they could do it; details were omitted though, as to how they did it.

There's a good chance that something like the Speak n Spell during that era stored its program in mask-programmed ROM. That's where the program is fabricated into the IC itself, and specified when you order a batch of chips.

A friend and I were doing layout on a Z80 system. Dual sided PCBs, therefore dense layout was a (relatively speaking) PITA. My friend just randomly hooked up both the address and data lines to the boot ROM. This freaked our boss out. He could understand the concept of rearranging data lines, but was concerned about rearranging address lines.

I wound up writing a C program that took in pristine "Intel HEX" (i.e. from the assembler) and swizzled both the address and data to correct for the board layout. I then spit out corrected Intel HEX. Simple, almost trivial.

Interesting that you were designing single sided PCBs. I certainly wouldn't have wanted to do that layout for anything at all complex. Hopefully you had high volume production to make it pay off.

OTOH, I'm surprised you opted for a socket adapter. Way too much hassle compared to a simple C program.

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