25 Nov
2009
25 Nov
'09
2:46 p.m.
On 11/25/09, Alexandre Souza - Listas <pu1bzz.listas at gmail.com> wrote:
Why use a handful of ttl chips if I can do the same with a pair? :o)
Because I can wire up a handful of chips in the time it takes to
design and build a microcontroller then write and debug the program
for it.
If I have to design a complete functional replacement, that's one
level of effort. If I can clone an existing design, that's a
different level of effort. In the case of a 4X MR78, I can easily
redesign the ROM field and slap on two upper address switches and not
worry about the timing between an IM6100 and an independent
microcontroller.
This device needs to sit on a wad of ROM and listen to the VT78 ask
for more bytes and feed them down the pipe. It would need to have
enough I/O lines to address the ROMs in the box and to listen to the
VT78 - I haven't analyzed the design, but for 16K of ROM, you need
either a lot of free I/O pins to strobe the address pins of the ROM or
you need to throw in a shift register or something similar to
externally address the ROM, adding to the complexity of the software
design. Either way, it's an entirely new peripheral at that point,
with lots of debugging ahead. Building some with 8-10 TTL parts is a
construction project much more than a design project. Someone who
designs hardware all day is free to go build an AVR-based MR78
replacement, but I don't know that I would ever find the time to spend
to take that route - I've worked with design engineers on testing and
refining commercial products - it's easier than it used to be, but it
still takes a lot of time and effort to ensure that what is happening
is what you want to happen and no more, no less.
It's not that I never work with microcontrollers. I have built
several AVR-based devices and am building an 8051-family device.
Those items require flexibility far beyond clocking a bunch of ROM
reads. They have to interpret command streams from larger processors
and have complex behavior (well over "here's your next byte"). They
need a microcontroller. I'm not sold on the design-time/construction
tradeoff for a peripheral for a VT78 - the market for that is small
enough I could probably hold all the devices in one hand - not worth
putting large amounts of effort into designing something completely
new.
Pick your battles and your toys. Sometimes, a 555 LED flasher is
easier and cheaper than a microcontroller even though you can do it
either way.
-ethan
I'd do that with a microcontroller, an EPROM
and like :) Nice project
:D
No microcontroller required - it's already just a handful of TTL chips.