Do any octal 3 to 8 demultiplexers exist?

[Michael Noone]

Hi - I'm looking for an octal 3-8 demultiplexer. Does such a beast even
exist? If not (which I suspect) - are there any duals or quads? I've been
unable to find any even dual 3 to 8s. I didn't expect to find any octals,
but it seems like a dual or quad would be out there... I really would like
to avoid having to use 8 16-pin single 3-8 demuxes...

Thanks!

-Michael J. Noone

 

[Keith Williams]

Hi - I'm looking for an octal 3-8 demultiplexer. Does such a beast even
exist? If not (which I suspect) - are there any duals or quads? I've been
unable to find any even dual 3 to 8s. I didn't expect to find any octals,
but it seems like a dual or quad would be out there... I really would like
to avoid having to use 8 16-pin single 3-8 demuxes...

That be a lot of pins (91 I/O, at least)! An FPGA would be a
possibility, though perhaps an expensive one.

 

[Michael Noone]

That be a lot of pins (91 I/O, at least)! An FPGA would be a
possibility, though perhaps an expensive one.

Yes it sure would be, and even more pins if I have to use 8 individual
chips! I should mention though, that I only need a chip with common control
lines - so really the ideal chip would only have 8 + 64 + 3 + 2 = 77 pins
(input, output, control, power). Honestly though I realize there's little
to no hope in finding such a thing, so I'm hoping to just find something
like a dual or even a quad to make things a bit simpler. I'd rather not use
an FPGA, as, as you said - it's a more expensive option, and also the
project I'm working on is intended to eventually be released to the public
(open source and all that nonsense) - and I don't think the average joe has
access to an FPGA programmer.

-Michael

 

[Keith Williams]

Yes it sure would be, and even more pins if I have to use 8 individual
chips! I should mention though, that I only need a chip with common control
lines - so really the ideal chip would only have 8 + 64 + 3 + 2 = 77 pins

Ah, yes. I see what you want.

(input, output, control, power). Honestly though I realize there's little
to no hope in finding such a thing, so I'm hoping to just find something
like a dual or even a quad to make things a bit simpler.

High pin-count devices aren't generally so specific. I highly doubt
you'll find such a thing. ...even a quad.

I'd rather not use
an FPGA, as, as you said - it's a more expensive option, and also the
project I'm working on is intended to eventually be released to the public
(open source and all that nonsense) - and I don't think the average joe has
access to an FPGA programmer.

An SRAM based FPGA would need a PROM programmer, but I suspect that's
your only choice, other than SSI. If this is for hobbyists, then
package type is going to be a major problem too. High pin-count
packages tend to be hobbyist unfriendly.

 

[Rich Grise]

....
Yes it sure would be, and even more pins if I have to use 8 individual
chips! I should mention though, that I only need a chip with common
control lines - so really the ideal chip would only have 8 + 64 + 3 + 2
= 77 pins (input, output, control, power). Honestly though I realize

So, do you want eight independent one-of-eights, or a one-of-64?
You could do that with 4X 74HC154, 4-to-16 decoder. That at least
reduces your chip count by half, albiet it's still 96 pins. And,
of course, you still have to select only one of them with a 2-to-4,
like a 74HC139.
http://www.standardics.philips.com/products/decoders/

Good Luck!
Rich

 

[Tim Shoppa]

I should mention though, that I only need a chip

with common control lines - so really the ideal chip would
only have 8 + 64 + 3 + 2 = 77 pins

I'm not sure exactly what you're trying to do, but if you're making
eight byte-wide addressable output ports you should know there are
easier and denser ways of doing it! Microcontrollers with 64 bits of
digital output will be the least-pins approach. But simply 8 octal
registers will be easier to lay out on a PCB than a bunch of
multiplexers.

Tim.

 

[[email protected]]

You could get a quad in a cheapish cpld.

 

[Michael Noone]

I'm not sure exactly what you're trying to do, but if you're making
eight byte-wide addressable output ports you should know there are
easier and denser ways of doing it! Microcontrollers with 64 bits of
digital output will be the least-pins approach. But simply 8 octal
registers will be easier to lay out on a PCB than a bunch of
multiplexers.

Tim.

Well OK - so I've been working on a microcontroller based servo controller.
Right now I just use an 8 bit port and software pwm, giving me 8 motors
that can be controlled. But I got to thinking about it - the PWM signal you
feed a servo has less than a 10% duty cycle, and a constant frequency.
Thus, if you were to put a 1:8 demux on a single pin from the
microcontroller, if you were very careful with timings, you could control 8
servos with just ONE pin (technically you could control 10, but the timings
would be getting close enough there that I wouldn't push it, and also
that'd be another control bit, so 8 it is). Thus, if you were to use a full
port - you could control sixty four servos! The software that I've written
will require just a couple very minor modifications to handle the
additional pins and the multiplexer, so mostly I just want to try it out
for shits and giggles, but I also think that this could be developed into a
very small, very effective servo controller if I could find the right
parts.

-Michael Noone

 

[Rich Grise]

I'm not sure exactly what you're trying to do, but if you're making
eight byte-wide addressable output ports you should know there are
easier and denser ways of doing it! Microcontrollers with 64 bits of
digital output will be the least-pins approach. But simply 8 octal
registers will be easier to lay out on a PCB than a bunch of
multiplexers.

There ya go - use a pic!
http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=1095&mid=10&lang=en&pageId=74

There's a couple of them with more than 64 I/Os.

Cheers!
Rich

 

[Michael Noone]

So, do you want eight independent one-of-eights, or a one-of-64?
You could do that with 4X 74HC154, 4-to-16 decoder. That at least
reduces your chip count by half, albiet it's still 96 pins. And,
of course, you still have to select only one of them with a 2-to-4,
like a 74HC139.
http://www.standardics.philips.com/products/decoders/

Good Luck!
Rich

I'd rather eight 1 to 8 demuxes that had their control bits tied together -
so essentially that'd be like a 8 channel 1 to 8 demux. If I'm not being
clear - I want to switch an 8 bit parallel signal to 8 different 8b buses.

-Michael

 

[John Fields]

Hi - I'm looking for an octal 3-8 demultiplexer. Does such a beast even
exist?
---
No.
---

If not (which I suspect) - are there any duals or quads?
---
No.
---

I've been
unable to find any even dual 3 to 8s. I didn't expect to find any octals,
but it seems like a dual or quad would be out there... I really would like
to avoid having to use 8 16-pin single 3-8 demuxes...

 

[Keith Williams]

I'd rather eight 1 to 8 demuxes that had their control bits tied together -
so essentially that'd be like a 8 channel 1 to 8 demux. If I'm not being
clear - I want to switch an 8 bit parallel signal to 8 different 8b buses.

Ok, what about tri-state line drivers, with the inputs tied together
and OE controlled by a 3-8 decoder? I believe you should be able to
get at least dual-octal drivers. ...perhaps quads. It's more pins
(and more input capacitance), but the parts might be more readily
available.

 

[Rich Grise]

I'd rather eight 1 to 8 demuxes that had their control bits tied together -
so essentially that'd be like a 8 channel 1 to 8 demux. If I'm not being
clear - I want to switch an 8 bit parallel signal to 8 different 8b buses.

Then you don't want demuxes at all - except the one to select your
port. Splitting up an 8-bit port into 8 ea. 8-bit ports using demuxes
would be an absolute nightmare to wire up/lay out.

If you need them latched, use 8X HC573; if they don't need to be
latched, then 8X HC244. That's the way they've been doing output
ports since time immemorial. If you need them bidirecitonal, use
HC245's.

Or, use one of those mongo PICs:
http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=1095&mid=10&lang=en&pageId=74

Good Luck!
Rich

 

[Si Ballenger]

Well OK - so I've been working on a microcontroller based servo controller.
Right now I just use an 8 bit port and software pwm, giving me 8 motors
that can be controlled. But I got to thinking about it - the PWM signal you
feed a servo has less than a 10% duty cycle, and a constant frequency.
Thus, if you were to put a 1:8 demux on a single pin from the
microcontroller, if you were very careful with timings, you could control 8
servos with just ONE pin (technically you could control 10, but the timings
would be getting close enough there that I wouldn't push it, and also
that'd be another control bit, so 8 it is). Thus, if you were to use a full
port - you could control sixty four servos! The software that I've written
will require just a couple very minor modifications to handle the
additional pins and the multiplexer, so mostly I just want to try it out
for shits and giggles, but I also think that this could be developed into a
very small, very effective servo controller if I could find the right
parts.

-Michael Noone

You may want to check out the cheap and available 74HCT259 3 to 8
demultiplexer chips (even many Radio Shack stores still carry
them). Sounds like you would like to send pulses to a servo to
get it positioned, then stop the pulses so it stays positioned,
send pulses to the next servo to position it, etc. If you have 8
bit control (like the data pins on the parallel port), you could
use 3 of the 8 bits for input to fifteen of the 259 chips and use
two more of the chips with another 4 bit input to two more of the
259 chips for fifteen output lines to turn on/off the other
fifteen chips. This would give you 120 control lines for the
servos. I haven't tried this with servos, so YMMV. Below are some
pages where I've used the 259 chips and servos for a webcam.

http://www.geocities.com/zoomkat/ppswitcher3.htm
http://www.geocities.com/zoomkat/output.htm
http://www.geocities.com/zoomkat/ezservo.htm

 

[[email protected]]

You could get a quad in a cheapish cpld.

Yep. Even assuming no shared pins, it should fit in a hand solderable
TQFP package.

Marc

 

[Matthias Melcher]

Well OK - so I've been working on a microcontroller based servo

controller.

Sounds familiar. I thought about demultiplexers, but favored a PIC only
approach. At a dollar and a half per piece, it's a great price. And if you
take the 16F628, you can use 14 pins out of 18 to control servos. The other
four pins are power, RxD and one input-only pin that I use to configure the
baud rate. Then you can chain as many as you like via a single serial
communication wire.

 

[Zak]

That be a lot of pins (91 I/O, at least)! An FPGA would be a
possibility, though perhaps an expensive one.

What about building a bus with 8 octal tri-state drivers?


Thomas