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Topic: What is the part I'm looking for called? (Read 1 time) previous topic - next topic

bobthebanana

I need some sort of chip that can be used to switch an 8 bit data bus between two outputs and I have no idea what it's called. I don't really know how to describe it other than 8 electronically operated SPDT switches in a chip so I drew a picture


where HIGH on 's' switches 'a1' to 'o1H' and LOW on 's' switches 'a1' to 'o1L'

I need there to be 8 of these in one chip with separate inputs 'a' and outputs 'o' for each, but the selection 's' for the switch needs to be common to all of the 8 switches so that I can switch the whole bus at once. The switches don't have to be bidirectional, but bonus points if they are. Needs to be 5v logic.

mmcp42

this sort of thing... http://uk.rs-online.com/web/p/multiplexer-switch-ics/6711239/
there are only 10 types of people
them that understands binary
and them that doesn't

Riva

Or this http://www.ti.com/lit/ds/symlink/sn74ahc157.pdf though they only come in nibble size not byte so you would need 2 per bus.

bobthebanana

#3
Oct 09, 2012, 10:07 am Last Edit: Oct 09, 2012, 10:30 am by bobthebanana Reason: 1
I've seen plenty of 4 bit ones and I suppose they would work although I'd rather use an 8 bit version. The FSHDMI04 could work although it's $3 on digikey (while 4 bit versions are 50 cents) and has ~20 repetitive/unused pins. It's kinda strange really, digikey has tons of single 4 bit switches and tons of dual 8 bit switches but they don't have any single 8 bit switches. Or maybe they're labeled wrong. The FSHDMI04 was labeled as a 3 x 2:1 instead of an 8 x 2:1.

I'm also looking for a 16 bit binary counter on digikey and can't find one. All the ones I've seen have 2-8 of the least significant outputs missing which sort of seems to defeat the purpose. I've settled for the CD4040BE for now.

Krupski

#4
Oct 09, 2012, 06:19 pm Last Edit: Oct 09, 2012, 06:25 pm by krupski Reason: 1

I need there to be 8 of these in one chip with separate inputs 'a' and outputs 'o' for each, but the selection 's' for the switch needs to be common to all of the 8 switches so that I can switch the whole bus at once. The switches don't have to be bidirectional, but bonus points if they are. Needs to be 5v logic.


Do you want a chip that has 8 inputs, 8 outputs and "transfers" (and latches) the 8 input states to the outputs when a pin is toggled?

If so, it sounds like you want a simple octal latch: http://www.nxp.com/documents/data_sheet/74HC_HCT573.pdf

They show two part numbers... the '573 version just has convenient pin placement (all inputs on one side and all outputs on the other).

It's 5 volt logic as well.....

Hope this helps.
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

KeithRB

He wants a chip that switches 2 8 bit busses to 1 8 bit buss, He could do it with *two* latches if they have high Z outputs.

CrossRoads

Just need the equivalent of two 74LS244, enable 1 or the other to drive the bus, or 74LS245 if need a transceiver.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

bobthebanana

KeithRB is correct in understanding what I need. I realize that there are many ways to accomplish this task using two latches, two 4 bit 2:1 demultiplexers, etc. but I would like it to be contained in one chip. If there is a chip which is the equivalent of two 74LS244 with common inputs but seperate outputs please let me know. Or if anyone can find an 8x2:1 demux on digikey that would be great as well.

KeithRB

You could use a picAxe 40x2 to do it, depending on how fast you wanted it to switch.
$11.00 from sparkfun

(Of course, you could use the ATMega2560)

CrossRoads

The parts are apparantly called "bus switch" these days at ti.com. I don't see any 2:1 parts . Use two parts such as
http://www.ti.com/lit/ds/symlink/sn74cbt3345.pdf
to connect each to the common bus.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

bobthebanana

Yea that seems to be the only alternative, problem is your layout ends up looking something like this:


It would be much nicer if they stuck two of these in one chip and tied the inputs together. It's quite surprising that they don't exist. I'll keep searching though.

CrossRoads

Two in one - now you're talking a big 24 pin+power pins+control pins chip. All the signals have to come to that area also.
And that only handles 2 buses.
If you have more than 2, seperate parts make way more sense.
If you're designing for 4 layer, 8 layer, 16 layer cards, there is way more flexibility in routing too. Parts can be spread out more for routing to more connectors.
A simplifed 1 layer routing doesn't really reflect much.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

bobthebanana

I suppose you're right. While this was originally intended to be nothing more than a simple, homebrew, single sided pcb, I just re-routed it with 2 layers(the most I can do at home) and it's certainly simpler; I can expand to an infinite number of chips easily. I'll probably be switching to 6 or more output busses so this is actually ideal.

Krupski


KeithRB is correct in understanding what I need. I realize that there are many ways to accomplish this task using two latches, two 4 bit 2:1 demultiplexers, etc. but I would like it to be contained in one chip. If there is a chip which is the equivalent of two 74LS244 with common inputs but seperate outputs please let me know. Or if anyone can find an 8x2:1 demux on digikey that would be great as well.


What about a programmable logic array like an Atmel PLD?

I use an ATF16V8B as an address decoder for a Motorola 68HC11 board - works like a charm. You would need a bigger part though (8 inputs, 16 outputs).

You can write the logic code for it with (free) WinCUPL from Atmel, then burn the JEDEC file to it with an eprom programmer. It's flash, so you can reprogram it.

OR..... this may sound crazy... if you can tolerate slow 100-ish nsec. propagation delay, use a 16 bit EPROM or FLASH memory chip and just program the bit pattern you need in it.

I did that too - years ago - to make a switch type joystick to motor driver decoder (the EPROM fed the H-bridges in the proper pattern so that the platform moved the same way the joystick pointed).

Good luck.
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

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