Cascading two 74LS90's doesn't work (needs deglitching)

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Jan Panteltje

Posted: Sat Apr 26, 2008 4:10 am

Guest

On a sunny day (Fri, 25 Apr 2008 20:04:05 -0700 (PDT)) it happened Rocky
<forums@internode.on.net> wrote in
<8d7d0ac2-dc7a-40c3-9c60-bd7d922baf89@p25g2000pri.googlegroups.com>:

Quote:

On Apr 26, 12:52 pm, rebel <m...@privacy.net> wrote:
So now I need to alleviate reflections causing the multiple triggers.
Do people have suggestions as to how I do that ?

Short cables ....

Surely that isn't my only option ! I'd prefer not to have to shorten
those wires.

Any other suggestions ? I can't be the first person wanting to
transmit digital signals over a 1m distance without reflections.
Perhaps I need some kind of impedance matching.

regards
Peter

You need some logic switch ICs close to the LS90, and use DC signals to the
switches.
It is more chips again, so a PIC would be better.

Arie

Posted: Sat Apr 26, 2008 6:52 am

Guest

"Rocky" <forums@internode.on.net> wrote in message
news:68d837c4-9528-4982-872f-2d90a7943691@h1g2000prh.googlegroups.com...

Quote:

So now I need to alleviate reflections causing the multiple triggers.
Do people have suggestions as to how I do that ?

ALWAYS use buffers and terminators to drive cables (or "electrically long"
traces).
If anything possible, use Schmitt trigger receivers.

When you have only one receiver at the end you should use a series
terminator at the driving side, or you might use the parallel terminator
(see below), with somewhat more dissipation.
Example: for driving TTL or CMOS levels over a flatcable with GND-signal-GND
(about 100 Ohm impedance) use (100 Ohm minus the output impedance of the
buffer) between buffer and cable.

When you have multiple receivers along the cable you MUST use a parallel
terminator at the far end.
Example: for driving TTL levels over a flatcable with GND-signal-GND use 150
Ohm to +5V, 330 Ohm to GND: idle level is about 3.3V, termination = 100 Ohm.
Example: for driving CMOS levels over a flatcable with GND-signal-GND use
220 Ohm to +5V (or +3V3), 220 Ohm to GND.

With systems like this I've driven many meters of cabling with very clean
signals at the receiver(s).

Regards,
Arie de Muynck

Frank Buss

Posted: Sat Apr 26, 2008 11:48 am

Guest

Rocky wrote:

Quote:

I am using two 74LS90 ICs cascaded in series to divide down a clock
signal (f=1/60 Hz). A 74LS90 has a divide by 2 and a divide by 5
section in it. The 1min period clock pulse is therefore divided up
into:
2min, 5min, 10min and 25min periods.

the 2, 5 and 10min periods work OK, but the 25min doesn't. It turns
out to also be 10min rather than 25min.

You'll need less parts and it would be more flexible and more stable with a
microcontroller. I've implemented it to test the PIC12F508, which I've
bought some time ago and it works well, the internal oscillator is very
accurate. Instead of a rotary switch, I'm using 4 buttons: If you push one
button, it is displayed with a LED and the other LEDs are turned off.

Just for fun I've routed it (no wonder for such a simple circuit, it is
possible with one bottom layer, only, without bridges) and rendered it with
Eagle3D:

http://www.frank-buss.de/PICPulseGenerator/index.html

--
Frank Buss, fb@frank-buss.de
http://www.frank-buss.de, http://www.it4-systems.de

Jamie

Posted: Sat Apr 26, 2008 6:09 pm

Guest

Rocky wrote:

Quote:

On Apr 26, 12:52 pm, rebel <m...@privacy.net> wrote:

So now I need to alleviate reflections causing the multiple triggers.
Do people have suggestions as to how I do that ?

Short cables ....

Surely that isn't my only option ! I'd prefer not to have to shorten
those wires.

Any other suggestions ? I can't be the first person wanting to
transmit digital signals over a 1m distance without reflections.
Perhaps I need some kind of impedance matching.

regards
Peter
Twisted pairs ? maybe?

--
http://webpages.charter.net/jamie_5"

Fred Bloggs

Posted: Sun Apr 27, 2008 9:49 am

Guest

Rocky wrote:

Quote:

On Apr 26, 1:58 pm, dpl...@radagast.org (Dave Platt) wrote:

Short cables ....

Or, termination (series or parallel) of the long wires, at the far
ends, with a suitable resistance that will absorb the pulse and
prevent the reflection (or reduce its amplitude enough to avoid
false-triggering).

--
Dave Platt

Hi Dave,
yes that is a good idea. The cable I am using is cut from a VGA cable
with the ends cut off. I am guessing its characteristic impedance is
about 75 to 100 ohms, so I will try termination resistors close to
that value at the load end to see if that helps. I found a useful
article detailing how to deal with/eliminate reflections here
http://www.ecircuitcenter.com/Circuits/tline1/tline1.htm

The 1m line is not too bad, it does not look like a true transmission
line to the LS but it does represent a troublesome reactance to both the
drive and receive side, and the negative incremental input impedance of
the LS exacerbates things. One approach to eliminating problems is to
isolate the reactance from the driver and swamp out the negative receive
impedance. This can be done like so:
View in a fixed-width font such as Courier.

.
.
.
.
. sw
. LS90 ---------
. --- | |
. |-[82]-- --|-> |
. | | | | |
. |-[82]--| 4 |--|-> | 1
. | |-----/----| | <---o-|---/----+-->
. |-[82]--| |--|-> | |
. | | | | | 220p|
. |-[82]-- --|-> | ===
. --- | | |
. --------- [82]
. |
. ---
. ///
.

Fred Bloggs

Posted: Sun Apr 27, 2008 9:52 am

Guest

Rocky wrote:

Quote:

You can't DC terminate the receive end- LS doesn't have enough drive to
develop a logic H across it, hence the ac-termination....

Rocky

Posted: Sun Apr 27, 2008 4:56 pm

Guest

On Apr 28, 12:49 am, Fred Bloggs <nos...@nospam.com> wrote:

Quote:

The 1m line is not too bad, it does not look like a true transmission
line to the LS but it does represent a troublesome reactance to both the
drive and receive side, and the negative incremental input impedance of
the LS exacerbates things. One approach to eliminating problems is to
isolate the reactance from the driver and swamp out the negative receive
impedance. This can be done like so:
View in a fixed-width font such as Courier.

.
.
.
.
. sw
. LS90 ---------
. --- | |
. |-[82]-- --|-> |
. | | | | |
. |-[82]--| 4 |--|-> | 1
. | |-----/----| | <---o-|---/----+--
. |-[82]--| |--|-> | |
. | | | | | 220p|
. |-[82]-- --|-> | ===
. --- | | |
. --------- [82]
. |
. ---
. ///
.

Hi Fred,
Thanks for that cct. After doing a lot of reading on this topic last
night your cct. makes sense to me. I will try it out later on
tonight. BTW what about the idea previously suggested by Arie and
cbarn24050 to include a series termination resistor (50 to 100 ohms)
on the driving(source) side? Detailed here:
http://www.interfacebus.com/Design_Termination.html. As Arie
highlighted and as the article mentions , "Series [Source] Termination
should only be used with one load on the line". I'm pretty sure that
applies in my situation.

Which option would be preferable (series termination vs. AC trace
termination (i.e. your cct.)) and why ? Power consumption via the
220p and 82R is not an issue for me as the clk frequency is so low.

BTW, another problem I found with my cct is that of cross talk between
the wires in the cable (resulting in unwanted CLK triggering). To
reduce that effect I will probably have to replace the round multicore
VGA cable with a flat ribbbon cable and ground every second conductor.

regards
Peter

Fred Bloggs

Posted: Mon Apr 28, 2008 9:29 am

Guest

Rocky wrote:

Quote:

That's a good reference...

Quote:

highlighted and as the article mentions , "Series [Source] Termination
should only be used with one load on the line". I'm pretty sure that
applies in my situation.

Series termination is close but not really a match for your situation
for a few reasons: 1) your line length is much shorter than the
transition time for LS logic and does not act like a true transmission
line, and 2) the output stage of the LS is not symmetric, over 100
hundred ohms when driving high and maybe a few hundred milliohms when
pulling low.

Quote:

Which option would be preferable (series termination vs. AC trace
termination (i.e. your cct.)) and why ? Power consumption via the
220p and 82R is not an issue for me as the clk frequency is so low.

You don't have a choice because it is not possible for you to series
terminate and your logic does not have enough drive to develop valid
logic levels across a resistor parallel termination.

Quote:

BTW, another problem I found with my cct is that of cross talk between
the wires in the cable (resulting in unwanted CLK triggering). To
reduce that effect I will probably have to replace the round multicore
VGA cable with a flat ribbbon cable and ground every second conductor.

Right- crosstalk results from the capacitive coupling between the lines
and high impedances at the source and receive ends. You told us you were
using bundled VGA cable, and those lines should be miniature 75 ohm
coaxial. If you ground the shields then crosstalk is greatly
attenuated, and by maintaining a low impedance at the receiver as I
suggested, the crosstalk is further attenuated. I went with 82 ohms
because it a common value and is less than 5% mismatch reflection for 75
ohm lines, which is more than tolerable. The 82 ohm in series with the
drivers does provide a series match on the logic low transition which
your destination triggers on but does not help much on the logic high
transition. It does help reduce crosstalk though. That ac-termination
should be right up between the center conductor and shield of the cable.

Fred Bloggs

Posted: Tue Apr 29, 2008 9:28 am

Guest

Rocky wrote:

Quote:

On Apr 28, 12:49 am, Fred Bloggs <nos...@nospam.com> wrote:

The 1m line is not too bad, it does not look like a true transmission
line to the LS but it does represent a troublesome reactance to both the
drive and receive side, and the negative incremental input impedance of
the LS exacerbates things. One approach to eliminating problems is to
isolate the reactance from the driver and swamp out the negative receive
impedance. This can be done like so:
View in a fixed-width font such as Courier.

.
.
.
.
. sw
. LS90 ---------
. --- | |
. |-[82]-- --|-> |
. | | | | |
. |-[82]--| 4 |--|-> | 1
. | |-----/----| | <---o-|---/----+--
. |-[82]--| |--|-> | |
. | | | | | 220p|
. |-[82]-- --|-> | ===
. --- | | |
. --------- [82]
. |
. ---
. ///
.

Hi Fred,
Thanks for that cct. After doing a lot of reading on this topic last
night your cct. makes sense to me. I will try it out later on
tonight. BTW what about the idea previously suggested by Arie and
cbarn24050 to include a series termination resistor (50 to 100 ohms)
on the driving(source) side? Detailed here:
http://www.interfacebus.com/Design_Termination.html. As Arie
highlighted and as the article mentions , "Series [Source] Termination
should only be used with one load on the line". I'm pretty sure that
applies in my situation.

Which option would be preferable (series termination vs. AC trace
termination (i.e. your cct.)) and why ? Power consumption via the
220p and 82R is not an issue for me as the clk frequency is so low.

BTW, another problem I found with my cct is that of cross talk between
the wires in the cable (resulting in unwanted CLK triggering). To
reduce that effect I will probably have to replace the round multicore
VGA cable with a flat ribbbon cable and ground every second conductor.

regards
Peter

I would skip routing the clocks through the selector switch altogether
and use a data selector 74LS153 like so, it can handle a dual rotary but
only one switch because the A B inputs are common to both halves:
View in a fixed-width font such as Courier.

..
..
..
.. translation to 2-bit binary
..
..
..
..
.. ROTARY SWITCH
.. --------- B | A
.. | | ----|----
.. | o-|-> P0 __ | __
.. | | P0 | PO
.. | o-|-> P1 __ |
.. VCC---|-o---> | ====> P1 | P1
.. | o-|-> P2 | __
.. 5V | | P2 | P2
.. | o-|-> P3 |
.. | | P3 | P3
.. --------- |
..
..
.. circuit for LS switch tranlation
..
..
.. P0 >--N/C
..
..
..
.. P1 >---------+------------------+------->A
.. | |
.. | |
.. | |
.. P2 >------------+--------+------|------->B
.. | | | |
.. .-|>|-' | | |
.. | | | |
.. P3 >---+ | [470] [470]
.. | | | |
.. '-|>|----' | |
.. ----+------+------
.. 2x 1N4148 GND
..
..
..
..
..
..
.. inputs to dual 4:1 selector
..
..
..
..
.. 75LS153
.. -----------
.. from >----| A |
.. translator | |
.. ckt | |
.. >----| B |
.. 4 | |
.. from >-----/------. | |
.. LS90 |\| C0 |
.. Q outputs | | |
.. |\| C1 Y|---> clk out
.. | | |
.. |\| C2 |
.. | | |
.. \| C3 |
.. | _ |
.. .--| G |
.. | -----------
.. ---
.. gnd
..
..
..
..
..
..
..
.. ------ ---------
.. | | 4 | 74LS153 |
.. |74LS90|---/----|SELECTOR |---> CLK OUT
.. | CTR | | |
.. | | | A B |
.. ------ ---------
.. ^
.. |
.. '---------.
.. ------- ------------- |
.. |rotary | 3 | switch | 2 |
.. | sw |-/-| translation |-/-'
.. | | | |
.. ------- -------------
..
..
..
..

Fred Bloggs

Posted: Tue Apr 29, 2008 9:52 am

Guest

Rocky wrote:

Quote:

You could do a 74LS09 quad AND OC, but it's not really all that common:
View in a fixed-width font such as Courier.

..
.. +5V
.. |
.. |
.. |
.. ROTARY SWITCH 74LS09 |
.. --------- --------- [1.2k]
.. | | | __ | |
.. | o-|-> P0-----------+--------|--| \ | |
.. | | [470] | | >--|-----+-->clk
.. | o-|-> P1-------. | .--|--|__/ | | out
.. VCC---|-o---> | | | | | | |
.. | o-|-> P2----. | | | | | |
.. 5V | | | | | | | __ | |
.. | o-|-> P3-. | +------------|--| \ | |
.. | | | |[470] | | | | >--|-----+
.. --------- | | | | |/-|--|__/ | |
.. | | | | | | | |
.. | | | | | | | |
.. | | | | | | __ | |
.. | +---------------|--| \ | |
.. | | | | | | | >--|-----+
.. |[470]| | |/-|--|__/ | |
.. | | | | | | | |
.. | | | | | | | |
.. | | | | | | __ | |
.. +------------------|--| \ | |
.. | | | | | | | >--|-----+
.. [470]| | | |/-|--|__/ |
.. | | | | | | |
.. | | | | | ---------
.. '--+--+---' |
.. | |
.. gnd |
.. |
.. 4 |
.. LS90 OUPUTS>---------/---------------
..
..
..
..

Fred Bloggs

Posted: Tue Apr 29, 2008 10:07 am

Guest

Fred Bloggs wrote:

Quote:

Rocky wrote:

On Apr 28, 12:49 am, Fred Bloggs <nos...@nospam.com> wrote:

The 1m line is not too bad, it does not look like a true transmission
line to the LS but it does represent a troublesome reactance to both the
drive and receive side, and the negative incremental input impedance of
the LS exacerbates things. One approach to eliminating problems is to
isolate the reactance from the driver and swamp out the negative receive
impedance. This can be done like so:
View in a fixed-width font such as Courier.

.
.
.
.
. sw
. LS90 ---------
. --- | |
. |-[82]-- --|-> |
. | | | | |
. |-[82]--| 4 |--|-> | 1
. | |-----/----| | <---o-|---/----+--
. |-[82]--| |--|-> | |
. | | | | | 220p|
. |-[82]-- --|-> | ===
. --- | | |
. --------- [82]
. |
. ---
. ///
.

Hi Fred,
Thanks for that cct. After doing a lot of reading on this topic last
night your cct. makes sense to me. I will try it out later on
tonight. BTW what about the idea previously suggested by Arie and
cbarn24050 to include a series termination resistor (50 to 100 ohms)
on the driving(source) side? Detailed here:
http://www.interfacebus.com/Design_Termination.html. As Arie
highlighted and as the article mentions , "Series [Source] Termination
should only be used with one load on the line". I'm pretty sure that
applies in my situation.

Which option would be preferable (series termination vs. AC trace
termination (i.e. your cct.)) and why ? Power consumption via the
220p and 82R is not an issue for me as the clk frequency is so low.

BTW, another problem I found with my cct is that of cross talk between
the wires in the cable (resulting in unwanted CLK triggering). To
reduce that effect I will probably have to replace the round multicore
VGA cable with a flat ribbbon cable and ground every second conductor.

regards
Peter

You could do a 74LS09 quad AND OC, but it's not really all that common:
View in a fixed-width font such as Courier.

.
. +5V
. |
. |
. |
. ROTARY SWITCH 74LS09 |
. --------- --------- [1.2k]
. | | | __ | |
. | o-|-> P0-----------+--------|--| \ | |
. | | [470] | | >--|-----+-->clk
. | o-|-> P1-------. | .--|--|__/ | | out
. VCC---|-o---> | | | | | | |
. | o-|-> P2----. | | | | | |
. 5V | | | | | | | __ | |
. | o-|-> P3-. | +------------|--| \ | |
. | | | |[470] | | | | >--|-----+
. --------- | | | | |/-|--|__/ | |
. | | | | | | | |
. | | | | | | | |
. | | | | | | __ | |
. | +---------------|--| \ | |
. | | | | | | | >--|-----+
. |[470]| | |/-|--|__/ | |
. | | | | | | | |
. | | | | | | | |
. | | | | | | __ | |
. +------------------|--| \ | |
. | | | | | | | >--|-----+
. [470]| | | |/-|--|__/ |
. | | | | | | |
. | | | | | ---------
. '--+--+---' |
. | |
. gnd |
. |
. 4 |
. LS90 OUPUTS>---------/---------------
.
.
.
.

Make that a 74LS38:
View in a fixed-width font such as Courier.

..
.. +5V
.. |
.. |
.. |
.. ROTARY SWITCH 74LS38 |
.. --------- --------- [1.2k]
.. | | | __ | |
.. | o-|-> P0-----------+--------|--| \ | |
.. | | [470] | | o--|-----+-->clk
.. | o-|-> P1-------. | .--|--|__/ | | out
.. VCC---|-o---> | | | | | | |
.. | o-|-> P2----. | | | | | |
.. 5V | | | | | | | __ | |
.. | o-|-> P3-. | +------------|--| \ | |
.. | | | |[470] | | | | o--|-----+
.. --------- | | | | |/-|--|__/ | |
.. | | | | | | | |
.. | | | | | | | |
.. | | | | | | __ | |
.. | +---------------|--| \ | |
.. | | | | | | | o--|-----+
.. |[470]| | |/-|--|__/ | |
.. | | | | | | | |
.. | | | | | | | |
.. | | | | | | __ | |
.. +------------------|--| \ | |
.. | | | | | | | o--|-----+
.. [470]| | | |/-|--|__/ |
.. | | | | | | |
.. | | | | | ---------
.. '--+--+---' |
.. | |
.. gnd |
.. |
.. 4 |
.. LS90 OUPUTS>---------/---------------
..
..
..
..

Rocky

Posted: Wed Apr 30, 2008 3:28 am

Guest

On Apr 30, 1:07 am, Fred Bloggs <nos...@nospam.com> wrote:

Quote:

Make that a 74LS38:
View in a fixed-width font such as Courier.

.
. +5V
. |
. |
. |
. ROTARY SWITCH 74LS38 |
. --------- --------- [1.2k]
. | | | __ | |
. | o-|-> P0-----------+--------|--| \ | |
. | | [470] | | o--|-----+-->clk
. | o-|-> P1-------. | .--|--|__/ | | out
. VCC---|-o---> | | | | | | |
. | o-|-> P2----. | | | | | |
. 5V | | | | | | | __ | |
. | o-|-> P3-. | +------------|--| \ | |
. | | | |[470] | | | | o--|-----+
. --------- | | | | |/-|--|__/ | |
. | | | | | | | |
. | | | | | | | |
. | | | | | | __ | |
. | +---------------|--| \ | |
. | | | | | | | o--|-----+
. |[470]| | |/-|--|__/ | |
. | | | | | | | |
. | | | | | | | |
. | | | | | | __ | |
. +------------------|--| \ | |
. | | | | | | | o--|-----+
. [470]| | | |/-|--|__/ |
. | | | | | | |
. | | | | | ---------
. '--+--+---' |
. | |
. gnd |
. |
. 4 |
. LS90 OUPUTS>---------/---------------

Fred, I think this latest suggestion of yours is a great one. I'm
going to order a few 74LS38's tomorrow.
For the record I tried out both the series termination and the AC
termination ccts. mentioned above. I had mostly encouraging results.
The AC termination (220pF+82R) at the load end of the cable turned out
to be the most effective just by itself. When I tried a series
termination of 82R at the source end (i.e. the Q outputs of the
74LS90) that value of 82R wasn't sufficient to snub the reflections
(viz. the 2nd 74LS90 had additional transitions on its Q3 output
(IIRC)).

I had to increase the series termination resistor from 82R to 180R
before it eliminated spurious triggers. Anyway once I placed BOTH
series and AC termination components in place, the two 74LS90 counters
seemed OK until I introduced a third counter in the cascade (74LS92).
I'd rather not go into details as I've decided it is not worth
refining the different termination methods given that I'm going to use
the 74LS38 now.

Oh, and before all the PIC fans continue telling me how I should have
used a PIC to solve this problem, well the CLK selection part of the
cct. (that we've been discussing) is actually independent of the CLK
generation part (i.e. PIC vs. 74LS90's). Additionally I actually have
8 different CLK signals 1min, 2min, 5min, 10min, 15min, 20min, 25min,
30min. I've just kept it down to 4 in these discussions to keep things
simpler.

Fred, what package do you use to generate the ASCII schematics ? The
are certainly very helpful. My schematics are currently drawn up
using PSPICE Schematics evaluation version (v9.1). I also have
LTSpice installed, but my circuit currently isn't laid out in that
editor.

It will be some time on the weekend before I can report back on my
success with the 74LS38's as per Fred's cct.

thanks again

regards
Peter

Rocky

Posted: Wed Apr 30, 2008 4:06 am

Guest

On Apr 30, 11:38 pm, Jan Panteltje <PNaonStpealm...@yahoo.com> wrote:

Quote:

Always state your real requirements.

Divide and conquer. Subsystems that are accurately specified can be
designed independently to other subsystems as long as someone with the
complete picture (i.e. a full system perspective) is ensuring on paper
and in practice that the subsystems integrate properly.

regards
Peter

Rocky

Posted: Wed Apr 30, 2008 4:10 am

Guest

On Apr 26, 7:10 pm, Jan Panteltje <PNaonStpealm...@yahoo.com> wrote:

Quote:

You need some logic switch ICs close to the LS90, and use DC signals to the
switches.

BTW Jan, since you were the first to suggest this solution I extend
the credit to you too and then to Fred for elucidating the idea and
drawing up the schematic.

regards
Peter

Jan Panteltje

Posted: Wed Apr 30, 2008 8:38 am

Guest

On a sunny day (Wed, 30 Apr 2008 06:28:44 -0700 (PDT)) it happened Rocky
<forums@internode.on.net> wrote in
<3fb938fc-b394-4ced-99de-46ff51530dad@a9g2000prl.googlegroups.com>:

Quote:

Oh, and before all the PIC fans continue telling me how I should have
used a PIC to solve this problem, well the CLK selection part of the
cct. (that we've been discussing) is actually independent of the CLK
generation part (i.e. PIC vs. 74LS90's).

Now that is vague is it not?

Quote:

Additionally I actually have
8 different CLK signals 1min, 2min, 5min, 10min, 15min, 20min, 25min,
30min.

One more reason to use a PIC.

Quote:

I've just kept it down to 4 in these discussions to keep things
simpler.

Always state your real requirements.

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