# 74C373 Octal Latch The Hard Way?

#### Longhair

Joined Dec 5, 2010
8
My simulator does not have the 74C373 Octal Latch and I don't know enough to make one by reading the data sheet

Could someone let me know parts and how to connect them to achieve the same thing?

Thanks

#### Ron H

Joined Apr 14, 2005
7,014
Just be aware that a latch modeled with individual gates may functionally match the truth table, but it would require a lot of tweaking to get it to reasonably match the timing specs.
The Fairchild 74C373 datasheet even has a schematic of the individual latches. The same caveat applies.

#### Longhair

Joined Dec 5, 2010
8
I'm using "Circuit Wizard" by New Wave Concepts.

What I am trying to accomplish is something like this: http://www.geofex.com/article_folders/fxswitchr/fxswitchr.htm (start at Building the footswitching section) with momentary switches.

I'll use LEDs for a better example.

Power on, all LEDs off.
Press SW1, LED 1 goes on.
Press SW2, LED 1 goes off, LED 2 goes on.
...
Press SW8, LED 7 goes off, LED 8 goes on.

Also, the switching order should not matter.

Press SWx, current LED goes off and SWx goes on.

#### Georacer

Joined Nov 25, 2009
5,181
I wonder how the LE input would behave when the diode's anode is on LOW. Would it be effectively tied on LOW potential?

#### Papabravo

Joined Feb 24, 2006
12,702
The transparent latch equation has three terms which can easily be seen from the Karnaugh map. Two of the terms are obvious from the requirements that Q = D when G is high and Q = Q when G is low. The third term is a covering term which appears redundant but prevents a timing hazard.

#### Ron H

Joined Apr 14, 2005
7,014
I wonder how the LE input would behave when the diode's anode is on LOW. Would it be effectively tied on LOW potential?
The LE pin needs a pulldown resistor. 100k to GND would probably work.

#### SgtWookie

Joined Jul 17, 2007
22,210
Linear Technology's LTSpice is a free and good PSPICE simulator. Googling "LTSpice download" will get you there.

It does not come with 74-series CMOS IC's by default, but the Yahoo! group LTSpice users group has a 74HC series library; not extensive, but it does have a 74HC373.

The 74HC models depend on LTSpice built-in functions, so they won't work on other simulators.

Also, you should really use the 74HC14 hex Schmitt-trigger inverters instead of the CD4049's.

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#### Longhair

Joined Dec 5, 2010
8
I've tried LTSpice in the past and it is too complicated for me because I am still a beginner when it comes to electronics. I am learning about what I need to know as I go...

It seems like the 74C373 is being phased out because it shows obsolete on Mouser. A couple other places have it for $8+ a single chip. Am I going to have to go with a programmed chip or is there an easier solution? #### SgtWookie Joined Jul 17, 2007 22,210 Forget the 74C series. Use a 74HC373: http://www.mouser.com/ProductDetail/NXP-Semiconductors/74HC373N652/?qs=sGAEpiMZZMutXGli8Ay4kE3wRMDwmh/%2bDLcA3NHS0W8=$0.57/ea if you're buying 1.

I modeled the circuit; it needs some changes. See the attached.

For one, note that U2a thru U2f and U3a, U3b are 74HC14's instead of CD4049B's. This change is important, as the 74HC14 will toggle it's outputs cleanly (a square wave). A 4049 is not a Schmitt-trigger.

Note the addition of C1, R1, D17, and R18 to the latch enable input of the 74HC373, which need some explaining.
C1 couples the leading edge of any one of the square wave outputs of the 74HC14's to the latch enable input. R1 pulls the LE input back to ground. D17 prevents the latch enable input from going more than 0.7v below ground (negative) in the event of multiple triggers (ignore the fact that it's tied to OE; they are both connected to ground, and it was just convenient to use the same ground for both.)

R18 is necessary as without it, the cathodes of D1-D8 could "float" high, which would prevent the latch pulse from being seen.

LTSpice doesn't come with pushbuttons, but it has the SW symbol. S1, S2, and S3 are "pushed" by V4, V3 and V2 respectively, momentarily shorting the inputs to U3a, U2d, and U2a (respectively) to ground.

At 10mS, U2a is shorted to ground for 1mS. You can see on the plot that output V(1) goes high and stays there until 30mS.
This is because at 30mS, S1 closes for 1mS, placing a logic low on U3a for 1mS.
Output V(7) stays high from 30mS to 70mS when V(4) goes high.

I'm also attaching the .asc file. If you want to run this simulation, you'll need the 74HC.lib and dview.lib & associated symbols from the LTSpice users group. You'll also need to either use a different LED model, or add this model to the simulation:

.MODEL RGYALED D(IS=93.1P RS=42M N=4.61 BV=5 IBV=10U CJO=2.97P VJ=.75 M=.333 TT=4.32U Iave=30mA Vpk=2.1 mfg=RGYA type=LED)

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#### Longhair

Joined Dec 5, 2010
8
Many thanks for the explanation on your changes and for the 74HC373

Question about LTSpice - does it show the current flow in the schematic? It is one of the nice things about the current simulator I am using.

#### SgtWookie

Joined Jul 17, 2007
22,210
Many thanks for the explanation on your changes and for the 74HC373
You're welcome.

Question about LTSpice - does it show the current flow in the schematic? It is one of the nice things about the current simulator I am using.
No, it is not "interactive" like that. However, you can show current flow vs time on the plot output by clicking on a component (if it has two leads) or a component lead (if >2 leads).

Having current flow showing in an interactive mode can be a helpful learning tool. I'll sometimes use Falstad's circuit simulator if I want to show someone how a fairly simple circuits' current flow works; but one has to have Java installed to use it: