74373 (chip) Octal D-Type Latch with 3 State Output

[bitfogav]

Hi all, I thought it was a good idea to start an introduction into the 74373 as it is used in alot of Brads projects.

The 74373 consists of 8 latches with 3-state outputs for bus system applications, it has 8bit D-type latches which is controlled by a latch enable input (LE) and an output enabled input (OE).

Also this chip is the same as the 74573 chip, the 74573 device functions identical but has different pinouts, the inputs and outputs are on the opposite sides of package allowing it easy connections to a microchip.

Example of the 74573 package:

74573.jpg (30.9 KiB) Viewed 25763 times

Example of the 74373 package:

74373pinout.gif (6.77 KiB) Viewed 26011 times

this is a slight introduction into the 74373 but I will add more when I get more time!




(Edited post to include a image of the 74573 package)

[brad]

Very nice introduction!

You are right, I use these 74373's alot in my projects - because they are just so handy when dealing with digital electronics - mainly data buses.

you can sort of think of these 74373's as 'data bus expanders'

E.G. if we have a 16f648a microcontroller with 15 i/o lines (plus one input only line) then we can actually expand our I/O lines to 48 by using six 74373's

HOW????

Well you would connect the eight input pins of all six 74373's to the eight pins of the microcontroller, PORTB. this means that all six 74373's will receive whatever data is on PORTB.

you then use PORTA to use the 'latch' function of the 74373's. you will need six pins from PORTA to do this (one pin for each individual 74373) then finally you will connect all six output enable connections of the 74373's together and then connect them to a seperate pin of PORTA. this means we have now used up all of our output pins of the microcontroller (leaving us with PORTA pin 5 which is an input only pin)

So now we actually have 48 outputs. so what you do is - place eight bits of data on PORTB (of which all of the 74373's will receive this data) BUT we only want this data to goto a specific 74373, so we send a pulse (I.E. logic 1 back to logic 0) to the latch enable pin of that specific 74373. this means that the only 74373 that has grabbed onto that data is the one that we sent the latch pulse to.

This is great if we need to control a whole heap of LED's (like with the pov project)

Once you have sent out all the data that you require, you then send a negative going pulse (logic 0 back to a logic 1) to the output enable pins and whatever data was stored in the 74373's - it will now be shown on the outputs of all 74373's

T

[bitfogav]

Thats a very good explanation Brad

So if you had 48 leds connected to each output of the six 74373 and you wanted to turn one led on at a time then im guessing you would send a logic 1 to each 74373 output pin in sequence and control it all through your microchip program

Heres a 74373 diagram of a 16F648a connected to a 74373

74373 2.jpg (22.73 KiB) Viewed 25994 times

[odessa]

Hi Guys,

Not 100% with you on this ....

Say I have connected 6 74373's together as you say. I send out my data on port B.

I have connected all the output enables together to a pin on port A and the the enable pins to seperate pins on port A

Soooo ... Once my data is sent on port B, I send a logic 1 to the specfic 74373 to grab it ( on latch enable), then a logic 0 to all of them ( as the output enables are all connected ) to display it ?

Is that right ?

Or ... do the chips store the data sent indivdually via the enable pin and port B then when I send then output enable they all display their stored data ?

Also, could these sink/source enough on each pin to drive an led ?

Thanks

Jay

[brad]

I guess what we were talking about with all of the output enables connected together is if you were using the 74373's t drive LED displays.

So if you had 3 of the 74373's (one for red, one for blue and on for green) you would send out a byte of data to each - always using the common data bus but using a separate enable line for each of the three. once all three have their data, you then enable all outputs at once so that all the colour data goes out to the led display.

So I guess the key thing is that you don't actually enable the outputs until they all have their data.

You may very well have an application where you want the output enable for each 74373 to be controlled separately and you can of course do this aswell.

As for sinking / sourcing capability, you should have no problem driving LED's as long as your series resistor is of a sufficient value to limit the current. If you are running on 5volts then don't go below 100 ohms would be a good rule of thumb.

[odessa]

Got it

Thanks for the reply !

[bitfogav]

odessa what are you looking at making then or was you just wondering how the 74373 chips work!?.

[odessa]

Ahhhh

Well, what I want to make is a display that uses more than 1 matrix, so i figure these would be ok to use to send the data out to the other displays. I am learning so much and being drawn in so many directions..every time I learn about 1 thing something else pops up and I want to try it out.. I love it

Jay

[bitfogav]

Sounds very cool

How many matrix's are you looking to put together? did you see my post topic 24x24 matrix? ive put nine 8x8 matrix's together and used the same sort of Octal D-type latch chips as the 74373's but I used 74573's instead, they do exactly the same thing as the 74373's but the pin layout is the only difference.

[odessa]

Yes I saw your 24x24 matrix..... Awesome !!!

I was going to start with say 3 then move on. I bought 20 of each from Dannel for £14 ( 373 and 573 ) so I have plenty to play with. Also I'm getting into swordfish so the coding should be easier too

I dont want to run before I can walk so am trying to take it all slowly ... Its just so much fun

Jay

[brad]

All in good time

Half the fun is learning something new along the way!