74HC595 driver or Darlington array ?

Areal Person

Hi,
·
I’m building a simple 8x24 led matrix multiplexer using· a TLC4950 which controls the led cathodes.
However, because it’s a common anode IC I need a way to select the desired anode for any one single led channel.
·
I’m not sure if I should (or can) use a 74HC595 or a Darlington array
to select the correct anode ?
·
I’ve been searching through the forum(s) and have found several code/example sources for the HC595 using the basic stamp.
e.g
http://forums.parallax.com/showthread.php?p=666544
http://forums.parallax.com/showthread.php?p=674940
and others… StampWorks experiment #23b
·
I know the HC595 can be used as a I/O port expander for the propeller.
However, I can’t find a Propeller driver for the 74HC595.
·
It’s currently my assumption that in order to use the 595 with the propeller, I would need to code a driver in ASM.
·
Is this correct ?
Is this my only option ?
·
I don’t have the skill or the project time to learn assembly (I think) for this.
I would like to write a driver, but it would take me a month or more. (I really
····· have no idea how long it would take me, because I know nothing about asm or driver coding).
·
What should I do ?
·
Thanks for the help,
-Areal

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Newzed

Areal, you wrote:

It’s currently my assumption that in order to use the 595 with the propeller, I would need to code a driver in ASM.

Wrong.· The 595 can be driven using the object BS2.Functions and the contained shiftout methods.

Sid



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Newzed@aol.com
·

Newzed

Areal, the 595 is rated at 70ma total.· You would be better off using the TPIC6595, which is rated at 250ma per each of the eight channels.· The 6595 is a combination shift register and Darlington.

Sid

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Yesterday is history, tomorrow is a mystery, and today is a gift.

That is why they call it the present.

Don't have VGA?
Newzed@aol.com
·

deSilva

(a) If you have 8 spare pins at the Prop you will need no additional logic
(b) Your application is a classical case for decoders and not for shift registers, there are encoders with open collectors that may simplify your business. Edit: No, they will most likely not
(c) One or two weeks ago there was a very instructive thread about using a 64 key matrix exactly using those shift registers.. The person has a "driver" and "know-how"

Post Edited (deSilva) : 10/17/2007 10:43:54 PM GMT

Peter Jakacki

By all means use the 595 and not the 6595. Why? Because you need anode drivers and the 6595 is an open-drain device suitable for driving the cathodes. Couple the 595 outputs to cheap pnp transistors (BC327) and you will have plenty of drive for the common anodes (maybe). That's a maybe because all we know is that they are leds and that there are 8x24, but we don't know the current needed to drive them. I assume if you push around 10ma max average that 24x10ma = 240ma/digit. If you are multiplexing then the peak current can be 8 times that to achieve the same brightness which means close to a 2A peak all on. Then there is the problem if the software stops multiplexing the leds and one digit gets stuck on with the peak current flowing continually (not pretty).

Does this help??

BTW, what's a TLC4950? (or do you mean TLC5940?)

*Peter*

Areal Person

I just wanted to say thank you to everyone!
·
I was able to successfully connect the 595 to the propeller using the BS2
functions. It was very easy to do.
·
I’m not sure if this will be the chip I will finally use with the TLC5940, but I’ll
let everyone know.
·
Thanks ! That really helped.
-Areal

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
I have a tree growing out of my head, but

what do you expect ? I'm a programmer.