What Type of Serial Should I Use?

jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

Hello All,

I plan on using LED Display driver chips to control a BUNCH of LEDs. Essentially they are shift registers. I will cascade up to 15 of them to control the LEDs.

My question is:

What type of serial lends itself to this type of output? I2C, SPI....just plain old bit banging?

Here is a link to the Data Sheet:

http://www.marktechopto.com/PDFs/toshiba/TB62706BNG_BFG_datasheet.pdf

Any advice or words of wisdom would also be much appreciated.

Thanks,

John

treitmey · Joined: 23 Jan 2004 Posts: 1094 Location: Appleton,WI USA

Depends on the speed. That depends on if your multiplexing the leds.

If just one out_n bar pin goes to just one led. Then I'd say bit bang it.
Then you can choose the pins to use.

If multiplexed, I'd do some experiments to test the speeds needed to make the leds pretty. Smile

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Those chips have an spi-like interface, but without a slave select signal.

You can use hardware or software SPI to clock the data into them.
The spi_write() function writes 8 bits at a time, so you would need
to call that function two times per chip. If you have 15 chips, then
30 calls must be made to spi_write() to shift out all the data.

There might be a problem with driving all the \Latch pins.
Page 3 of the data sheet shows equivalent circuits for the various pins.
The \Latch pin is shown with a series resistor and a pull-down resistor.
On page 5 they specify the pull-down as 100 ohms minimum, but I
don't see a spec for the series resistor. They also have no spec for
high-level input current for that pin. So fan-out might be a problem.
I'm sure you wanted to drive the \Latch pins on several Toshiba ships
with one PIC pin. You're going to have to check if that can be done.

jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

Thanks!

Great that answers my question, I'll start thinking SPI.

Thanks for the poop on the LATCH pin, I would have never caught that! I'll try to get an answer. I would assume that the series resistor must be significant....

BTW, one other complicating issue is that the Toshiba chip will be at 5V and the PIC at 3V and a MAX3371 will be handling the level translation...

I'll post any new data I can get a hold of.

John

Humberto · Posted: Fri Aug 26, 2005 1:09 pm

This drivers are technically a sink driver type and when they are multiplexed mostly
works in conjuntion with others source drivers type (like Allegro UCN5891) wich has
a built in constant current control that ovoid the series resistors and keep stable the
brightness across the display.

Take a look:
http://www.allegromicro.com/techpub2/an/an295031.pdf

Humberto

jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

FAE Report:

Value could not be quoted, however he assured me there is no problem driving many of those pins when cascading this part.

Thanks for the heads up.

(Humberto, thanks. PCM was referring to the logic pins LATCH and ENABLE.)

cmdrdan · Joined: 08 Apr 2005 Posts: 25 Location: Washington

John --

I saw some 3 volt parts on the Marktech site. Check out the TB62726A parts....

Dan

jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

Thanks!

I wasn't paying atttention as I was browsing those parts. I missed that there were 3v parts available. That saved me a bunch of time and some money on the MAX3371's.

Now, if I could only find a 3V LCD character display in 16x2, 16x4, 20x2, or 20x4, I'd be set.

John