# 13-Pins But 30 Required.

#### Yako

Joined Nov 24, 2011
245
I am going to be using an μC with only 13 IO pins to drive peripherals requiring around 30 or so. Namely starburst / alpha numeric LED displays. It has already been suggested that some 74HC595 serial shift registers could be used to do this, but are there other ways, cheaper ways, more professional ways.

This is the LED display that I will be using:
multiplexed x 8

As you can see, there are many segments that need to be driven. I require letters as well as numbers. I also have to interface some tactile switches, a buzzer and an IR receiver, which will all be at the realm of a very cheap one-dollar μC. True that I could go with a slightly larger μC, but it works out being more expensive with the 40-pin types hovering around three-dollars. So I am hoping that I can do it all with the one-dollar MCU for two-dollars if you know what I mean. These discrete logic ICs such as the 74HC595 are bubble gum cheap at probably well under 20-cents if I buy 100 pcs or so.

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#### thatoneguy

Joined Feb 19, 2009
6,359
Take a look at the MAX5691ANI 2 wire driven 20 port I/O expander with digitally controlled LED current sink on outputs, no need for resistors. Through Hole package too.

--ETA: Sample 2 from Maxim if you work for a tech company/own a tech business/etc.

--ETA2: Suggest 16F690, 20 pin IC, enhanced Mid Range Instruction set (Linear memory), C optimized instruction set, internal 8Mhz OSC freeing up 2 more pins for a total of 17 I/O + 1 Input only.

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#### Yako

Joined Nov 24, 2011
245
Runs off the fast IC2 bus. No current buffers or resistors required. Sounds like a top-notch IC. Sounds expensive too.

If space was a problem and or I was going into mass production of a board, then the IC that you mention would of course be my choice.

I am only looking to do a very small run of about 50 pcs.

I offer the product as a kit or pre-built.

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#### Yako

Joined Nov 24, 2011
245
Take a look at the MAX5691ANI 2 wire driven 20 port I/O expander with digitally controlled LED current sink on outputs, no need for resistors. Through Hole package too.
You've got me thinking now that I am not going to be able to find cheap enough current buffer transistors for the 74HC595's outputs (or similar schema) to drive the displays and keep it under my target of two-dollars.

#### Wendy

Joined Mar 24, 2008
22,544
What does the bottom of the part look like?

#### Yako

Joined Nov 24, 2011
245
What does the bottom of the part look like?
I have never used this display before, so it is new to me. I THINK it has all pins to the left and the right like a wide 'ish dil IC.

This display is not very common. I am struggling to get a good price on this one.

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#### Yako

Joined Nov 24, 2011
245
Take a look at the MAX5691ANI 2 wire driven 20 port I/O expander with digitally controlled LED current sink on outputs, no need for resistors. Through Hole package too.
How much is this IC in low volume?

#### Yako

Joined Nov 24, 2011
245
If I go with a larger MCU such as the PIC16F877a (I know this one well too) -- then I have 33 IO pins. The star burst displays have 17 segments, and I will be multiplexing 8 of them. Plus I have other parts to interface too. This MCU is around 3-dollars. I can direct drive the rows and columns without the need for current buffers as well.

The cost of the current buffer transistors will be the killer of my two-dollar goal.

#### thatoneguy

Joined Feb 19, 2009
6,359
They also offer a 28 port expander. Don't see them at Digikey or mouseryet, I was sent two as samples with datasheets and they seem extremely useful. Designed for exactly what you are doing (Driving starburst displays), and having some extra I/O for uCs.

Probably $2-$3 each, but considering board space, transistors and resistors saved, it'd be an overall cheaper solution, especially if selling in bulk.

#### Yako

Joined Nov 24, 2011
245
Hmmm...

Was sort of hoping to create a commodity for some of this obsolete technology, and some people like building big projects. I mean, any twit can solder a few parts on a board, but there is skill and reward after spending many sessions building something large and complex.

Not too sure which road to take.

#### MMcLaren

Joined Feb 14, 2010
853
...The cost of the current buffer transistors will be the killer of my two-dollar goal.
How much do the displays cost? Wouldn't eight of them cost more than your $2.00 goal? Last edited: #### AlexR Joined Jan 16, 2008 732 Probably the MAX6955 is better suited to your application as its specifically designed to drive 7, 14 and 16 segment LED displays. Thread Starter #### Yako Joined Nov 24, 2011 245 How much do the displays cost? Wouldn't eight of them cost more than your$2.00 goal?
errr I mean the interface to drive them for under $2. The starburst displays are not cheap at$2.25 ea.

#### Yako

Joined Nov 24, 2011
245
Probably the MAX6955 is better suited to your application as its specifically designed to drive 7, 14 and 16 segment LED displays.
I just go with the larger 40-pin MCU.

So it costs less because I don't have to learn something new.

I can buy 16F877a @100pcs very cheap I think.

#### Yako

Joined Nov 24, 2011
245
I am a bit embarrassing when I barter people down I have been told.

I get good prices or I walk, and they know this.

I know retail and the mark ups.

They buy for 3-cents and sell for 3-dollars.

#### SgtWookie

Joined Jul 17, 2007
22,221
Why use the 16F877A?
Use the 16F887 instead. Much less money, and a more modern part. You can use the internal clock, or you can use a ceramic resonator or xtal if you really want to.