Cost sensitive project. Considering ATTiny85 with ATTiny84 option.

Folks,
OK... I rewrote the question below... after several of you responded.... so look for my response a few lines down.... but I've been asked to paraphrase some of what I wrote before so that the comments folks offered to me made sense... Sorry all, I was foolish to delete this first post in this thread...it's a disservice to the community to try to intepret responses to content that has disappeared.

The original idea was to develop a single pcb that would fit a higher end and lower end implementation of my product. This thread is only dealing with the remote annunciator panel portion of my product... which is smaller than a Tin of Altoids Mints. (My switch/sensor array is much more complex..and I've taken as much cost out of it as posible (5X reduction)).

Lower end would use ITTiny 85, Higher end would use ITTiny84 (I mistakenly wrote ITTiny44 in the original post).

I also mentioned using pulldown resistors on my board.. not knowing the ITTiny84 and ITiny85 implemented internal pull-up resistors. Have not worked with either before... but pretty sure they fit what I need...

The comments folks posted below were to help me:

  1. reduce component costs (internal pullups)
  2. use the same ITTiny85 for both units.

The original write up was confusing because I presented the product as a stretch (to the ITTiny84), when in fact it's a shrink (to the ITTiny85).

Has anyone done something like this before? Or should i just develop the 2nd PCB seperately?

Not sure what you are asking. If you are developing the product then you must already know everything you need to know about the processors so what are you asking (besides has anyone done this before ?) I guess I should rephrase the question:why are you asking this (as in , what are your concerns that make you want to ask?) I am not familiar with the ATTINY44, so I can't see readily what would prompt you to ask this question.

ODwyerPW:
ATTiny85 in the SOP package seems ripe to use. Feed it 5v and use the appropriate pull down resistors for switches and voltage dividing resistors for the LEDs. (and yes the .1uF capacitor for the Tiny).

So - I am puzzled. why would you ever make a design using pull-down resistors and in fact, why would you not use the internal pull-ups provided in the ATTiny85?

Hi,

You could connect either 3 or 5 buttons to the same analog input pin on the '85 by using a resistor network and analogRead ().

You could light 3 or 5 leds using 3 output pins on the '85 using charlieplexing.

So the '85 could do both standard and plus models on the same pcb, with the plus model having a few more components populated. You could probably get a single version of sketch to run both models without alteration.

Paul

OK,
I reread my original post. I could not have written it in a more confusing manner had that been my primary objective from the onset. I really appreciate everything that all of you have written so far. So let me try again.

#1... everywhere ATTiny44 appeared, should have been ATTiny84...

Let me put it in reverse.
I have a Product based on the ATTiny84...
I have 5 switches feeding it. The switches should make in a given pattern. Using a microcontroller permits me to do error checking for stuck switches or improper operation.
Different combinations of switches will illuminate different LEDs to indicate the operation. I have 6 LEDs (one is power, always on, 2 must be able to pulse).
Without multiplexing or using resistor networks, I need the pin count of something like the ATTiny84. It's size fits the package as well.

However, I see a greater opportunity to remove two switches, two LEDS and reduce conductor count through my cable and produce a lower cost alternative. These changes alone result in a BOM savings of about $3..
My thought was.. perhaps I could substitute the ATTiny84 (which has a cost of about $2.25 in bulk) with an ATTiny85 (which has a cost of about $.91). That would result in another $1.30 saved on the BOM...
That's a total of $4.30 saved (plus some labor). This yields a real world reduction of cost at the retail level of about $13. That's pretty compelling and ultimately the lower cost model would outsell the higher cost ítem 10 to 1... I would then bump the cost of the higher end ítem to realize greater profits from those customers who dont' mind paying more to have more. So there is a place for both models...one a higher volume... one a higher profit..

Hence, my question about the ATTiny85 substitution for the lower end model. And yes..the sketch would be nearly the same.... but I'd produce seperate ones.

The real question is.... CharliePlexing and Resister Networks.... Occurs to me it could add component cost (material and labor) to the point that it erases allot of the benefit of the ATTiny84 to ATTiny85 substitution.

Paul, Resistor network is easy (and quite clever).. I'll educate myself on CharliePlexing.

Other Paul, I'll look at the Internal Pullups of the ATTiny84/85. Starting to feel like two different PCB designs for each useage scenario...

The real question is.... CharliePlexing and Resister Networks.... Occurs to me it could add component cost (material and labor) to the point that it erases allot of the benefit of the ATTiny84 to ATTiny85 substitution.

Sounds like a basic front end design decision to analyze and compare the cost of a 84 chip + external components Vs using a chip with more pins that might require less external components, a process that most any project should probably go through.

Lefty,
That's exactly where I'm at....
I'm redesigning this portion (annunciation panel) of the product... Previously, I was using a $3 Arduino Pro Mini Clone. Took up too much space in the enclosure.... and drove up costs.
Our volume will be small for a while..so there will be a bit of bench top soldering by myself and a few others. Fewer components to mount the better.

Please reinstate the text of your original post so that we can make sense of the replies to it.

It is not helpful to the Arduino community to make significant modifications to earlier posts.

...R

Got it Robin. Did my best to add some context to the replies that follwed the first post.

Folks,
I've sourced ATTiny84A in bulk at sub $1 a piece costs ...shipped.
Now to pick up a programmer for this guy and learn about it more thoroughly.
It's advertised specifically as the ATTiny84A SSUR... Guess I'm going to have to learn how to work with this format.

This pricing was from Digikey, so it's not an instance where supply could be suspect.

(Looks like ATTiny85 is not needed (I have a mess of them in DIP8 format... oh well good for students)).