If you are making an array of LED modules why not use ole reliable serial data?

Hmm ! that's sound quite wonderful.
I think you know what I had in mind.

If it is not too much to ask, can you explain me how to trigger using one wire, please

 

...
The project is a secret. I have strict space limitations. ...

There's very little difference in size between the 6 pin and 8 pin PIC once you use the SMD versions.

...
How much space is required for coding I2C in software and give a fade out delay of 1 to 2 seconds?
...

You need to say if the I2C comms is master or slave. I'm guessing you have an I2C sensor? So the PIC is the master?

If you are not forced to use I2C (and comms are only one direction) I would second Ernie's suggestion to change to a simple serial system. You can do a software UART TX or RX with a few lines of code. And it saves you one pin and one transmission wire (significant saving if there are multiple devices).

 

There are multiple device. And using one pin saves a lot I must say.

The SOT package is smaller than a SMD PIC, right ? This saves a lotta space too.

I do not need Tx at the LED control PIC's as the SOT PIC's connected to led only need to look for trigger. That is it will only trigger and do a LED light and fade out. Tx will be needed at the master PIC which controls the SOT PIC's and the master does not need to know what is triggering or not, it just needs to send out address telling to trigger which and when.

Could it be done ?

 

Hmm ! that's sound quite wonderful.
I think you know what I had in mind.

If it is not too much to ask, can you explain me how to trigger using one wire, please

Well it's kinda the only reason you want to add a PIC to simplify things.

I'm just stopping home for a sec (wife and I are helping our neighbors run a party) but in the meantime search out the Microchip sight: they have an application note for a DMX transmitter and receiver: much of what that has is what you need.

Several parts can safely be left out, unless you need the system to operate reliably over hundreds of feet.

 

There are multiple device. And using one pin saves a lot I must say.

The SOT package is smaller than a SMD PIC, right ? This saves a lotta space too.

Well because it's so secret we really don't know your size constraints. To me it's hard to imagine a situation where a SOT-6 will fit and a SO-8 won't... There's only a couple of mm difference involved. But it's your baby.

...
Tx will be needed at the master PIC which controls the SOT PIC's and the master does not need to know what is triggering or not, it just needs to send out address telling to trigger which and when.
...

Sounds like the master just needs to send a device address, and then if a device hears its own address it does its flashy task.

You can do that by sending a single byte as serial TX from the master, onto a single wire "bus". That can trigger up to 256 devices.

There may be some other issues, like if you need to trigger multiple devices at exactly the same instant that will be a problem. There will be a delay between triggering each device (the delay it takes to send each address).

 

If you can get them in MSOP packaging an 8 pin (4.9x3mm) is only a tiny bit bigger than a SOT-23-6 (2.8x2.95mm). A little bit harder to solder, the pin spacing is 0.65mm. If you fancy soldering a DFN version it is only 2x3mm for 8 pin.

 

I can do SOT's pretty well.

I believe I have all the info I need.
It's not a DMX but I will look into it.
The point where triggering both is some what bad. Cause I might need to trigger two devices at once.

Distance will be less than 5 feet. no more.

Size constraint is say the size of a 5mm LED. The chip needs to be no bigger than the 5mm that is smaller than or around that. It will be soldered on flex type cables beneath the LED.

 

...
The point where triggering both is some what bad. Cause I might need to trigger two devices at once.
...

"At once" has different meanings in the computer world.

If this needs to be VISUALLY "at once" then a 1mS or a 5mS delay probably will not be seen by the user.

If you can send the control signal to a single device in 1mS, then there will only be 1mS delay between devices lighting up.

The fact that they fade the light works in your favour, it's very hard for the human eye to detect a tiny time difference in fading lights.

 

So this is doable.

Time to buy some PICs