Selecting a Microchip

Hi Jeosist,

"Now I need help to select a right Atmel microchip which I can use in this type of cards and of course cost is very important. Once right microchip has been selected then I will focus on programming "

I think the first thing you will have to decide is what communication protocol you intend to use with each card. The communication protocol decided upon will then, to a certain extent, decide upon the chip you will use.
Then you need to know the maximum number of relays per card as this will define the number of port pins required to control the relays.
Do you require the cards chip to be doing autonomous functions not controlled by the PC? If so then you will need to consider the amount of programme and variable memory you will need for the chip.
Do the cards require input from the external world AtoD / DtoA, Digital inputs? This again will define more of the chips hardware needed.

You mention using USB from the PC to communicate so probably you will want an Atmel chip with USB communications capability. Once you have all chip requirements in front of you then the next step is to look at Atmels site at the parametric table of devices and search out the most suitable chip.

Sorry that this is not an answer, but this is the common dilemma all designers have to go through, even then it can often result in a second design.

Best
Crusty :juggle:

Crusty has made very relevant questions that you shouldn't ignore, since without answering them people will not be able to give relevant answers to your questions.

Following on from Merlins comments.

If you are new to micro programming then I would be tempted to use old fashioned RS232.
Whilst most modern PC's do not have a serial port they do have USB ports and there are abundant low cost USB to serial plugin dongles for PC's. I use one for programming via USB and an Atmel STK500 board.
It's almost certain that all the Atmel chips you are likely to use will have a UART or can be bit bashed to provide an RS485 multi drop serial communication.

Depending on the distance between cards and PC there are other options such as I2C and SPI which do not come with so much cost overhead as discussed by Merlin.

First question to you from us would be, how many relay cards(addresses)?
Second question is what distances will the cards communications connection be from the PC?

If we know this the people like Avra and Merlin will be able to help your quest

Best crusty

I was think about thi,s after the last post.

By the sounds of it your chip will need two comms ports one that is in and one that is used as out to the next in chain.
This is especially important if the cards have a unique identity assigned in eprom and can be inserted into any position on the comms cable. Trying to work out their place on the communications cable is very difficult without a two port approach.
The in port is always on and listening, the out port is only switched on when the in port gets a signal message from the card in front, or if first in line from the computer. Once place is identified the cards become receivers for their own message and pass through transmitters of the message if it for cards further down the line.

So there we have it you will probably need a chip with the capability of two Uarts.
What speed of comms will you need as this will decide if you use a crystal or internal clock on the chip.
Be aware that you will have to accommodate in your software and design for any mismatch in the baud rate between cards, as this might make some cards work in some slots and not other slots.

I think that this would temp me to design around using a SPI communication chain as the clock is physically part of the communications scheme. As long as you can read the clock and data lines you can work with the signal.

Next Questions are what power supply are you providing to the cards and what drive load will the ports need to give to the relays, do you intend to isolate the relays optically or direct drive.
These questions if answered correctly will save you hours of hardware and software debugging.

Do you need a watch dog?

I sorry if I sound like a school master but the list of questions you must ask yourself, as the designer, is endless. Especially if you are to get a chain of cards working first time, every time and without failures. Trying to debug problems on the clients property is not to be advised.

Best
Crusty :schlaumeier: