On a '328P based card, 11-12-13, MOSI, MISO, SCK - are connected to the ICSP header.
Ultimately, I would like to put this project onto the Uno, but once I throw in an RTC and a temp sensor,... As is, I have 4 available digital pins left and 5 analog. I know the temp sensor will go on the Analog end of it, but I keep expanding my idea as time goes on, so I want to make this as tight as possible.
On a '328P based card, 11-12-13, MOSI, MISO, SCK - are connected to the ICSP header.You don't gain any pins.Since they are the SPI pins tho, you can connect other SPI devices in parallel (like an SPI based RTC (vs I2C) ) and control it with another chip select line.A4/A5 are the I2C pins.All the analog pins can also be used as digital pins (A0-A5 are D14-D19).
If you start with a Mega, you might as well stay with a Mega. They don't cost much more, and trying to make it as tight as possible is probably a fatal mistake. Unos can do a good job of handling a lot of not very much but I suspect you are you are working towards a little of a lot. In that event, it's not the pins that are the problem, it's the memory. As you add a clock, an SD card, etc., you add libraries. As a result, your 13k project transforms into a 33k project with amazing speed - and then you are either stuffed, or glad you've got a Mega.
Another option is ATMega1284 - 32 IO, 128K flash, 16K SRAM (twice that of Mega2560), dual hardware serial.DIP component, so easy to work with.http://www.crossroadsfencing.com/BobuinoRev17/I have kits with onboard USB/Serial module as an option, or use male pins and connect an off-board FTDI Basic/compatible USB/Serial for downloading sketches when USB is not needed for an embeedded use.
As I say at the website:"Bare boards $5. Kits less the MIKROE-483 USB module $22, most folks go with offboard FTDI module to 6-pin header. $5.25 for USPS mailing to US address. Assembled boards available upon request."$10 more for the MIKROE-483. I have kits on hand, and I think 1 '483 module (I usually keep a few on hand, need to order some more I guess).paypal to cardinalflyer at comcast dot net.