It really depends on how fast you want it to be. If you need a respectable speed then get yourself a low cost single chip type modem platform from the likes of Rockwell and marry that to a DAA device fromt he likes of Cermatek. This whole mess will add about $50 to the production cost your hardware. (Usually that is enough for the customer to quiet down and be satisfied instead with a local notebook computer with a comm port and a web site access to download a new programming file).

For a slower approach where you don't mind how long it takes to downlaod the new data you might consider sending bytes via DTMF tone bursts. You can acheive data rates of about 3 bytes per second with this scheme but the hardware impact increase in the microcontroller is simply a DTMF detector and possibly one of those cheap suction cup acoustic pickups if you don't mind manually handling the phone call to do the update.

An in the middle solution is to purchase a standalong modem and connect it to the microcontroller via the serial port. Then all you have to implement, as part of the IAP code, is some support for the modem to implement the autoanswer code to establish the modem connection. Once the modem connection is made you could easily transfer data in an almost normal serial fashion like you would do with a direct connect design. However over the phone line, via a modem, you might like to consider making a transfer using a protocol like XMODEM. Code can be found in C source on the WEB that implements the XMODEM stuff and with it you can arrange to transfer packets as small as 128 bytes. Perfect for record at a time IAP flash re-programming.

Mike Karas