Interfacing memory mapped device to ATMega64

Hi Chris, Do you have to decode at all? If your external device is the only one on the bus, there is typically no need. Just connect the chip select signal to one of the address pins. Make sure that the addresses you use to access the device have this specific address-chip select pin low (because I assume that the chip select pin is active low). You have to verify that the chip does not toggle the address lines randomly when no external accesses are made, but I don't think the ATmega64 does that.

An alternative way is to use a one-gate SOT23 chip. You can get such a chip for much less than 5 cents.

I hope my suggestions will help you. Regards, Anders Rosvall

How many devices do you have and big is the memory area for each?

Of course, you could use general-I/O and do the chip selects in software. However, this has some downsides:

- you need additional time/code to switch CS

- code gets ugly

- needs more development time for software

- you are likely to forget some switches ;-(

- you could damage chips, if you activate multiple CS at once

Jan-Hinnerk

Hi, I've used the ATMega128 with an Infineon CAN controller on the external address bus. No glue logic needed. The mega128/64 will only drive the !RD and !WR pins if the address is not used internally so the !CS line can be held low. The external IC just maps into memory multiple times. You would olny need to do address decoding if you needed more than 1 IC on the external bus.

TTFN

If you only have one device, then just use an address line as Chip Select. If you have more than one, then a 74HC 2 to 4 line or 3 to 8 line decoder is quite cheap, especially in the newer small package sizes. I have found that a 74HC device in a TSSOP package can be as much as 5x cheaper than a DIP package.

Regards Anton Erasmus