----- Original Message ----- From: Kieren Johnstone To: Sent: Monday, August 19, 2002 1:05 PM Subject: Re: [EE]: More advanced system design.. > I was really enquiring about the memory access methods too... am I then to > understand it works like this?: > > * Each device is hooked onto the address/data bus lines from the CPU > * When the device recognises an address in it's range on these lines, it > performs a read (and puts the data on the data bus) or write (data bus -> > internal register / RAM / etc) > > If this is so, how easy would it be to attach say any old parallel EEPROM? > How would you assign it an address range? Would you recognise an > appropriate pattern in the 8 upper bits, if it was correct for the EEPROM, > feed the bus lines to the EEPROM (use the lower 24 bits of the address bus > to specify the address for the EEPROM access)? > Actually each device has one or more chip enable inputs allong with its address inputs. The address inputs allow it to decode and respond to an address in the range 0 to N (where N might be 255 for a device which has 8 address lines). So if you put together 8 such devices they would all respond to the same addresses (0 to 255). This is where the chip enable inputs come in. You would (in this example) take address lines 8, 9 and 10 to another chip (glue logic) that would convert these 3 address lines into 8 chip enables. Each of these chip enables would be tied to a seperate device. The address lines 0 to 7 would be connected to all 8 of the devices. Each device would then only respond to its own allocated address range e.g. 000 0000 0000 to 000 ffff ffff for device 0 001 0000 0000 to 001 ffff ffff for device 1 010 0000 0000 to 010 ffff ffff for device 2 011 0000 0000 to 011 ffff ffff for device 3 101 0000 0000 to 101 ffff ffff for device 5 etc Some times this is complicated by the fact that a device actually responds to a larger number of address bits than it has address lines. This is achived by stobing the address into the device in chunks. Regards Sergio Masci http://www.xcprod.com -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu