Actually, guys -- this question is pretty clear to me.

Praveen: In general, say you have a design that can be built in more than one way. An example would be a micro with different memory-size options. You lay out the PCB to support, say four memory chips, and since memory chips tend to have the same footprint even though they have different sizes, you can support all sorts of different configurations.

So, how does your system know how much memory is installed? Simple. You can put a '541-type buffer out there in the memory space. The output (read) side of the buffer lives on your micro's data bus (and the '541's 3-state buffer is enabled by some address decode logic), and the write side is simply eight inputs. What you do is to provide pads to allow for resistors to both pull up and down each of the '541's input pins. You decide that if that port reads back 0x00, then memory config 0 is chosen, and if it reads back 0x01, then memory config 1 is chosen, etc. At PCB assembly time, you stuff either a pull-up or pull-down resistor for each '541 input pin so as to create an 8-bit word.

When your micro initializes, the firmware can read from that 8-bit port and then it'll know how much memory and how it's configured.

Now, if you're using an FPGA, then there are a handful of configuration-option-select pins on the FPGA that determine whether the FPGA is doing master serial config, slave serial config, parallel config, JTAG config, whatever. In most cases, those pins are permanently pulled high or low, so there are no stuffing options. But maybe one customer wants to always configure things with JTAG (no on-board serial EEPROM) but the rest of the customers want the EEPROM. So you provide stuffing options: in the normal case, you stuff resistors to pull up/down the proper pins to enable slave serial config (reading the EPROM), and for your special customer config, you stuff the resistors that enable JTAG loading.

At my last job, we did this sort of "read a port with pull-up/down resistors" all the time, because we had so many build-time options. (Of course, if you have a CPLD or FPGA on board, then these configuration option resistors can go away and you can create a read-only register in the CPLD/FPGA that accomplishes the same thing.)

-a