Connecting Configuration Pins to Ground or VCC

[hello188]

Hi,

Some of the digital IC's uses external pins to set configuratinos.

For example, MCP4725 allows external address pin to be connected to either VCC or GND to distinguish last bit in its I2C slave address.

In general, should I connect it to VCC or GND THROUGH RESISTOR? or can i just connect it directly to VCC or GND?

I know it is safe to use resistor as it limits current.

However, in case I am using a tightly packaged Ball Grid Array with more than half of the configuration pins to be grounded, it becomes a little bit cumbersome to add a resistor to each of the configuration pins.

Any ideas??

Thank you

[Ttelmah]

It is perfectly normal/safe to directly connect, _provided_ you are careful with the board layout. The problems appear, when you directly connect to (say) a power line that runs to other devices, or has significant capacitance nearby, and the potential exists during power up/down, or as other devices operate, for this to actually go 'above' (or below) the rail powering the chip involved.

If you look at some of the example circuit layout's from people like TI, for chips with such configuration pins, they run these without resistors, as a 'tree' from the actual power pins on the device concerned, not connecting them at any other point, or to anything else. This way the voltages can't go outside that present on the chip.

Also remember you can save a lot of space, if you have several pins, by using just one resistor. So Vdd - resistor - new 'pull up' rail for whatever pins need it.

Best Wishes