Electronic – Enabling Freescale IMX6 pull down registers on an input pin

I would go for the buffer. You would never have to worry about not delivering enough current or any damage to your CPLD.

Now you can use 300 transistors if your really want to but I would suggest using Darlington transistor arrays. They come in many different flavours, most giving you 7 inputs and outputs at a few hundred mA per channel.

See here: http://uk.rs-online.com/web/c/semiconductors/discretes/darlington-transistor/?searchTerm=darlington+transistor+array&sort-by=default&sort-order=default&applied-dimensions=4294868855&lastAttributeSelectedBlock=Configuration

You would still need about 40 something arrays but if you are able to use surface mount components it should not become too unwieldy.

Using Darlington transistors:

The Darlington array is usually packaged as follows:

Logic:

Internally:

Now, it may look complicated but it is actually quite simple. Pins labelled "B" are inputs (transistor base) and pins labelled "C" are outputs (transistor collector). All that is needed is for you to use the exact same circuit as you posted above, but instead of a MOSFET, usuing the Darlington pair. Connect the base to the 5V output terminal of your MCU and the collector to the collector resistor (in your case 5k as you indicated). For this package the emitter is common and is connected to ground, also as you have shown.

If you are interested: The Darlington array effectively multiplies the gain of the two transistors it contains resulting in a very high gain. This high gain allows very small currents to flow in through the base while still driving or sinking much higher currents.

I've done it several times when I couldn't figure out in advance which I really need. So I would put both on the PCB and only solder down one. In this way, if I was wrong I can just remove the resistor and solder the other one down.

These days, split termination isn't done so much but it used to be a popular form of signal termination. To the unfamiliar, split termination looks like both a pullup and pulldown where the resistor values are usually less than 400 ohms.

For analog inputs, sometimes an input needs a DC bias. This can be done using a simple resistive voltage divider-- which also looks like a pullup+pulldown. Normally in this case there is also something that blocks the DC, like a cap in series with the signal, before the resistors.

In my opinion, you never actually need a pullup and pulldown at the same time, since it just doesn't make electrical sense. Using both at the same time will create a conflict and the end result is not an up or down, more like a pull-sideways! :) But there are lots of things, like the 3 that I mentioned, that will appear like a pullup and pull down at the same time. These are very common, and I know of new engineers who confuse them for pullups+pulldowns.