digital-logicnoisepulldownpullupresistance
Is it OK to have pull up or pull down resistors which aren't all the same value (if you're running low on components for example)? I can't think of a reason it should affect simple logic circuits.
I'm mostly using 74HC series ICs but plan to experiment with some Z80-based projects. Is there a possibility that things might start to act strangely at higher clock speeds of around 4MHz?
Well, I'm studying electrical engineering right now and I can tell you that such jumps as you described take around two years of lectures at my university.
First thing which is important is to know which elements are passive and which are active. Then you need to know which elements are linear and which aren't. Next step is to get equivalent schematics for elements which you have and to see how they behave.
For example, let's take the switch. In off state, it functions as an open circuit, while in on state it functions as short circuit. Next, if you have sensitive equipment, you'll be able to notice that the switch isn't actually short circuit because it has some resistance, but that it's very low. Now let's take a look at the diode. Diode isn't linear component, so it doesn't have resistance in the classical sense in which for example resistors have. Instead there's the V-I curve of the diode. On a resistor, it's a linear function and we can use resistance as its characteristic, but on diode, it looks exponential.
As you can see from the image, certain voltage is needed for diode to start working properly and when you trigger the switch, that voltage disappears. That means, that the "resistance" of the diode just became huge. To get a feeling for this, use the parallel resistor calculation for say 1 mΩ resistor and 1MΩ resistor and take a look how much current goes through each of them. This is the way the circuit you mentioned behaves.
TTL has a threshold between low and high that is closer to ground than to the positive rail, so it is better when the stronger transistor pulls the output down against the relatively weaker resistor.
In general ground is presumably a better (e.g. more stable) reference voltage than a power rail.
You can use open collector/drain outputs as voltage converters, if you connect the resistor to the positive rail of the target voltage.
The ancient resistor transistor logic even used this as its working principle throughout.
That said, some microcontrollers have configurable internal pull-ups and pull-downs, e.g. the NXP LPC1xxx.
Best Answer
By today's standards 4MHz is by no means a high clock speed. Most often pull up resistor values are not that critical, people often use 10k for address or data lines without even thinking about it (just like 100nF is used for decoupling). Using for instance 6k8 instead of 10k won't do any harm. Just make sure the pull up is on the trace itself, and not on a stub.