Did you ever attempt to diagnose, understand or repair a mechanical or electro-mechanical mechanism (perhaps a toaster that won’t toast or even a gearbox from a car or motorbike)? Once you’ve removed the covers and are able to look at what you have, if you are lucky you may immediately see a broken component, but more than likely it will be hidden from immediate view. That’s when the engineers’ instinct to start moving the parts of the mechanism will become difficult to resist. Say you turn an input shaft and it gets stuck, you can use that as a starting point and ask “why is it stuck?” That’s a new track that may well lead to the answer you are looking for. Interacting with a problem, not simply observing is much more powerful as a problem solving method.

XJAnalyser is an ideal engineers’ tool that will allow you not only to observe, but also to interact with your board to debug or fault-find problems. Because it’s a “plug and play” tool, it’s always there when you need to explore board level issues in an intelligent, interactive and inspirational way. It won’t hold back your natural problem solving instincts by requiring a lengthy set-up.

The tool allows you to make use of the inbuilt JTAG test access. XJAnalyser allows you to not only to control circuit nets with a click of the mouse, but also show you the result on a clean graphical view. Just like the mechanical fault analogy, you are able not only to observe, but also to manipulate.

You can quickly put your circuit into a state that will test your hunch. This might be impossible (or just very time consuming to achieve) within the “mission mode” operation of the circuit, perhaps requiring board mods or even special fault finding software to be written. Just like being able to turn the shaft of a gearbox to answer instinctive curiosity, with XJAnalyser you can either provoke some aspect of the problem or eliminate one of a number of possible explanations.

In fact, the analogy with mechanical systems understates the full power of XJAnalyser because it gives you visibility (and control) of circuit nodes which increasingly in modern electronics are entirely hidden from view and physical access. Once used XJAnalyser will quickly take its place alongside oscilloscope, logic analyser and multi-meter as an indispensable part of your work bench.

Back to basics

In most case a range of values will work fine for pull resistors, which is why we don’t give them much consideration. There are two limits for the range of values that will work correctly.

The lower limit on resistance can be calculated as follows:

• V_supply = 1.8 V
• V_OL = 0.4 V the worst case ( highest output voltage when an output is driven low)
• I_OL = 12 mA the worst case drive current for the worst case driver on the net

Using V = IR ( R= V/I)

• R= (1.8 – 0.4) / 0.012 = 116 Ω

So a pull up resistor which is stronger (i.e. lower value ) than 116 Ω many not work reliably.

The upper limit on resistance can be calculated as follows:

• V_supply = 1.8 V
• V_IH = 1.17 V ( lowest V_IH off all the devices on the net)
• I_leakage = 20 μA Sum of all the worst case leakage currents

and therefore

• R= (1.8 – 1.17) / 0.000020 = 31.5 kΩ

So a pull up resistor which is weaker (i.e. greater value) than 31.5 kΩ many not work reliably.

So what should I use?

Most datasheets specify the values across temperature, voltage and process. So the above limits should also work across temperature, voltage and process. You may however want to adjust the limits slightly to take into account resistor tolerances. So where in the above range should you choose?

For lowest power, choose 31.5 kΩ. Apart from that it doesn’t really matter. If the last milliWatt isn’t important it can be useful to choose a value that is used elsewhere on the board. E.g. in the PSU the feedback resistor might be 27 kΩ so use that instead of the standard 10 kΩ that you might have chosen.

NB. One last thing: some internal pull resistors are very strong, especially on the Spartan3 devices. If you are trying to overcome them the effective leakage current is much stronger – in the mA range is possible.