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I am an embedded SW developer who wants to get more familiar with HW design. I have an LPCXpresso board with an LPC1343 on it. I want to build some circuit around that. I thought I'll start by finding out what is the maximum current I can get from the GPIOs.

This is where I got confused. I went to Static characteristics (pg34) in the datasheet.

  1. The table contains the minimal value for IOH but not the maximal value. What is that supposed to mean?

  2. Datasheet tells under static characteristics that VOH is Vdd-0.4, so that is a maximum of 3.2V. But user manual tells in 7.3.2 (pg89) that for LPC1343 max output voltage is 2.6V. That seems to contradict...

  3. Why are no HIGH level output current values for I2C pins in datasheet (pg34)?

  4. I did not find the value of pull-up and pulldown resistors anywhere in the documents... why is that? :)

Chetan Bhargava
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imzs
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1 Answers1

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The table contains the minimal value for IOH but not the maximal value. What is that supposed to mean?

Note that this is the "current with voltage drop less than 0.4 Volts". The Minimum value is the only one of interest in this case - this has to be taken into account for the following logic. The maximum cannot be higher than the short circuit current, btw.

user manual tells in 7.3.2 (pg89) that for LPC1343 max output voltage is 2.6V

The chapter talks about the weak Pullup resistors in input mode. See Fig. 9 just above that chapter.

Why are no HIGH level output current values for I2C pins in datasheet

I²C is open-drain, pulling the bus lines high is forbidden except for the external pullup resistors.

I did not find the value of pull-up and pulldown resistors anywhere in the documents... why is that?

They are not resistors. In Fig. 9 you don't see resistors but transistors as weak pullup/pulldown. You cannot make "real" resistors efficiently on silicon chips.

Also look closer at the currents:

Ipd_min:  10µA at 5 V = 500 kOhm pulldown resistor
Ipd_max: 150µA at 5 V =  33,3 kOhm pulldown resistor

That would not look nice in a datasheet - especially when compared to common 1% resistors.

Turbo J
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  • Does that mean that I can get the short circuit current without damaging the device? – imzs Feb 10 '13 at 10:00
  • Within the limits of Iss/Idd and Ptot (see Limiting values in datasheet) you usually have headroom for one pin in short circuit. – Turbo J Feb 10 '13 at 11:59