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9 years, 10 months ago.
My Mbed's logic level 1 does not correspond to 3.3 V ?
Logic 1 for my Mbed LPC11U24 is around 3.190 V . This affects my calculations. Any idea for resolution?
2 Answers
9 years, 10 months ago.
Output high voltage isn't always a good indicator, what matters is the voltage on the ADC reference input not how high it can drive it's output pins.
That said looking at the schematic there is a BAT60A Schottky diode between the 3.3V regulator and the CPU power input. A quick google give a the BAT60A has having a typical forward voltage drop of 0.12V which would result in a nominal CPU VCC of 3.18V.
The voltage regulator used has a tolerance of +/- 1% at low currents and 25C or 2% for heavy loads or other temperatures. The diode voltage drop listed is for currents of 10mA rising to 0.2V for 100mA and 0.3V for 1A.
So from the documentation you should take the LPC11U24 ADC reference as being 3.18V but expect a few percent error depending on unit to unit variation, temperature and system loading.
9 years, 10 months ago.
Make a circuit which isn't that critically affected by voltage is probably the best solution. With very few exceptions such a small difference shouldn't hamper the performance of your circuit.
Anyone else having this problem ? Or are you getting 3.3 V output for logic HIGH ?
posted by 16 Jun 2014Seeing as my circuit is part of a much bigger project, I'm afraid that would not be possible/acceptable.
posted by 17 Jun 2014Especially in such a case I would then really change the requirements of those other blocks. For example also every IC is at least supposed to work with a +/- 10% range on its supply voltage, this is significantly less. If you really need a very accurate voltage there, you need a bandgap reference. But in general a circuit should be tolerant for errors in a supply voltage, but also in errors in components. On your PCB resistors will be fairly accurate, but for example capacitors also have often huge ranges, they can differ by a factor 2!
posted by 17 Jun 2014
How does this affect your calculation??
posted by Martin Kojtal 16 Jun 2014When i use AnalogIn I need to multiply by 3300 right ? Here I'll have to multiply by 3190. And I'm not sure if all the microcontrollers we have give the same 3190 value.
posted by Bharath S 16 Jun 2014