Open Drain High Current mode

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Open Drain High Current mode

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ianbenton
Senior Contributor I

The LPC812 claims "High current sink driver (20mA) on two true open-drain pins". 

How does one select this feature? 

It appears from the manual as though they are only enabled in high-speed I2C mode.

There is also mention of setting FUNC to 000, but it doesn't say where "FUNC" is.

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alan_ning
NXP Employee
NXP Employee

Hi Lan,

High-current sink if I2C Fast-mode Plus is selected in the I/O configuration register.

We will update the document to clarify this.

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ianbenton
Senior Contributor I

Having tested it out, it seems as though the user manual is wrong as well:

Pins PIO0_10 and PIO0_11 operate as high-current sink drivers (20 mA) independently of
the programmed function. 

is not correct. They only operate as high-current drivers if the fast-plus mode is set in IOCON. The operate as normal I/O even when programmed to I2C Fast-plus mode if they are not connected to the i2C module by the Switch-matrix.

Setting normal I2C mode and setting I/O mode both seem to have the same effect - it gives a 4mA output.

Any comments NXP?

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vojtechhavlicek
Contributor III

Still the same story... Question was asked XY times and no meaningful response from NXP. We will be forced to move to another manufacturer because of this lack of support. 

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ianbenton
Senior Contributor I

Another thing I have found is that, unlike all the other port pins, this one is set as an output and held low during processor reset (all the others are set as inputs and pulled high). 

If the high-current drive is driving an external load then while ever the processor is in reset (or halted an main: waiting to start) the output will be ON; and that's not at all helpful.

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ianbenton
Senior Contributor I

I see that 267 people have read this, but no-one from NXP has come along to say what the correct spec is. 

There's no denying that there's an error - the two documents contradict each other; but come NXP - let us all know which is right - we already know that you've got one of them wrong!

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ianbenton
Senior Contributor I

Indeed it does! 

. . . and that makes me wonder why it is specified at 3.5mA in the datasheet.

It says exactly the same thing in the LPC15xx manual (page 109), but the LPC15xx doesn't claim to have high-current sink outputs at all.

They can drive my opto triacs without an extra transistor.

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ianbenton
Senior Contributor I

Perhaps someone at NXP could clarify whether the datasheet is correct or whether the user manual is correct.

To quote vojtech havlicek's recent discussion " everywhere on the forum are the questions which should be answered from the NXP experts (bugs in MCUs, non-standard behaviour of some peripherals),..."

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vojtechhavlicek
Contributor III

Hi

I´ve found:

Pins PIO0_10 and PIO0_11 operate as high-current sink drivers (20 mA) independently of
the programmed function.

So I think, that if pins are not used for I2C, they can sink 20mA as their default.

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ianbenton
Senior Contributor I

Thanks.

Did you find this by measuring it? Or did you find it in some documentation somewhere?

In the LPC812 datasheet (page 34) it specifies IOL as 3.5mA in the standard mode, and only as 20mA in the I2C fast-mode. The LPC15xx (which appears to have the same I2C peripheral) says the same thing.

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vojtechhavlicek
Contributor III

LPC81x User manual
Rev. 1.4 — 7 February 2014

page 70

I didn´t measure this, just tried to find in documentation.

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