@NXP: output drive strength - question

[lpcware]

Content originally posted in LPCWare by Martin84 on Wed Mar 26 06:25:37 MST 2014
Hi,

there are Pins which can be used as normal drive strength outputs and some can be used as high drive strength outputs.

Can you help me reading the datasheet values and determining the maximum output current for the different output pins?

For example: I_OH is the HIGH-Level Ouput Current with min. -4mA and the I_OL is min. +4mA. What do these values mean? Where can I find the maximum output current of a pin? Is it the HIGH-Level short-circuit output current and the LOW-Level short-circuit output current? 

Another question:

The HIGH-drive strength pins are graduated in standard drive mode, medium drive mode, high drive mode and ultra-high drive mode. In the pin configuration I just find an indication for normal drive strength [2] and high-drive strength [3]. But nothing about the different modes.

How can I choose between the modes or which pins have the different modes?

[lpcware]

Content originally posted in LPCWare by embd02161991 on Fri Mar 28 00:24:18 MST 2014
Hi Martin,

I_OH refers to the maximum current flowing out of an output pin when the input conditions are such that the output is in logic high state. It shows the current sourcing capability of the output pin. It gives you an idea of the number of inputs the logic function can drive when it is in the logic high state.
I_OL refers to the maximum current flowing into the output pin of a logic function when the input are such that the output is in logic low state. It shows the current sinking capability of the output. It also gives an idea of the number of inputs the logic function can drive when its in the logic low state.

If a 4 mA current is drawn at the output pin, the amount by which the VDD will drop is VDD-0.4V. Similarly , if a 4 mA current flows into the output pin, the voltage rise from VSS will be 0.4V.   The maximum output current depends on the application. if IOH is 4mA the VDD reduces by 0.4V. So on increasing the current drawn at the pin , the VDD would reduce to an extent that the device attached to that pin would not be able to detect a HIGH level on that pin.



Thanks ,

NXP Technical Support

[lpcware]

Content originally posted in LPCWare by starblue on Thu Mar 27 10:23:11 MST 2014

---

Quote: Martin84
Can you help me reading the datasheet values and determining the maximum output current for the different output pins?

For example: I_OH is the HIGH-Level Ouput Current with min. -4mA and the I_OL is min. +4mA. What do these values mean? Where can I find the maximum output current of a pin? Is it the HIGH-Level short-circuit output current and the LOW-Level short-circuit output current?

---

Yes, and you can also look at the diagrams under "10.3 Electrical pin characteristics".

---

Quote: Martin84

The HIGH-drive strength pins are graduated in standard drive mode, medium drive mode, high drive mode and ultra-high drive mode. In the pin configuration I just find an indication for normal drive strength [2] and high-drive strength [3]. But nothing about the different modes.

How can I choose between the modes or which pins have the different modes?

---

In the SCU registers for high-drive pins there is a bit field EHD for choosing between the 4 modes (user manual page 404).

[lpcware]

Content originally posted in LPCWare by MarcVonWindscooting on Thu Mar 27 09:55:10 MST 2014
Have a look at the data sheet, not the user manual.

From an offset of 0.4V from the rail and 4mA of current, I'd expect R=U/I = 0.4V/4mA = 100Ohms output impedance (regular pin).

I personally would not try to source/sink more that 20mA from a high-drive pin. The maximum of 50mA is stated in the data sheet.
This is not a power device. If you need much more, what do you plan to drive? A relay (inductive!) ? Don't!
A good indication is the IO latch-up current: 100mA. You exceed that for a very short time only and your LPC dies.
SC70 or SOT23 FETs extend the current capabilities beyond everything you really need. Look at PMF170XP, PMV30UN, PMV65XP as a few examples. These won't consume up all your PCB's space, I promise  ;-)

Edit: Ooops! I thought I was in the LPC800 forum. What I said applies to LPC800. Probably similar to LPC43xx