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I'm having this S32K142 Microcontroller. 64 pin package.

I have some trouble with the logic level compatibility between the above Microcontroller and the A80604-1 Backlight Boost LED Driver.

Performing the IO Logic compatibility:

From the Table 19, In Microcontroller datasheet, we get the values of Vih and Vil.

Question 1: Why is the value of the Voh and Vol not provided? Can someone tell me how to find the value of the Voh and Vol pin (incase the GPIO is not configured in Open Drain fashion. I assume in the case of the pin being Open-drain the Voh and Vol will be determined by the voltage supply outside the pin from which the pull-up is connected.) ?

If I perform a Ctrl+F search "Vol and Voh), I see from the other tables that in certain places (Table 17 and Table 19), Voh appears to be (Vdd - 0.8V) and Vol appears to be 0.8V. Can we take these as the Voh and Vol ? Voh = Vdd - 0.8V & Vol = 0.8V ?

Question 2: I have an Enable Signal coming from the Microcontroller to the LED Driver. From the datasheet of the LED Driver, I found the Minimum Input Logic Low for the Enable pin is 0.4V as below.

enter image description here

In the above case, if the Vol(max) of the Microcontroller is 0.8V and the Vil(max) of LED Driver is 0.4V, they are not voltage compatible right? Is my understanding correct or am I missing something?

If that's the case, how to solve this voltage compatibility issue?

And just by the way, how can the Vol of the Microcontroller be as high as 0.8V?

Can someone help to clarify the above 3 questions, please.

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3 Answers 3

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Voh and Vol are provided, just look at the datasheet for GPIO DC specifications.

They are just based on the pin drive strength and at the rated current for that pin strength. At the rated current the drop is 0.8V. With less current, the drop is less. With no or very low DC load, the drop is almost nothing, 0V.

Since the LED driver is a high impedance input, it draws almost no current. If the MCU GPIO supply voltage is for example 3.3V, it can easily output 0V and 3.3V to the LED driver pin. There is no compatibility issues.

All microcontrollers and other chips with CMOS outputs have some voltage drop due to the output impedance of the output driver, nothing unusual going on here. For example another MCU could be rated to have 1V voltage drop at 20mA, which equates to output impedance of about 50 ohms.

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  • \$\begingroup\$ Thank you very much for the answer. So, are you saying that there is no compatibility issues between the MCU GPIO and the LED Driver Input? \$\endgroup\$
    – user220456
    May 8, 2021 at 5:57
  • \$\begingroup\$ I also received the below answer from @Elliot Alderson who says there is a compatibility issue. What conclusion can I come to ? \$\endgroup\$
    – user220456
    May 8, 2021 at 5:57
  • \$\begingroup\$ One more question, from the LED Driver datasheet, how did you come to a conclusion that the LED EN pin is a high impedance input? I saw the functional block diagram and saw that the LED EN pin is connected to a 100k internal pull down inside the IC. So, if my Supply is 3.3V, The current from the microcontroller on the LED Pin would be 33uA. In this case, how can I find the Vol on the Microcontroller line? \$\endgroup\$
    – user220456
    May 8, 2021 at 6:09
  • \$\begingroup\$ The datasheet says the enable pin is a logic level input, with 100k pulldown. If your MCU would have problems driving logic level inputs with 100k, it would be pretty useless MCU and nobody would buy it. The voltage drop is guaranteed to be below 0.8V at the rated maximum current for any selectable pin strength. Do you think the 33uA is anywhere near the maximum IO pin current to be anywhere near the full 0.8V drop? No, because the rated current is in the order of milliamps. \$\endgroup\$
    – Justme
    May 8, 2021 at 7:53
  • \$\begingroup\$ Ok. But how did you find out that the EN pin is a High Impedance input? Is it because of the 100k pull-down? \$\endgroup\$
    – user220456
    May 8, 2021 at 8:07
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The V(ol) is specified at the maximum sink current for the pad, at worst case process and temperature.

If you’re not loading the pad significantly there probably isn’t an issue, as it is a CMOS output. With no DC sink draw the output V(ol) will be GND. The pad in the low state will ‘look’ like a resistance to GND of no more than 40 ohms.

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  • \$\begingroup\$ Thank you for the answer. Could you tell me how you came to a conclusion on How the pad output is a CMOS Output? Where is it given in the datasheet? I tried to search the word CMOS in the datasheet, but couldn't find any instances. And how do you say , "The pad in the low state will ‘look’ like a resistance to GND of no more than 40 ohms." . From where did you get the 40 ohms resistance value? Just trying to understand how you came to the conclusion, please explain. \$\endgroup\$
    – user220456
    May 8, 2021 at 6:21
  • \$\begingroup\$ And could you tell me how there won't be an issue, as the output is of CMOS type? What are the output types of Output that might cause a compatibility issue? Like, I found there is a push-pull type output also. In that case, there won't be an issue ? Please help \$\endgroup\$
    – user220456
    May 8, 2021 at 6:22
  • \$\begingroup\$ CMOS pads can swing ‘rail to rail’, compared to bipolar outputs (like TTL) that can’t. TTL will have one Vce(sat) - about 0.2 ~ 0.4V - between GND and the I/O because it’s an NPN driver. MOSFETs don’t have that, they only present a resistance. \$\endgroup\$ May 8, 2021 at 16:24
  • \$\begingroup\$ As far as the FET ‘on’ resistance, it’s Ohms law: R=E/I. 0.8V worst-case / 20mA = 40 ohms. \$\endgroup\$ May 8, 2021 at 16:27
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Yes, Vol is 0.8V and Voh is (Vdd-0.8V).

Yes, that is going to be a compatibility problem with your LED driver. You need to look for a buffer to go between these two devices, where the buffer input characteristics match the microcontroller and the buffer output characteristics match the LED driver.

How can Vol be as high as 0.8V? Ask the manufacturer. It is what it is.

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  • \$\begingroup\$ Thank you for the answer. I also received an answer from @Justme who says there is no compatibility issue. What conclusion can I come to ? \$\endgroup\$
    – user220456
    May 8, 2021 at 6:03
  • \$\begingroup\$ My answer assumed that you were building a real product and cared about reliability. The other answers are telling you that it might work. You can do what ever you want with that information. \$\endgroup\$ May 8, 2021 at 11:53
  • \$\begingroup\$ What do you think about the EN pin of the LED Driver. It has a 100k internal pull down. If my Micro Supply is 3.3V, only 33uA goes into the EN pin of the LED Driver from the microcontroller , right? In that case, the output low voltage would be much less than the mentioned 0.8V, right? \$\endgroup\$
    – user220456
    May 8, 2021 at 11:55
  • \$\begingroup\$ You can make all of the assumptions you want, but they are not guaranteed by the manufacturer. \$\endgroup\$ May 8, 2021 at 11:57
  • \$\begingroup\$ Ok, I am trying to understand here. When the datasheet says, "I/O current sink capability measured when pad Vol = 0.8 V is 3mA", does it mean , the current flows into the Microcontroller pad or flows out of the pad? \$\endgroup\$
    – user220456
    May 8, 2021 at 12:00

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