Circuit for HVI digital input from normal IO in S12ZVL

[rajivbandodkar]

Hi,

I am using S12ZVL 32 pin LQFP for my project. The MCU has only one HVI input. I need additional 2 in digital mode. I used the HVI style circuit with a resistor divider of 56K and 100K  to ground (with cap across 100K for filtering) on pins PAD2 and PAD3. I am also enabling the internal pull up as need, wake-up from sleep mode also. Functionally there I found no problem.

I found the voltage of 5.8V on the I/O for voltages up to 20V input.

Are there any diodes to Vdd and ground on the I/Os?

Is this recommended?

Regards,

Rajiv Bandodkar.

[RadekS]

Hi Rajiv,

all GPIO pins are protected by ESD diodes to power supply rails.

So, this 5.8V presents 5V at VDDA plus 0.8V drop at internal ESD diode. In this case, you injected current into pin, ESD diode to VDDA/VDDX rail.

The maximum allowed continuous injection current per single pin is 2.5mA (Total device limit, a sum of all injected currents is 25mA).

Regarding your resistor divider values, the injected current will be small ((20V-5.8V)/56k=0.25mA). However, we have to be carefully especially in power save modes. If the sum of injected currents will be bigger than VDDX/VDDA power consumption, the regulator may be not able to keep stable 5V output voltage level.

See chapter A.1.2 Current Injection in RM. Optionally you may look also on our application notes:

AN4731 Understanding Injection Current on Freescale Automotive Microcontrollers

http://www.nxp.com/files/32bit/doc/app_note/AN4731.pdf

or older AN2434 Input-Output Pin Drivers on HCS12 Family MCUs

http://www.nxp.com/files/microcontrollers/doc/app_note/AN2434.pdf

Note: If you use PAD pins as digital inputs, please do not forget to enable appropriate digital input buffers by DIENADx registers.

What is supposed voltage range of these additional high voltage inputs? Maybe, the choosing of different voltage divider values may partially prevent against injection current effects…

I hope it helps you.

Have a great day,
Radek

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