For S12ZVMBA does port ADx have input diodes for protection against over voltage

rajivbandodkar · ‎04-13-2018

Hi,

Using S12ZVMBA-48pin device. Using Port AD3 (pin17) and AD4(pin16) as digital input. The circuit has a series resistor of 47K to limit the current. The input voltage can be up to 18V. Do I need to use a zener to limit the voltage to the pin, since there does not seem to be any diode protection to the port?

Regards,

Rajiv Bandodkar.

lama · ‎04-16-2018

Hi,

there are ESD diodes but these diodes are not allowed to be used for application purposes. They are connected to Vdda and Vssa. (Then to VDDX and VSSX ). You are not allowed to exceed required operational values for pins defined in the data sheet so the connection you want to use is not suitable.

Such a solution is possible (allowed injection current max for single pin <= +/-2.5mA) but not recommended. The issue can appear when you want to use low power mode with signal connected. Better would be to use external divider with 47k resistor on upper side of divider which ensures that nothing happens with the MCU even low side resistor of divider is in failure.

If you use Zener diode then it must be 6.2V because 5.1V Zener diode enters nonlinearity into measured signal (depends on on a current conditions, better to test and measure in reality).

For purpose to measure higher voltages it is better to use high voltage inputs with internal dividers connected to ADC. See chapter 2.4.6. and Port L description.

...or external divider...

AN4761 Understanding Injection Current on Freescale Automotive Microcontrollers

http://cache.freescale.com/files/32bit/doc/app_note/AN4731.pdf

Best regards,

Ladislav