Hello,
it was some kind of a shock for us to see that newer chips of the HC08 (QC) and most (all?) of the HCS08 have a much smaller limit for current injection on port pins.
The limit for HC08 was usually +-2mA
Newer chips now tolerate only 0.2mA
We used 220..470 Ohm resistors to protect ports for years, we never had problems. In many of our our applications you might reverse a connector, and then about 5V are at the port pins without any supply to the MCU. Again, so far no chip failed.
But with 0.2mA this seems to be far more dangerous. We stopped a MC9S08GT16 design already and will use a good old (but expensive) MC908GR16 instead.
Any comments or experiences? Thank you!
The S08 (QG8/GT16) datasheet seems to show contradictory information. First it shows +/- 25 mA (@ abs max ratings) as max current input and after (@ DC characteristics) it shows DC current injection as 2mA (greater than VDD) or even worse -0.2mA (negative input).
I don't believe that FS reduced the input clamp diodes, that would be nonsense and the +/- 25 mA info seems to confirm that. I really think that the clamp diodes withstand the 25mA but you have to provide a path for the current flow, e.g. place a shunt regulator (or even a zener diode between VDD and VSS pins, but be careful) so the current that flows thru the internal clamps can be absorbed by the external element, so the resulting voltage between VDD and VSS could be kept under max VDD. Using the DC current injection probe in automotive modules used to work like that.
The designs I've been working with rely on these premises. I hope they are right. Pls do send your comments.
Regards,
Celso
That's why we keep on using the internal clamp diodes. Latchup is avoided limiting the current with the input series resistor. The problem happens when the current injection generated e.g. by a load dump flows thru several input pins and there is low power consumption. There has to be a dummy load to absorb the current e.g a shunt regulator of any kind. Also ADC readings can be affected by the VDD change if your shunt regulator is not good enough.
Maybe we should ask for a freescaler to help us understand. Do you know how call their attention?
Cheers,
Celso