Unused pins for HCS12 DP512

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Unused pins for HCS12 DP512

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prog_ram
Contributor III
I have question regarding the unused pins.
Since there are internal pull up resistors for most of the pins (core and PIM pins),
do I have to connect an external pull up resistor to those that already have internal one?
(the Application notes recommend to connect every unused port to one pull up resistor).
for the PORT E pins, I am not using any of them (exccpt MODA,MODB), should I connect those also
to an external pullup resistor (although they already have one)?
and what about the TEST pin?
one more thing please,
when we connect an internal pullup resistor to a pin and read it's state, it's not one. is that
why we have to connect an external pull up resistor?
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Alban
Senior Contributor II
Hi,

The TEST pin MUST be tied to ground directly. If this pin was ever to read a one during reset, the MCU will start in test mode and your application would be at risk.

Other pins are advised to be pull-up or down for current consumption reasons. A floating input would create a high leakage current, especially if you have 10s of them.
The alternative for you would be to switch them as outputs. It is the best way for I/Os which are not available outside the package.
Output will also offer a better noise immunity to inputs.

You don't necessarily read the pin level when you do the read. You can read the content of the register instead. This depends on the DDR status (output or input), explanation is in the datasheet.

Cheers,
Alban.

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prog_ram
Contributor III
So, you recommend me to configure the unused pins as outputs and not use external (or even internal) pull up resistors?
in this case, the pins are not high impedance and thus they are exposed to alot of external noises, unless the MCU can afford having them like this (in outputs the internal pull up devices are disabled automatically)?
 
what do you think?
 
thank you Alban,
 
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Alban
Senior Contributor II
Hello,

My proposition as output means that you could save the cost of external resistors.
You can still add the resistors in outputs.

Pins are subject to external noise, but an input would get the noise in the internal circuitry. An output is driving the pin.

My worry with inputs is that you cannot say for sure that all your non-bonded pins will have internal pulling resistors. That's why outputs are a safer general comment.

I don't see why the MCU would not be able to have all pins as outputs, without anything on them.

Alban.

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Lundin
Senior Contributor IV
What if the pins don't have internal pull-up enabled by default, and they are exposed to ESD before the program sets them as outputs?

Because of this possible(?) risk I have always used external resistors on those ports where pull-up isn't enabled out of reset, better safe than sorry, since I really have no idea what protection the input pins have internally. Would you concider this unnecessary?

Message Edited by Lundin on 2007-05-0308:53 AM

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Alban
Senior Contributor II
Hello,

As a general rule, we are never too safe, you are right.
The I/Os already have a circuitry to prevent damages from ESD. Qualification of the µc means it passed the Human and Charge Body models (HBM and CDM).
Internal protection include clamping diodes to rail.

My explanations/reasons were primarily for Stop Mode Idd (current) limitation.

I don't consider external resistors as being useless. Removing them is usually a question of budget.
The other advantadge with external resistors is that you can easily do modifications on the same PCB.
This could be interesting during failure analysis, for instance, to pinpoint which component is faulty.

Cheers,
Alban.

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prog_ram
Contributor III
Hi Alban,
ok, here is what I understood and please correct me where it's necessary.
for unused pins that can be configured as outputs, it's better to configure them as outputs and save pull up resistors (AN2727 recommends to enable the pull-down resistor on the outputs).
for unesed pins that can only be configured as inputs, it's necessary to connect them to pull up resistors to prevent them from oscillating (noise). AN2727 recommends to connect they to pull up resistors if an internal pull up ones are avilable, when they are not , connecting an external pull up resistor is a must.
 
as far as I know, the only GIOP pins that can't be configured as outputs are the ATD pins, and those (as AN2434 says ) they can be left open or connect to ground.
 
thank you Alban
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