Unused port pins on 20 pin package vs 28 pin.

[JimDandy]

MCUs in question are '908JK3 vs '908JL3. The JK3 does not have PTA0-5 and PTD0-1 available whereas the JL3 does. If I were using the JL3 I would either configure unused port pins as output and let them float, or preferably, as input as tie them to ground. What though of the port lines PTA0-5 and PTD0-1 that are not available on the 20 pin package? Is it bad to have them as inputs and let them float? I think maybe it is. I have done this in the past and generally not had any obvious trouble, but now I am thinking it would be better to just make them outputs seeing I cannot physically tie them to anything. Or for that matter, are the io ports implemented on the die used in the 20 pin package?

[Monica]

On behalf of vicentegomez

Hi Pratt

There is a recommendation for the HCS08 MCU and as Xu Weiping already said the HC08 and HCS08 family are similar.

"Set the pins to a known state. For this family of devices—Never leave floating pins. Floating pins affect the overall

power consumption"

You can find this recommendation on the application note AN4441How to Use Low Power Modes on the Newest S08 Devices

http://www.freescale.com/files/microcontrollers/doc/app_note/AN4441.pdf

I hope this will help you.

Vicente

[Monica]

On behalf of weapon

Hi Pratt, I believe it's good pratice to configure all unconnected pins in a fixed state.  In S08 family, it's mentioned:

Not all general-purpose I/O pins are available on all packages. To avoid

extra current drain from floating input pins, the user’s reset initialization

routine in the application program must either enable on-chip pull-up

devices or change the direction of unconnected pins to outputs so the pins

do not float.

As HC08 and S08 are similar parts, I think it should be also applicable in HC08 family.

But in our latest Kinetis family, it mentions:

Do not modify pin configuration registers associated with pins

not available in your selected package. All un-bonded pins not

available in your package will default to DISABLE state for

lowest power consumption.

hope those information will be helpful.

[bigmac]

Hello,

It is very likely that the same die will be used for the different package sizes, where the unused I/O remains unbonded for the lower pin count.

All I/O pins have power-up default as inputs.  Whenever an input pin remains floating, it is possible for its voltage to drift to a mid-way point. When this occurs, the current draw of the device will increase.  This would be particularly detrimental if you were using a low power mode.

The most common methods used for controlling the state of unused and unbonded pins would be -

1. Set the data direction as output, (and possibly set the output state to low for the unused pins), or
2. Retain the data direction as input, but enable the internal pullup.

Regards,

Mac

[JimDandy]

Hi bigmac,

Thanks for that. Later on I found this statement in the datasheet:

***************************

20-pin devices have non-bonded pins. These pins should be configured

either as outputs driving low or high, or as inputs with internal pullups

enabled. Configuring these non-bonded pins in this manner will prevent

any excess current compsumption (sic) caused by floating inputs.

***************************

Regards,

Graham

[john_suchyta]

Hi Graham and bigmac,

Thanks for the advice (from bigmac) and the datasheet reference (Graham). The JK3E is the 20-pin version of the JL3E (same die), so the unbonded/unused pin advice applies. I generally recommend configuring the unbonded I/O pins as outputs driving low, and unused I/O pins as outputs driving low - and not connected to any potential on the PCB. We have seen runaway code overwrite data & control registers which consequently shorted unused pins that were tied to ground when the data registers were inadvertently written high.

Regards,

John Suchyta

Freescale