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I wonder if I can upgrade a device that is currently using a 48 pin MCU to a 64 pin MCU?

I can do this by altering the PCB pin layout but I will have floating pins.

The reason why is because it's a more powerful and faster MCU.

I'm a total newbie at this but want to do some crazy stuff.

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  • \$\begingroup\$ Leaving unconnected I/O pins on a microcontroller is usually not a problem. How are you planning to do "altering the pcb" ? Do you have the schematic and layout files for the original 48-pin version? \$\endgroup\$ Commented Nov 22, 2015 at 4:15
  • \$\begingroup\$ Though if they are input pins and power consumption is a concern it is often best to tie the pins via resistor to either GND of VCCIO to avoid floating CMOS inputs. \$\endgroup\$ Commented Nov 22, 2015 at 4:32

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You can deal with unused pins by leaving them open and setting them to output mode (if possible) or by tying them to GND or Vdd, preferably through individual resistors.

Typically they will float for a brief interval during power-up while RESET is asserted then assume the output mode. Usually that's not a problem but in some cases it may thought to violate 'best practices' or there may be some consequence of having a brief period where power consumption is increased, or perhaps RESET can be asserted for more than a brief period. In those cases, you can set the unused pins as inputs (typically the default) and use pull up or pull down resistors.

The reason for using individual resistors is that if the input were to become an output inadvertently (due to a hardware glitch or firmware error) then the output could be shorted.

You may have to do some thinking before redoing the PCB layout- most MCUs have pins that are reserved for certain purposes, and they may be shared with other similarly reserved pins with or without some kind of re-mapping ability.

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This type of swap can be done but there are a lot of things to consider.

1) Almost for sure you will have to re-do the MCU software as well as the PCB layout.

2) If the devices to be swapped are in the same MCU family from the same manufacturer then it is more feasible to do this from a hardware viewpoint.

3) If devices are not the same MCU family then there will be quite a few re-design considerations including supply voltages, GPIO signal compatibility, crystal or oscillator requirements, reset signal requirements, analog pin assignments and so on.

4) If you have the original PCB design files and have the rights to use/modify them then this is feasible. Without such you are basically looking at a whole new PCB design from the ground up.

5) Be aware that if the new MCU is a unit that runs its software faster that there are likely to be timing dependant parts of the code that will have to change.

6) If you have the original software source code and have the rights to use and modify it then this may be feasible. Without this then you will have to do a full software development for this new device.

7) If there is any change to the software programmers model between the original MCU and the target device be prepared to do software adjustments.

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  • \$\begingroup\$ Ok , that scared me. Although this is very possible with the necessary skills and enough time witch I don't really have, then I'll still won't get it right from the first time. I rather let someone do this for me and reviewing the schematics and trying to understand them afterwards? I have the schematics in eagle/ altium. Who can tell me where to go from here? I need 100% expertise. Are there any '' experts '' that can do this for me at fairly prices /h or where to look for it online? Schematics are open source. I can pay with paypal/regular bank transfer. Thanks you for your replies. greets \$\endgroup\$
    – jmnoi
    Commented Nov 22, 2015 at 14:22

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