VDDA on MC9S12XDP512

[mfarr]

I am using the MC9S12XDP512.

VDDR is connected to a 3.3Volts voltage regulator.

I hoped to connect the VDDA to 5Volts so as to be able to connect to external devices which use 5Volts analog signals.

However when I connect the 5Volts supply to the VDDA and VRH inputs the VDDR becomes some 4V!

Is it not allowed to connect the VDDA to a voltage which is higher than VDDR?

I have searched all over in the datasheets and cannot see anything which states this.

What is also interesting is that if I leave these pins (VDDA and VRH) unconnected there is a voltave of about 2.68Volts on these pins.

Can someone explain this behavour please?  I am sure that I am missing something

Thanks a lot

Michael

[mfarr]

Hi Alvin,

Thanks for replying to my post.

So it seems that I have to put level converters between the CPU and my analog inputs.

I hoped to be able to work at 5Volts.  I think with 3 Volts the analog inputs are more subject to noise generated on the PCB itself.  A noise of 1mV will now be compared to 3Volts instead of the former 5Volts - that is 1.6 times more!

Regards

Michael

[bigmac]

Hello Michael,

Also bear in mind that the digital signals on the PCB will also be correspondingly smaller, and should result in a lower "noise" level, all other things being equal.

To minimize noise, the PCB layout should keep the analog traces well separated from any digital traces.  If ground traces are used for shielding purposes, these should typically originate directly from the VssA pin.  It is also probably a good idea for the buffer circuitry (voltage divider)  to reduce the analog swing to 3 volts maximum, to be located as close to the analog input pins of the MCU as practicable.  However, this would not be the case if the "raw" analog signal required amplification.

Regards,

Mac

[Nabla69]

Hi !

You probably put yourself in a "current injection" situation.

No value anywhere is supposed to stay >Vdd.

To protect the MCU, you have diodes to Vdd and ground. "Clamp diodes"

Therefore once the voltage is above Vdd+0.3 to Vdd+0.6V the diode starts to conduct.

Be careful not to go greater than the injection current specified in the datasheet not to damage the MCU.

The several voltages you have are for the I/O structures (3.3-5V), for the core itself (CPU and internal logic), the analogue supply and analogue reference.

There is a AppNote to help you translate levels. And also the subject has been discussed on the 8-Bit board.

Cheers,

Alvin.

AN2433 - 5V to 3V Design Considerations | pdf
The move from 5V to 3V technology creates a challenging scenario for system designers. This application note will address the benefits that such a change brings and the design considerations that make the transition as smooth as possible.