Steve French

VSSA and VDDA not routed to elevators...?

Discussion created by Steve French on May 31, 2011
Latest reply on Jun 1, 2011 by Tom Moulton


I am a PCB designer who is looking ahead to when we end up making our own PCB for the MCF52259 chip eventually.  I am slightly confused by two things related to the TWR-MCF5225X board as it pertains to VDDA (analog power) and VSSA (analog ground) for the tower system layout and usage.  According to the schematic "TWR-MCF5225X-SCH.pdf", Freescale specifically filters the main power and GND using L8 and L2 and a handful of capacitors to create VDDA and VSSA.  So...


1) Why are these signals not passed to the elevators?  I have a team of college students and I am trying to teach them that not all grounds are created equal.  I am trying to explain that when they build their own analog circuit on the protoboard that they need to specifically use the VDDA and VSSA signals which are clean and filtered references for the analog circuits....but interestingly, these signals are not available on the elevators and thusly dont make it to the protoboard.  The only signals available on the protoboard are the unfiltered digital power and gnd.  Am I missing something?


2) While trying to find a place where we can tap onto the VSSA and VDDA signals, I discovered that they dont go anywhere except the MCU itself (no headers or places we can tap onto).  So, the MCU ADC inputs use VDDA and VSSA, but the on-board accelerometer with analog outputs does not use VSSA as its reference.  Am I wrong, or should the accelerometer use VSSA as it's reference?


Thank you for any input on this question!!!


Frenchy (Steve French)