Dan White

Need to know if VSS1 and VSS2 should be grounded on MC9S12XDT256

Discussion created by Dan White on Dec 5, 2006
Latest reply on Dec 6, 2006 by Steve Mcaslan
CPU: MC9S12XDT256
 
This question is in regards to the four core power pins VDD1, VSS1, VDD2 and VSS2.
 
I know that the VDD1 and VDD2 pins can only be connected to decoupling caps and can not be connected to an external power supply but do VSS1 and VSS2 need to be isolated or connected to our ground plane?
 
The user manual is a little ambiguous on this. It says that no static load is allowed on these pins which suggests that these shold only connect to the other side of the decoupling caps and there should be no external VSS connection. But, it also says in appendix C to connect VSS1 and VSS2 to VSSR.
 
We suspect that the correct solution is that the VSS side of these decoupling caps needs to be grounded. Can anyone give us a definitive answer on this?
 
From the MC9212XDP512V2 user manual:
 

1.2.4.3 VDD1, VDD2, VSS1, VSS2 — Core Power Pins

Power is supplied to the MCU through VDD and VSS. Because fast signal transitions place high,

short-duration current demands on the power supply, use bypass capacitors with high-frequency

characteristics and place them as close to the MCU as possible. This 2.5-V supply is derived from the

internal voltage regulator. There is no static load on those pins allowed. The internal voltage regulator is

turned off, if VREGEN is tied to ground.

NOTE

No load allowed except for bypass capacitors.

 

From appendix C:

Use low ohmic low inductance connections between VSS1, VSS2, and VSSR.

 

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