Posting for a customer:
I’ve run into some ambiguous documentation I could use some clarification with. I’m doing the schematic for the AM3351BZCE60 in my design. I’m looking at the power section. There is a diagram in the Data Sheet (SPRS717J – October 2011) on page 96, figure 5-1, that shows the power supplies and an idea of decoupling capacitors. While there are many power domains, there is usually a common ground. I noticed that there are several peripherals that seem to have their own ground pins. USB 1.8V and 3.3V have an associated signal VSSA_USB which, on the block diagram is not shown connected to ground. This kind of implies that it is tied inside the part and I should just put an external decoupling capacitor across the signals. The type of signal, however, listed in the Pin Attributes table (Table 4-1) is GND for those pins. The ADC power is that same situation. Do I connect VSSA_USB and VSSA_ADC to ground in the system or just use them for decoupling?
There are similar issues with the oscillators for the processor and the RTC, but there are subtle differences. They both seem to have their own VSS pins, VSS_OSC and VSS_RTC. The pin type of these signals in table 4-1, however, is A or analog. In table 4-1 there is a reference note for these signals which refers you to section 6.2.2 in the document. In that section there is a figure 6-9. In that figure, the VSS_OSC pin is shown connected to ground. Is that correct? Farther down in the document, figure 6-11 shows an external oscillator driving the CPU oscillator input and VSS_OSC tied to ground. Should the oscillator and RTC VSS pins connect to ground?
As a point of reference, I used the schematics for two actual designs, Beagle Bone Black and The TI evaluation module TMDSSK3358. In the Beagle Bone schematic, it shows the USB and analog VSS pins tied to ground and the Oscillator and RTC VSS pins are connected to decoupling connectors, but are isolated from ground. In the SK3358 schematic, the USB VSS pins are tied to ground and the Oscillator and RTC VSS pins are only connected to the decoupling caps, but not to ground.
So, the question is: Which of the following signals on the AM3351BZCE tie to ground?
VSSA_USB
VSSA_ADC
VSS_OSC
VSS_RTC
Thanks for any help.
