ProASIC3/E SSO and Pin Placement Guidelines
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the board alongside the SSO bus for the whole length of the SSO traces and on the same board
layer. These shielding traces should be connected to board ground at both ends of their length.
Shielding Using Other Pins
The type of shielding pins is not restricted to GND or virtual grounds. The shielding pins can also be
VCCI, VCCII, virtual VCCI, unused I/Os, or used I/Os that are not sensitive to SSO effects (e.g., outputs
driving LEDs).
How to Use This Document
The rest of this document is divided based on three different SSO effects: on outputs, on inputs,
and on Clock Conditioning Circuits (CCCs). Each section includes tables that identify the
recommended maximum number of SSOs and the required shielding if the number of SSOs exceeds
the recommendation. The tables are categorized by device/package type (e.g., A3P600-FG484) and
I/O configuration of the SSO bus (i.e., drive strength and slew rate). If the desired device/package
combination cannot be found in the tables, choose the SSO recommendation for the closest
package type and the next smaller die size. The following example describes two scenarios in which
the SSO recommendation for another device/package can be used for a member of the ProASIC3/E
family:
1. SSO guidelines for A3P250-PQ208 can be used when designing for A3P400-PQ208.
2. SSO guidelines for A3PE600-FG484 can be used when designing for A3PE1500-FG676.
You should study this entire document, consider the desired device/package combination, define
the worst-case SSO scenario, and use the SSO guidelines or shielding recommendations described in
the tables. At the end of each section, guidelines are given on how to mitigate the effects of SSOs.
Note that the data presented in this document is collected at nominal operating conditions (1.5 V
core voltage and room temperature). CMOS transistors switch faster when cold, and therefore the
edge rates become faster, so SSO effects are usually worse at lower temperatures.
At the end of this document, some general board-level design guidelines are included. Actel
recommends that you follow these guidelines when designing boards.
SSO Effects on Outputs
categorized by ground bounce, VCCI bounce, and push-out. The following sections give the
characteristics of SSO effects on outputs and provide guidelines on how to mitigate these effects.
Ground and VCCI Bounce
The most widely known effects of SSOs are ground and VCCI bounce. This section characterizes
ProASIC3/E ground and VCCI bounce in the presence of SSOs. Since outputs with higher drive
strength or faster slew rate source/sink higher current at the time of switching, SSOs are more
disruptive when they are configured at higher drive strength and high slew rate.
Table 21-1 onpage 21-5 lists the number of SSOs causing specified levels of ground and VCCI bounce for various device, package, and SSO bus configurations. A disruptive ground bounce is one with a 1.25 V peak
and 1 ns width—enough to trigger a high-speed input to change its value from zero to one.
Similarly, a disruptive VCCI bounce causes oscillations on the quiet output (driving HIGH) with a
magnitude of 2 V and width of 1 ns. These values are chosen based on Actel bench experiments
using typical CMOS input sensitivity.