2.3.2.2 Simultaneous Switching Outputs (SSO)
When multiple output drivers switch simultaneously, they induce a voltage drop in the chip/package power distribution. This simultaneous switching momentarily raises the ground voltage within the device relative to the system ground. This apparent shift in the ground potential to a non-zero value is known as Simultaneous Switching Noise (SSN) or more commonly, ground bounce.
SSN becomes more of an issue in high pin count packages and when using high performance devices such as the Axcelerator family. Based upon testing, Microchip recommends users not to exceed eight Simultaneous Switching Outputs (SSO) per each VCCI/GND pair. To ease this potential burden on designers, Microchip has designed all of the Axcelerator BGAs3 to not exceed this limit with the exception of the CS180, which has an I/O to VCCI/GND pair ratio of nine to one.
For more information, see the Simultaneous Switching Noise and Signal Integrity application note.
1
The user should note that in Bank 8 of both AX1000-FG484 and AX500-FG484, there are local violations of this 8:1 ratio.