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ProASIC3/E Flash Family FPGAs
v2.1
2-43
5 V Output Tolerance
ProASIC3 I/Os must be set to 3.3 V LVTTL or 3.3 V LVCMOS
mode to reliably drive 5 V TTL receivers. It is also critical
that there be NO external I/O pull-up resistor to 5 V, since
this resistor would pull the I/O pad voltage beyond the
3.6 V absolute maximum value, and consequently cause
damage to the I/O.
When set to 3.3 V LVTTL or 3.3 V LVCMOS mode,
ProASIC3 I/Os can directly drive signals into 5 V TTL
receivers. In fact, VOL = 0.4 V and VOH = 2.4 V in both
3.3 V LVTTL and 3.3 V LVCMOS modes exceeds the
VIL =0.8 V and VIH = 2 V level requirements of 5 V TTL
receivers. Therefore, level '1' and level '0' will be
recognized correctly by 5 V TTL receivers.
Simultaneous Switching Outputs (SSOs)
and Printed Circuit Board Layout
SSOs can cause signal integrity problems on adjacent
signals that are not part of the SSO bus. Both inductive
and capacitive coupling parasitics of bond wires inside
packages and of traces on PCBs will transfer noise from
SSO busses onto signals adjacent to those busses.
Additionally, SSOs can produce ground bounce noise and
VCCI dip noise. These two noise types are caused by
rapidly changing currents through GND and VCCI
package pin inductances during switching activities
Ground bounce noise voltage = L(GND) × di/dt
EQ 2-1
VCCI dip noise voltage = L(VCCI) × di/dt
EQ 2-2
Any group of four or more input pins switching on the
same clock edge is considered an SSO bus. The shielding
should be done both on the board and inside the
package unless otherwise described.
In-package shielding can be achieved in several ways; the
required shielding will vary depending on whether pins
next
to
the
SSO
bus
are
LVTTL/LVCMOS
inputs,
LVTTL/LVCMOS outputs, or GTL/SSTL/HSTL/LVDS/LVPECL
inputs and outputs. Board traces in the vicinity of the
SSO bus have to be adequately shielded from mutual
coupling and inductive noise that can be generated by
the SSO bus. Also, noise generated by the SSO bus needs
to be reduced inside the package.
PCBs perform an important function in feeding stable
supply voltages to the IC and, at the same time,
maintaining signal integrity between devices.
Key issues that need to considered are as follows:
Power and ground plane design and decoupling
network design
Transmission line reflections and terminations