EP2C8T144I8N Datasheet, PDF (312/470 Pages) Altera Corporation – Cyclone II Device Handbook, Volume 1

Pad Placement and DC Guidelines
If the bidirectional pads are all controlled by the same output enable (OE)
and there are no other outputs or voltage referenced inputs in the bank,
then there is no case where there is a voltage referenced input is active at
the same time as an output. Therefore, the output limitation does not
apply. However, since the bidirectional pads are linked to the same OE,
all the bidirectional pads act as inputs at the same time. Therefore, the
input limitation of 30 input pads (15 on each side of the V_REF pad) for
FineLine BGA packages and 20 input pads (10 on each side of the V_REF
pad) for QFP packages applies.
If the bidirectional pads are all controlled by different OEs, and there are
no other outputs or voltage referenced inputs in the bank, then there may
be a case where one group of bidirectional pads is acting as inputs while
another group is acting as outputs. In such cases, apply the formulas
shown in Table 10–8.
Table 10–8. Input-Only Bidirectional Pad Limitation Formulas
Package Type
FineLine BGA
QFP
Formula
(Total number of bidirectional pads) – (Total number of pads
from the smallest group of pads controlled by an OE) ≤9
(per VCCIO and ground pair)
(Total number of bidirectional pads) – (Total number of pads
from the smallest group of pads controlled by an OE) ≤5
(per VCCIO and ground pair).
Consider a FineLine BGA package with four bidirectional pads controlled
by the first OE, four bidirectional pads controlled by the second OE, and
two bidirectional pads controlled by the third OE. If the first and second
OEs are active and the third OE is inactive, there are 10 bidirectional pads,
but it is safely allowable because there would be 8 or fewer outputs per
VCCIO/GND pair.
When at least one additional voltage referenced input and no other
outputs exist in the same V_REF bank, the bidirectional pad limitation
applies in addition to the input and output limitations. See the following
equations:
Total number of bidirectional pads + total number of input pads ≤30
(15 on each side of your V_REF pad) for Fineline BGA packages
Total number of bidirectional pads + total number of input pads ≤20
(10 on each side of your V_REF pad) for QFP packages
10–30
Cyclone II Device Handbook, Volume 1
Altera Corporation
February 2008