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PALCE26V12 Family

fMAX Parameters

The parameter fMAX is the maximum clock rate at which

the device is guaranteed to operate. Because the flexi-

bility inherent in programmable logic devices offers a

choice of clocked flip-flop designs, fMAX is specified for

three types of synchronous designs.

The first type of design is a state machine with feedback

signals sent off-chip. This external feedback could go

back to the device inputs, or to a second device in a

multi-chip state machine. The slowest path defining the

period is the sum of the clock-to-output time and the in-

put setup time for the external signals (tS + tCO). The re-

ciprocal, fMAX, is the maximum frequency with external

feedback or in conjunction with an equivalent speed de-

vice. This fMAX is designated “fMAX external.”

The second type of design is a single-chip state ma-

chine with internal feedback only. In this case, flip-flop

inputs are defined by the device inputs and flip-flop out-

puts. Under these conditions, the period is limited by the

internal delay from the flip-flop outputs through the inter-

nal feedback and logic to the flip-flop inputs. This fMAX is

designated “fMAX internal”. A simple internal counter is a

good example of this type of design, therefore, this pa-

rameter is sometimes called “fCNT.”

The third type of design is a simple data path applica-

tion. In this case, input data is presented to the flip-flop

and clocked through; no feedback is employed. Under

these conditions, the period is limited by the sum of the

data setup time and the data hold time (tS + tH). How-

ever, a lower limit for the period of each fMAX type is the

minimum clock period (tWH + tWL). Usually, this minimum

clock period determines the period for the third fMAX,

designated “fMAX no feedback.”

fMAX external and fMAX no feedback are calculated pa-

rameters. fMAX external is calculated from tS and tCO, and

fMAX no feedback is calculated from tWL and tWH. fMAX in-

ternal is measured.

LOGIC

CLK

(SECOND

CHIP)

SCO

t S

fMAX External; 1/(tS + tCO)

LOGIC

CLK

LOGIC

CLK

fMAX No Feedback; 1/(tS + tH) or 1/(tWH + tWL)

fMAX Internal (fCNT)

16072E-18

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