chapter 8.pdf

Section 8.3 Sequential PLDs 593DOt CO This parameter appliesNOTto registered outputs. It is the propagationCOPYdelay t from the rising edge of CLK to a primary output.tDOCF This parameter also applies to registered outputs. It is the propagation tNOT COPYCFdelay from the rising edge of CLK to a macrocell’s registered output thatconnects back to a feedback input. If specified, t CF is normally less thant CO . However, some manufacturers do not specify t CF , in which case youmust assume that t CF = t CO .tDOSU This parameter applies to primary, bidirectional, and feedback inputs tNOT COPYSUthat propagate to the D inputs of flip-flops. It is the setup time that theinput signal must be stable before the rising edge of CLK.t H This parameter also applies to signals that propagate to the D inputs of t Hflip-flops. It is the hold time that the input signal must be stable after therising edge of CLK.fDOmax This parameter appliesNOTto clocked operation. It is theCOPYhighest frequency f maxat which the PLD can operate reliably, and is the reciprocal of theminimum clock period. Two versions of this parameter can be derivedfrom the previous specifications, depending on whether the device isoperating with external feedback or internal feedback.DO NOT COPYExternal feedback refers to a circuit in which a registered PLD output is external feedbackconnected to the input of another registered PLD with similar timing; forproper operation, the sum of t CO for the first PLD and t SU for the secondmust not exceed the clock period.DOInternal feedback refersNOTto a circuit in which a registeredCOPYPLD output is fed internal feedbackback to a register in the same PLD; in this case, the sum of t CF and t SU mustnot exceed the clock period.Each of the PLDs that we described in previous sections is available inseveralDOdifferent speed grades.NOTThe speed grade is usually indicatedCOPYby a suffixon the part number, such as “16V8-10”; the suffix usually refers to the t PDinput orfeedbackDO NOTt SUCOPYt Hinput orCLKCLKfeedbacktDOPDtcombinationaloutputNOTCOtCOPYCF t SUregisteredregisteredoutputfeedback(a) (b)1/ f max(c)1/f max(external feedback)(internal feedback)Figure 8-25 PLD timing parameters.Copyright © 1999 by John F. WakerlyCopying Prohibited