20
ispMACH 4A Family
ispMACH 4A TIMING MODEL
The primary focus of the ispMACH 4A timing model is to accurately represent the timing in a ispMACH
4A device, and at the same time, be easy to understand. This model accurately describes all combinatorial
and registered paths through the device, making a distinction between internal feedback and external
feedback. A signal uses internal feedback when it is fed back into the switch matrix or block without having
to go through the output buffer. The input register specifications are also reported as internal feedback.
When a signal is fed back into the switch matrix after having gone through the output buffer, it is using
external feedback.
The parameter, tBUF, is defined as the time it takes to go from feedback through the output buffer to the
I/O pad. If a signal goes to the internal feedback rather than to the I/O pad, the parameter designator is
followed by an “i”. By adding tBUF to this internal parameter, the external parameter is derived. For
example, tPD = tPDi + tBUF. A diagram representing the modularized ispMACH 4A timing model is shown
in Figure 15. Refer to the application note entitled MACH 4 Timing and High Speed Design for a more detailed
discussion about the timing parameters.
SPEEDLOCKING FOR GUARANTEED FIXED TIMING
The ispMACH 4A architecture allows allocation of up to 20 product terms to an individual macrocell with
the assistance of an XOR gate without incurring additional timing delays.
The design of the switch matrix and PAL blocks guarantee a fixed pin-to-pin delay that is independent of
the logic required by the design. Other competitive CPLDs incur serious timing delays as product terms
expand beyond their typical 4 or 5 product term limits. Speed and SpeedLocking combine to give designs
easy access to the performance required in today’s designs.
(External Feedback)
(Internal Feedback)
INPUT REG/
INPUT LATCH
tSIRS
tHIRS
tSIL
tHIL
tSIRZ
tHIRZ
tSILZ
tHILZ
tPDILi
tICOSi
tIGOSi
tPDILZi
Q
tSS(T)
tSA(T)
tH(S/A)
tS(S/A)L
tH(S/A)L
tSRR
tPDi
tPDLi
tCO(S/A)i
tGO(S/A)i
tSRi
COMB/DFF/TFF/
LATCH/SR*/JK*
S/R
IN
BLK CLK
OUT
tPL
tBUF
tEA
tER
tSLW
Q
Central
Switch
Matrix
*emulated
17466G-025
Figure 15. ispMACH 4A Timing Model