Altera HardCopy II Clock Uncertainty Calculator - Manual (Page 29)

Altera Corporation 3–3
HardCopy II Clock Uncertainty Calculator User Guide
Design Case Exceptions
Clock Divider
Figure 3–4 shows an example of a clock divider for intra-clock transfer, in
which CLK1 is accounted for in the clock uncertainty calculator, but not
CLK2. You should add 25 ps to both the setup and hold clock uncertainty
values.
Figure 3–4. Clock Divider for Intra-Clock Transfer
Ripple Clock
Figure 3–5 shows a ripple clock as an intra-clock transfer example. A
ripple clock is similar to a divided clock, but uses a different calculation
to account for extra clock uncertainty value.
CLK0 is accounted for by the clock uncertainty calculator, but not CLK1
and
CLK2. You need to add 25 ps uncertainty for the CLK1 network and
also add 25 ps uncertainty for the CLK2 network. Therefore, you should
add 50 ps on both setup and hold clock uncertainty for the example
shown in
Figure 3–5 .
Figure 3–5. Ripple Clock for Intra-Clock Transfer
PLL
INBUF
Source
Clock
Destination
Clock
Source
Register
Destination
Register
CLK1
CLK2
CLK3
CLK1
clk_a
clk_b
CLK1
CLK0
reg_c reg_d
CLK2