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Ignore paths which use shared clock lines.

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Quartus calculates fmax (the theoretical maximum clock rate) based on
the entirety of the logic between registers. In the case of the pclk_*
lines, this includes some invalid paths which cross between the
3x <-> 2x and 5x <-> 4x clock domains. This is because these clocks
share output pins from the PLL, but the PLL is configured to output only
one of these clocks at a time, and the correct output from the logic is
selected via a multiplexer. Therefore these paths cannot co-occur.

This has the effect of increasing the calculated fmax of these paths to:

pclk_3x: 107.98 MHz -> 132.52 MHz
pclk_5x: 162.23 MHz -> 170.33 MHz
This commit is contained in:
Russell Harmon 2020-06-01 00:17:26 +00:00
parent b1892079d8
commit d80a9fbb0c
1 changed files with 4 additions and 0 deletions
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@ -81,6 +81,10 @@ set_false_path -from [get_registers {scanconverter_inst|H_* scanconverter_inst|V
# Ignore paths from registers which are updated only at leading edge of hsync
#set_false_path -from [get_registers {scanconverter:scanconverter_inst|line_idx scanconverter:scanconverter_inst|line_out_idx* scanconverter:scanconverter_inst|hmax*}]
# Ignore paths that cross clock domains from 3x to 2x and 5x to 4x, since they share a clock line, but cannot co-occur.
set_false_path -from [get_clocks {pclk_3x*}] -to [get_registers {scanconverter:scanconverter_inst|*_2x*}]
set_false_path -from [get_clocks {pclk_5x*}] -to [get_registers {scanconverter:scanconverter_inst|*_4x*}]
# Ignore paths to latency tester sync regs
set_false_path -to [get_registers {lat_tester:lt0|mode_synced* lat_tester:lt0|VSYNC_in_* lat_tester:lt0|trigger_*}]