2018-11-15 19:25:26 +08:00
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# Constraints
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There are three types of constraints available for end users of nextpnr.
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2018-11-23 05:55:46 +08:00
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## Architecture-specific IO Constraints
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2018-11-15 19:25:26 +08:00
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Architectures may provide support for their native (or any other) IO constraint format.
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The iCE40 architecture supports PCF constraints thus:
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set_io led[0] 3
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and the ECP5 architecture supports a subset of LPF constraints:
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LOCATE COMP "led[0]" SITE "E16";
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IOBUF PORT "led[0]" IO_TYPE=LVCMOS25;
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## Absolute Placement Constraints
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nextpnr provides generic support for placement constraints by setting the Bel attribute on the cell to the name of
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the Bel you wish it to be placed at. For example:
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(* BEL="X2/Y5/lc0" *)
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## Clock Constraints
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There are two ways to apply clock constraints in nextpnr. The `--clock {freq}` command line argument is used to
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apply a default frequency (in MHz) to all clocks without a more specific constraint.
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The Python API can apply clock constraints to specific named clocks. This is done by passing a Python file
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specifying these constraints to the `--pre-pack` command line argument. Inside the file, constraints are applied by
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calling the function `ctx.addClock` with the name of the clock and its frequency in MHz, for example:
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ctx.addClock("csi_rx_i.dphy_clk", 96)
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ctx.addClock("video_clk", 24)
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ctx.addClock("uart_i.sys_clk_i", 12)
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