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