libs: arch: interchange: add comment on constant blocks

Signed-off-by: Alessandro Comodi <[email protected]>

Author: acomodi

libs/libarchfpga/src/read_fpga_interchange_arch.cpp

@@ -2040,6 +2040,31 @@ struct ArchReader {
         }
     }
 
+    /** @brief The constant block is a synthetic tile which is used to assign a virtual
+     *         location in the grid to the constant signals which are than driven to
+     *         all the real constant wires.
+     *
+     * The block's diagram can be seen below. The port names are specified in
+     * the interchange device database, therefore GND and VCC are mainly
+     * examples in this case.
+     *
+     * +---------------+
+     * |               |
+     * |  +-------+    |
+     * |  |       |    |
+     * |  |  GND  +----+--> RR Graph node
+     * |  |       |    |
+     * |  +-------+    |
+     * |               |
+     * |               |
+     * |  +-------+    |
+     * |  |       |    |
+     * |  |  VCC  +----+--> RR Graph node
+     * |  |       |    |
+     * |  +-------+    |
+     * |               |
+     * +---------------+
+     */
     void process_constant_block() {
         std::vector<std::pair<std::string, std::string>> const_cells{arch_->gnd_cell, arch_->vcc_cell};