add write_cmd_spll for designs without PLLs

docs/source/bitstream/commands.rst

@@ -204,6 +204,9 @@ CMD_SPLL
 
 There are two configuration sets, that could be set for each PLL.
 
+.. warning::
+    Value 0x00 is special case and it is used for writing to all 4 PLLs, same as 0x0F
+
 CMD_WAIT_PLL
 -------------
 

libgm/src/Bitstream.cpp

@@ -961,6 +961,10 @@ Bitstream Bitstream::serialise_chip(const Chip &chip, const std::map<std::string
                 size = Die::PLL_CFG_SIZE + Die::CLKIN_CFG_SIZE;
             if (!die.is_glbout_cfg_empty())
                 size = Die::PLL_CFG_SIZE + Die::CLKIN_CFG_SIZE + Die::GLBOUT_CFG_SIZE;
+            // Since it is not possible to skip writing to PLL configuration
+            // we overwrite B configuration of last PLL with all zeros in this case.
+            // This is needed for reconfigurable bitstreams not to affect PLL of bootloader.
+            wr.write_cmd_spll(0x80);
             wr.write_cmd_pll(0, die_config, size);
         }