PC820/PC821 firmware’s that have been released between 6/29/2016 and 5/2/2017 do have a clock boundary crossing issue on the irq_signal input of the axi_pc820_8l_host_if / axi_pc821_8l_host_if. This causes the interrupt pulse to not always being captured by the host interface. Because of this, read/writes of slow interfaces (for example I2C and SPI) that make use of this interrupt signal do not always work.
This issue can be fixed by updating the pc820_8l_host_if.vhd or the pc821_8l_host_if.vhd file of the host interface. These files can be found at the following location in your star library:
<CONSTELLATION_NAME>star_lib\axi_pc820_8l_host_if\vhdl
<CONSTELLATION_NAME>star_lib\axi_pc821_8l_host_if\vhdl
The Vivado project needs to be re-generated with StellarIP after this update.