Wiener VM-USB User Manual page 23

Because of the architecture of the VM-USB, writing of the IRQ mask and reading it back is
not as easy as writing to other registers. This is because the mask resides in the Interface
CPLD (top XC95144XL CPLD, named U7) and there are no dedicated connections for this
purpose.
To write the IRQ mask:
1) Write the IRQ mask to bits 0-6 of the global register and set bit 15 to 1
2) Set bit 1 of the action register to 1.
3) While bit 15 of the global register is set, bits 24-30 of the Firmware ID register will
return the IRQ mask
4) Set bit 15 of the Global register to 0
5) Reset the proper bits to the global register to define the operational parameters
desired.
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