Revision 1.02 – June 27, 2006
S5935 – PCI Product
Data Book
Servicing an Add-On mailbox interrupt (IRQ#):
1. Identify the interrupt source(s). Multiple interrupt sources are available on the S5935. The interrupt service routine must
verify that a mailbox generated the interrupt (and not some other interrupt source).
AINT
AINT
Bit 16
Bit 17
Add-On incoming mailbox interrupt indicator
Add-On outgoing mailbox interrupt indicator
2. Check mailbox status. The mailbox status bits indicate which mailbox bytes must be read or written.
AMBEF
AMBEF
Bits 31:16
Bits 15:0
Empty Add-On outgoing mailbox bytes
Full Add-On incoming mailbox bytes
3. Access the mailbox. Based on the contents of AMBEF, mailboxes are read or written. Reading an incoming mailbox
byte clears the corresponding status bit in AMBEF.
AIMBx
Bits 31:0
Bits 31:0
Add-On incoming mailboxes
Add-On outgoing mailboxes
AOMBx
4. Clear the interrupt source. The Add-On IRQ# signal is deasserted by clearing the interrupt request. The request is
cleared by writing a ‘1’ to the appropriate bit.
AINT
AINT
Bit 16
Bit 17
Clear Add-On incoming mailbox interrupt
Clear Add-On outgoing mailbox interrupt
In both cases, step 3 involves accessing the mailbox. To allow the incoming mailbox interrupt logic to be cleared, the mailbox
status bit must also be cleared. Reading an incoming mailbox clears the status bits. Another option for clearing the status bits
is to use the Mailbox Flag Reset bit in the MCSR and AGCSTS registers, but this clears all status bits, not just for a single
mailbox or mailbox byte. For outgoing mailbox interrupts, the read of a mailbox register is what generated the interrupt; this
ensures the status bits are already clear.
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DS1527
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