®
MAILBOX OVERVIEW
S5935
FUNCTIONAL DESCRIPTION
MAILBOX OVERVIEW
Figure 1 shows a block diagram of the PCI to Add-On
mailbox registers. Add-On incoming mailbox read ac-
cesses pass through an output interlock latch. This
prevents a PCI bus write to a PCI outgoing mailbox
from corrupting data being read by the Add-On. Fig-
ure 2 shows a block diagram of the Add-On to PCI
mailbox registers. PCI incoming mailbox reads also
pass through an interlocking mechanism. This pre-
vents an Add-On write to an outgoing mailbox from
corrupting data being read by the PCI bus. The fol-
lowing sections describe the mailbox flag functionality
and the mailbox interrupt capabilities.
The S5935 has eight 32-bit mailbox registers. The mail-
boxes are useful for passing command and status infor-
mation between the Add-On and the PCI bus. The PCI
interface has four incoming mailboxes (Add-On to PCI)
and four outgoing mailboxes (PCI to Add-On). The Add-
On interface has four incoming mailboxes (PCI to Add-
On) and four outgoing mailboxes (Add-On to PCI). The
PCI incoming and Add-On outgoing mailboxes are the
same, internally. The Add-On incoming and PCI outgo-
ing mailboxes are also the same, internally.
The mailbox status may be monitored in two ways.
The PCI and Add-On interfaces each have a mailbox
status register to indicate the empty/full status of
bytes within the mailboxes. The mailboxes may also
be configured to generate interrupts to the PCI and/or
Add-On interface. One outgoing and one incoming
mailbox on each interface can be configured to gen-
erate interrupts.
Figure 1. Block Diagram - PCI to Add-On Mailbox Register
MAILBOX
REGISTER
ADD-ON
BUS
"INCOMING
MAILBOX"
SELECT
ADD-ON
BUS
"INCOMING MAILBOX"
OUTPUT
INTERLOCK
LATCH
OUTPUT
DRIVER
PCI BUS
"OUTGOING MAILBOX"
D
Q
D
Q
LOAD ENABLE
EN
EN
ADD-ON
RD#
READ ENABLE
SELECT#
MAILBOX
FULL
S
"O"
D
Q
EMPTY/FULL FF
SELECTED READ ENABLE
Figure 2. Block Diagram - Add-On to PCI Mailbox Register
PCI BUS
OUTPUT
INTERLOCK
LATCH
PCI
MAILBOX
REGISTER
ADD-ON
BUS
"OUTGOING
MAILBOX"
"INCOMING MAILBOX"
"INCOMING
MAILBOX"
SELECT
Q
D
Q
D
EN
PCI READ PULSE
WR#
SELECT#
ADD-ON WRITE PULSE
MAILBOX
FULL
S
Q
"O"
D
REGISTER
DECODE OF
ADR[6:2]
BE[3:0]#
EMPTY/FULL FF
SELECTED
READ PULSE
9-109
AMCC [ APPLIED MICRO CIRCUITS CORPORATION ]
