Cirrus Logic CS8900A - Manual (Page 83)

DS271F5 83
CS8900A
Crystal LAN™ Ethernet Controller
CIRRUS LOGIC PRODUCT DATASHEET
tecting the incoming frame's End-of-Frame
(EOF) sequence.
Like all Event bits, RxDest and Rx128 are set
by the CS8900A whenever the appropriate
event occurs. Unlike other Event bits, RxDest
and Rx128 may be cleared by the CS8900A
without host intervention. All other event bits
are cleared only by the host reading the appro-
priate event register, either directly or through
the Interrupt Status Queue (ISQ). (RxDest and
Rx128 can also be cleared by the host reading
the BufEvent register, either directly or through
the Interrupt Status Queue). Figure 22 pro-
vides a diagram of the Early Interrupt process.
5.2.3.3 Acceptance Filtering
The third step of pre-processing is to deter-
mine whether or not to accept the frame by
comparing the frame with the criteria pro-
grammed into the RxCTL register (Register 5).
If the receive frame passes the Acceptance fil-
ter, the frame is buffered, either on chip or in
host memory via DMA. If the frame fails the
Acceptance filter, it is discarded. The results of
the Acceptance filter are reported in the Rx-
Event register (Register 4).
5.2.3.4 Normal Interrupt Generation
The final step of pre-processing is to generate
any enabled interrupts that are triggered by
the incoming frame. Interrupt generation oc-
curs when the entire frame has been buffered
(up to the first 1518 bytes). For more informa-
tion about interrupt generation, see
Section 5.1 on page 78.
5.2.4 Held vs. DMAed Receive Frames
All accepted frames are either held in on-chip
RAM until processed by the host, or stored in
host memory via DMA. A receive frame that is
held in on-chip RAM is referred to as a held re-
ceive frame. A frame that is stored in host
memory via DMA is a DMAed receive frame.
Pass
DA Filter? Discard Frame
Destination
Address Filter
Check:
- PromiscuousA?
- IAHashA?
- MulticastA?
- IndividualA?
- BroadcastA?
Receive Frame
Yes
No
Yes No
Generate Interrupts
Check:
- RxOKiE?
- ExtradataiE?
- CRCerroriE?
- RuntiE?
- RxDMAiE?
Pre-Processing
Complete
Generate Early
Interrupts if Enabled
(see next figure)
Acceptance Filter
Check:
- RxOKA?
- ExtradataA?
- RuntA?
- CRCerrorA?
Status of receive
frame reported in
RxEvent register,
frame discarded.
Status of receive
frame reported in
RxEvent register,
frame accepted
into on-chip RAM
Pass
Accept.
Filter?
Figure 21. Receive Frame Pre-Processing