am79c972b Advanced Micro Devices, am79c972b Datasheet - Page 40

• Manufacturer Part Number: am79c972b
• Description: Pcnet?-fast+ Enhanced 10/100 Mbps Pci Ethernet Controller With Onnow Support
• Manufacturer: Advanced Micro Devices

When the preemption occurs after the counter has
counted down to 0, the Am79C972 controller will finish
the current data phase, deassert FRAME, finish the
last data phase, and release the bus. Note that it is im-
portant for the host to program the PCI Latency Timer
according to the bus bandwidth requirement of the
Am79C972 controller. The host can determine this bus
bandwidth requirement by reading the PCI MAX_LAT
and MIN_GNT registers.
Figure 20 assumes that the PCI Latency Timer has
counted down to 0 on clock 7.
Master Abort
The Am79C972 controller will terminate its cycle with a
Master Abort sequence if DEVSEL is not asserted
within 4 clocks after FRAME is asserted. Master Abort
is treated as a fatal error by the Am79C972 controller.
40
DEVSEL
FRAME
TRDY
STOP
C/BE
IRDY
REQ
GNT
CLK
PAR
AD
1
DEVSEL is sampled
Figure 18. Target Abort
2
ADDR
0111
3
PAR
4
DATA
0000
5
PAR
6
21485C-21
7
Am79C972
The Am79C972 controller will reset all CSR locations
to their STOP_RESET values. The BCR and PCI con-
figuration registers will not be cleared. Any on-going
network transmission is terminated in an orderly se-
quence. If less than 512 bits have been transmitted
onto the network, the transmission will be terminated
immediately, generating a runt packet. If 512 bits or
more have been transmitted, the message will have the
current FCS inverted and appended at the next byte
boundary to guarantee an FCS error is detected at the
receiving station.
RMABORT (in the PCI Status register, bit 13) will be set
to indicate that the Am79C972 controller has termi-
nated its transaction with a master abort. In addition,
SINT (CSR5, bit 11) will be set to 1. When SINT is set,
INTA is asserted if the enable bit SINTE (CSR5, bit 10)
is set to 1. This mechanism can be used to inform the
driver of the system error. The host can read the PCI
Status register to determine the exact cause of the in-
terrupt. See Figure 21.
Parity Error Response
During every data phase of a DMA read operation,
when the target indicates that the data is valid by as-
serting TRDY, the Am79C972 controller samples the
AD[31:0], C/BE[3:0] and the PAR lines for a data parity
error. When it detects a data parity error, the controller
sets PERR (PCI Status register, bit 15) to 1. When re-
porting of that error is enabled by setting PERREN
(PCI Command register, bit 6) to 1, the Am79C972
controller also drives the PERR signal low and sets
DATAPERR (PCI Status register, bit 8) to 1. The asser-
tion of PERR follows the corrupted data/byte enables
by two clock cycles and PAR by one clock cycle.
Figure 22 shows a transaction that has a parity error in
the data phase. The Am79C972 controller asserts
PERR on clock 8, two clock cycles after data is valid.
The data on clock 5 is not checked for parity, since on
a read access PAR is only required to be valid one
clock after the target has asser ted TRDY. The
Am79C972 controller then drives PERR high for one
clock cycle, since PERR is a sustained tri-state signal.
During every data phase of a DMA write operation, the
Am79C972 controller checks the PERR input to see if
the target reports a parity error. When it sees the PERR
input asserted, the controller sets PERR (PCI Status
register, bit 15) to 1. When PERREN (PCI Command
register, bit 6) is set to 1, the Am79C972 controller also
sets DATAPERR (PCI Status register, bit 8) to 1.