Bit
Name
Description
match due to delays in the part
used to make up the final IPG.
Changes should be added or sub-
tracted from the provided hex val-
ue on a one-for-one basis.
31-4 RES
Reserved locations. Written as
zeros and read as undefined.
3
RPA
Runt Packet Accept. This bit
forces the Am79C978 controller
to accept runt packets (packets
shorter than 64 bytes).
CAUTION: Use this parameter
with care. By lowering the IPG
below the ISO/IEC 8802-3 stan-
dard 96 bit times, the
Am79C978 controller can inter-
rupt normal network behavior.
This bit is read accessible al-
ways; write accessible only when
STOP is set to 1. RPA is cleared
by H_RESET or S_RESET and is
not affected by STOP.
These bits are read accessible al-
ways. Write accessible when the
STOP bit is set to 1. IPG is set to
60h (96 Bit times) by H_RESET
or S_RESET and is not affected
by STOP.
2-0
RES
Reserved locations. Written as
zeros and read as undefined.
CSR125: MAC Enhanced Configuration Control
7-0
IFS1
InterFrameSpacingPart1. Chang-
ing IFS1 allows the user to pro-
gram the value of the InterFrame-
Bit
Name
Description
31-16 RES
15-8 IPG
Reserved locations. Written as
zeros and read as undefined.
SpacePart1
timing.
The
Am79C978 controller sets the de-
fault value at 60 bit times (3ch).
See the subsection on Medium
Allocation in the section Media
Access Management for more
details. The equation for setting
IFS1 when IPG ≥ 96 bit times is:
Inter Packet Gap. Changing IPG
allows the user to program the
Am79C978 controller for aggres-
siveness on a network. By chang-
ing the default value of 96 bit
times (60h) the user can adjust
the fairness or aggressiveness of
the Am79C978 integrated MAC
on the network. By programming
a lower number of bit times other
then the ISO/IEC 8802-3 stan-
dard requires, the Am79C978
controller will become more ag-
gressive on the network. This ag-
gressive nature will give rise to
the Am79C978 controller possi-
bly “capturing the network” at
times by forcing other less ag-
gressive nodes to defer. By pro-
gramming a larger number of bit
times, the Am79C978 home net-
working MAC will become less
aggressive on the network and
may defer more often than nor-
mal. The performance of the
Am79C978 controller may de-
crease as the IPG value is in-
creased from the default value.
IFS1 = IPG - 36 bit times
Note: Programming of the IPG
should be done in nibble intervals
instead of absolute bit times due
to the MII. The decimal and hex
values do not match due to de-
lays in the part used to make up
the final IPG.
Changes should be added or
subtracted from the provided hex
value on a one-for-one basis.
Due to changes in synchroniza-
tion delays internally through dif-
ferent network ports, the IFS1
can be off by as much as +12 bit
times.
These bits are read accessible al-
ways. Write accessible only when
the SPND bit or the STOP bit is
set to 1. IFS1 is set to 3ch (60 bit
Note: Programming of the IPG
should be done in nibble intervals
instead of absolute bit times. The
decimal and hex values do not
times)
by
H_RESET
or
S_RESET and is not affected by
STOP.
144
Am79C978
AMD [ AMD ]
