User Manual
BCM1250/BCM1125/BCM1125H
10/21/02
B r o a d c o m C o r p o r a t i o n
Document
1250_1125-UM100CB-R
Section 8: PCI Bus and HyperTransport Fabric Page
257
The transmitter registers work similarly. Bits in the Numerator indicate that data should be inserted into the
transmit FIFO from the internal clock. In the BCM1250 or BCM1125H, the clock ratio is fixed to 1/4. Data is
inserted into the FIFO 8 bytes wide on the internal clock and removed byte wide on each edge of the
HyperTransport transmit clock. These cancel, so the timing registers must be configured to allow data to be
inserted every internal cycle. The defaults of
SriTxNum
= 32’hFFFF and
SriTxDen
= 16 are suitable values.
Receive Pointer Margin Control in SriCmd Register
The receive FIFO is used to separate the link receive clock domain from the internal HyperTransport interface
clock domain. To allow for the data to become stable and valid some time must be provided between writing
an entry in the FIFO and reading it. This is done by having an offset (or margin) between the load and unload
pointers. The initial separation is configured in the
SriCmd
register.
The value set for the initial margin is computed from the minimum margin that is needed once the interface is
running, adjusted for the movement of the load and unload pointers during the initialization sequence. (For
implementation reasons the value programmed in the configuration register is twice the basic computation,
hence the extra doubling in the discussion below.)
The configuration register must be set to twice the offset that the unload pointer should have from the load
pointer when the link is reset. If the pointers did not move during the initialization process this would simply be:
2*( - margin for data to be stable)
This is negative because it is measured from the load pointer to the unload pointer, which must always be
behind. (Since the pointers cycle around the FIFO the offset must be taken mod 8 to get the actual value
programmed.) In asynchronous mode the load and unload pointers are stationary during initialization so the
correct value can be obtained directly from this computation. In synchronous mode the load and unload
pointers are always being moved according to the ratio set in the
SriRxNum
and
SriRxDen
registers, so a
correction must be applied.
In synchronous mode, the received SYNC pattern indication from the link passes through four synchronization
stages clocked by the internal HyperTransport clock on its way in to the link initialization logic. The load pointer
for the receive FIFO will advance every two receive clocks (the incoming data is on every edge of the clock,
so the four byte FIFO width builds up in two clocks). The minimum advance of the FIFO load pointer during the
SYNC synchronization is therefore:
(time for SYNC propagation to initialization logic)/(time per load pointer increment)
= (4 * internal clock period)/(2 * receive clock period)
= 2*f
RX
/f
LDTINT
The FIFO wraps every eight entries, so this value is taken modulo 8.
The pointer offset must also be adjusted to take into account the possible movement of the unload pointer. The
worst case movement of the unload pointer must be used, since that brings it closest to the load pointer. The
maximum unload pointer movement can be computed from the
SriRxNum
value. If there are two adjacent 1s
anywhere (including over the wrap from bit [SriRxDen-1] to 0) in the Numerator then it is possible for two (32
bit) words to be extracted from the FIFO during SYNC. If there are only 10 and 01 patterns for adjacent bits
then only one word can be extracted during SYNC.
On the BCM1250 or the BCM1125H, the recommended time allowed for the data to become stable is two
cycles.
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