BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
258
Section 8: PCI Bus and HyperTransport Fabric
Document
1250_1125-UM100CB-R
Putting the three terms together, the value to be programmed for the RxMargin field in synchronous mode is:
RxMargin = 2*(load pointer updates - unload pointer updates - margin for data to be stable)
= 2*(2*f
RX
/f
LDTINT
- (1 or 2) - 2)
In asynchronous mode the RxMargin field should be programmed to twice the number of receive clock cycles
for the data to settle. The settle time recommended for the BCM1250 or BCM1125H is 7ns, so in asynchronous
mode:
RxMargin = 2*7ns*f
RX
Transmit Pointer Initial Offset in the SriCmd Register
The transmit FIFO pointers must be offset to take account of the seven stages of synchronization using the
HyperTransport transmit clock that are applied to the reset pulse, and to allow time for data settle margin. The
minimum number of internal HyperTransport clocks taken for the synchronization time is given by:
minInternalSyncClocks
= 7*transmitClkPeriod / internalClkPeriod
= 7*f
LDTINT
/f
TX
= 7/4 = 1 (rounding down)
The minimum advance of the load pointer can be found from the
SriTxNum
value. In this case the Numerator
is all 1s so the minimum advance is 2, if the Numerator had 10 or 01 pairs of bits then the minimum advance
would be 1.
The recommended data settle margin is 2. This gives:
TxInitialOffset
= minInternalSyncClocks - minPointerAdvance - Margin
= 1 - 2 - 2 = -3 MOD 8 = 5.
Error Control Register
The Error Control Register sets the error handling behavior of the HyperTransport interface for both the
standard HyperTransport errors and the additional ones reported on the BCM1250 or BCM1125H. The
possible responses to errors are: to raise the ldt_fatal_int interrupt, to raise the ldt_nonfatal_int interrupt and
to flood the link with SYNC packets. Any of these may be selected as the response to any of the errors. (The
CRC error causing SYNC flooding is set in the standard Link Configuration Register).
Transmit Control Register
The transmit control register must be configured to set the minimum number of packets that will be sent
between NOP packets that are reporting buffer releases to the device on the other end of the link. If the
transmitter is idle, the NOP packet will be sent as soon as the buffer is free. If the transmitter is busy and
becomes idle the NOP packet will be sent when the link idles. If the transmitter is continually busy the NOP
packets will be spaced by the number of packets set in this register.
If the value is set too low then the NOP packets will frequently interrupt transmission of data, so the link
utilization will fall. If the value is set too high then buffer releases will not be reported in a timely manner, so the
device the other end of the link may throttle its transmissions unnecessarily.
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