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
256
Section 8: PCI Bus and HyperTransport Fabric
Document
1250_1125-UM100CB-R
Configuration Flags in the SriCmd Register
There are three flags in the
SriCmd
register. Two of these should be set at the start of initialization.
The ReduceSyncZero flag should only be set during debugging; it reduces the number of bit times of zeros
during link initialization from the standard 512 to 128.
The SyncPtrCtl should be set if the interface is running in Synchronous Clock Mode (using the same reference
clock source as the other end of the link) and clear for Asynchronous Mode. Selection of the correct mode
depends on the system design, it will normally be coded in to the boot ROM but could also be set on the
software configuration bits in the system control register.
The third flag is the SipReady flag. This should be left clear while the initialization is done. Once all the registers
(including the
SriCmd
register) have been setup this bit should be set.
The SipReady bit is sticky. Once it has been set the only way to clear it is a cold reset. Configuration code
should check the state of SipReady to determine if the full initialization sequence or an abbreviated version is
needed.
Timing Registers: SriRxDen, SriTxDen, SriRxNum and SriTxNum
The receive timing registers are used when the interface is running in synchronous clocking mode, the transmit
registers must always be configured. The registers describe the relationship between the receive clock from
the HyperTransport link (f
RX
)and the internal HyperTransport interface clock (f
LDTINT
), and the relationship
between the internal HyperTransport interface clock and the HyperTransport transmit clock (f
TX
). Note that the
receive and transmit clocks used in this discussion are the actual clocks on the link and therefore are half of
the data rate.
The transmit clock frequency is set in the LinkFreq register in the HyperTransport Capability block. The
100 MHz reference clock is multiplied up by the HyperTransport PLL to give the requested frequency. The
internal HyperTransport interface clock is always one quarter of the transmit clock frequency: f
LDTINT
=f
TX
/4.
The receive clock frequency is set by the device the other end of the link. Data is inserted into the receive FIFO
byte wide sent on both edges of the receive clock, it is removed from the FIFO 8 bytes wide clocked on the
rising edge of the internal HyperTransport interface clock. The data rate ratio is therefore: f
RX
/4*f
LDTINT
= f
RX
/f
TX
.
The
SriRxDen
and
SriRxNum
registers encode this receive ratio. The Numerator is a bit pattern in which a 1
indicates that data should be extracted from the receive FIFO. The Denominator sets the number of bits in the
Numerator that are used. Two bits from the Numerator pattern are examined each internal cycle, starting from
bit zero and ending at bit (Denominator-1), to determine how many 32 bit words should be read out of the FIFO
in that cycle.
For example if the receive clock from the link (i.e. the transmit clock set in the LinkFreq register of the device
the other end of the link) f
RX
=300 MHz and the transmit clock (set in the LinkFreq register) f
TX
=400 MHz then
the receive data rate ratio is 3/4. The
SriRxNum
should therefore be set to 32’b1101 (or some similar pattern
with three ones and a zero) and the
SriRxDen
should be set to 4 to indicate only the bottom four bits of the
Numerator pattern are used.
In the common case where the clock frequency is the same at both ends of the link then the default values of
SriRxNum
= 32’hFFFF and
SriRxDen
= 16 will work.
Содержание BCM1125
Страница 18: ...BCM1250 BCM1125 BCM1125H User Manual 10 21 02 Broadcom Corporation Page xviii Document 1250_1125 UM100CB R ...
Страница 28: ...BCM1250 BCM1125 BCM1125H User Manual 10 21 02 Broadcom Corporation Page xxviii Document 1250_1125 UM100CB R ...
Страница 515: ...BCM1250 BCM1125 BCM1125H User Manual 10 21 02 Broadcom Corporation Page vii Index Document 1250_1125 UM100CB R ...