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
132
Section 6: DRAM
Document
1250_1125-UM100CB-R
This figure shows where in time the first DQS edges must occur for various settings of the parameters [tCrD,
tCrDh, tFIFO]. The following table describes the opening and closing of the windows for different parameter
settings:
Note that both the MCLK and the DQS are observed at the pins of the controller. When a memory system is
analyzed the loop delay must be considered: the clock is driven from the controller and takes time to propagate
to the SDRAM, the SDRAM will drive DQS in some window around when it receives the clock edge and DQS
will take time to propagate back. In a heavily loaded channel (or with lower drive strength) there may also be
a contribution to the delay from the rise/fall time of the clock and DQS. The DQS signals are all being driven
by different SDRAM chips and may have different propagation delays (since the board trace length difference
between the 8 data bits and their DQS is more tightly controlled than the difference across the DQS bundles).
In systems with DIMMs there may be a non-negligible difference in response between a device on the first
DIMM and a device on the last DIMM of the channel, increasing the uncertainty in DQS arrival time.
The positions of the windows are affected by the settings of the DLLs described in the previous section. They
are shown for the center DLL position (
M
=4’b1000). Increasing the value of addr_skew will move the windows
to the left (this is not true on the initial pass1 prototypes with the earlier address DLL arrangement, in those
parts increasing dqo_skew will move the window to the left). Increasing the value of dqi_skew will move the
windows to the left (this is true on all parts).
The positions of the windows are also affected by the settings of the clock_class and clock_drive parameters
in the memory clock configuration register. The windows in the figure are shown for clock_class=1 and
clock_drive=7 (i.e. maximum drive strength SSTL_2 Class II). Values other than these will move the windows
to the left.
If the memory system is running slower than 95MHz, it is possible that the DLLs in the memory controller will
saturate (i.e., reach their maximum possible delay). When this happens, the windows will move to the right.
A few of the settings are not useful in practice. There is no reason to use the setting [n,1,1] since it is a subset
of the [n,0,1], [n, 0, 2] and [n, 1, 2] settings and has a narrower window that all nine DQS signals must hit. The
setting for [n-1,0,2] is also not particularly useful. It is only shown for completeness.
The setting for [n-1,1,2] is probably only useful to systems running with such a slow memory clock that the
DLL's have saturated. This setting is misleading the controller about the actual CAS latency of the SDRAM.
The controller will therefore expect the SDRAM to release the bus earlier, which must be compensated for by
always setting r2wIdle to 1 to restore the cycle in a read to write turnaround.
Table 71: First DQS Window Opening and Closing (Typical)
Setting
Window opening
Window closing
[n, 0, 1]
1.5ns before rising edge
3.5ns before rising edge of cycle n+1
[n, 1, 1]
1.5ns before falling edge
3.5ns before rising edge of cycle n+1
[n, 0, 2]
1.5ns before rising edge
2.0ns before falling edge of cycle n+1
[n, 1, 2]
1.5ns before falling edge
2.5ns before falling edge of cycle n+1 plus ¼ cycle