
PXB 4330
Operational Description
Data Sheet
4-60
09.99
4.10
Empty Cell Rate Calculation Example
The internal SDRAM refresh generator uses empty cell cycles to perform its refresh cy-
cles. Thus the refresh function is closely related to the scheduler output rate
configuration. The empty cell rate generator guarantees a minimum number of empty
cell cycles required for refresh cycles. A maximum value for parameter T can be derived
by the following term:
with:
ABM core clock SYSCLK = [1/s]
SDRAM RefreshPeriod = [s]
SDRAM RefreshCycles requirement
The empty cell rate parameter definition for fractional and integer parts of T is according
to equations (5) and (6).
Example:
Given values: RefreshPeriod = 64ms, RefreshCycles = 4096, SYSCLK = 50 MHz
T
max
= 24.414
Thus the value of T that is represented by programmed device parameters IntRate and
FracRate according to equations (6) and (7), must be less or equal to T
max
to guarantee
sufficient refresh cycles according to the SDRAM specification.
In case of additional bandwidth needs to be reserved (e.g. for multicast operation in sub-
sequent devices), a second maximum condition for parameter T can be derived
depending on the empty cell rate required for multicast bandwidth reservation. In this
case the value of T must conform the following equation:
4.11
Traffic Classes
4.11.1
These connections should use the real-time bypass of the respective Scheduler. How-
ever, if two priority levels for real-time connections must be offered, a slightly lower real-
time performance can be achieved by using the WFQ Multiplexer with maximum weight.
CBR Connections
T
max
SYSCLK
32
RefreshCycles
×
-------------------------------------------------------------------------
8
( )
=
T
Min T
equation 5
{
T
MCreservation
T
max
,
,
}
9
( )
=