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CHAPTER 5 SAR FUNCTION
151
x (32 bits)
y (32 bits)
p (8 bits)
Variable
c (8 bits)
S (scan, 1 bit)
R (round robin, 1 bit)
Flag
A (active, 1 bit)
Cautions 1.
Do not set the enable (E) bit of the scheduler register to 1 before the variables and
flags used by the
μ
PD98405 have been cleared to 0 and the parameters (such as I, M,
P, C, and Priority) have been set up.
2.
When operation begins, the variables and flags in the scheduler register must all be
set to 0.
3.
The contents of the scheduler register cannot be changed if the A bit is set to 1 and
the relevant shaper is active. A shaper becomes inactive when there is no linked VC
table. Before changing parameters I, M, P, and C, make sure that the relevant shaper
is inactive (that the A bit is set to 0) and reset the enable (E) bit to 0. Also, clear all
the variables and flags used by the
μ
PD98405 to 0.
4.
The host can change only the Priority parameter at any time, even when the shaper is
active (while the A bit is set to 1). In this case, set only the byte enable B3 bit of the
Indirect_Access command to 1; be careful not to overwrite any other fields.
(a) Setting the transmit rate parameters
The
μ
PD98405 performs cell scheduling according to the I, M, P, and C parameters set in the
scheduler register. This section explains the setting of these parameters. The I, M, P, and C
parameters are all set in VC units (except in Aggregate mode).
I, M (average rate)
The
μ
PD98405 usually transmits cells at the average rate set with the I and M parameters. The
average rate is specified as I/M, where I cells are transmitted for every M cells. Note, however,
that the settings of the P and C parameters take priority over those of the I and M parameters. For
example, if I/M = 1/1, P = 2, and C = 2 are set, cells are transmitted at a rate of 1/3.