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CHAPTER 5 SAR FUNCTION
227
The Use-it-or-lose-it detection conditions are as follows.
C
link
*Nrm/ACR <= Ta*b
C
link
: Link capacity (149.76 Mbps)
b: b = 1 - 1/2
b0
(b0: Register setting)
The a and b parameters used for Use-it-or-lose-it control can be set to a value within the following ranges.
a: (1, 0.5, 0.25, 0.125, 0.0625, 0.03125, 0.015625, 0.0078125)
b: (0, 0.5, 0.75, 0.875, 0.9375, 0.96875, 0.984375, 0.9921875)
Every time a larger value is set for a, the actual rate is newly calculated and applied. Similarly, every time
a smaller value is set for b, if a difference arises between ACR and the actual rate, the margin by which
ACR is reduced to ACR*15/16 is enlarged.
NEC recommends that the Use-it-or-lose-it parameters be set to a = 1 and b = 0.96875 (a0 = 0, b0 = 5).
The format of the register used for the Use-it-or-lose-it function is shown below. The default values are
a0 = 1, b0 = 1. NEC recommends setting a0 = 0 and b0 = 5.
ULR (Use-it-or-lose-it register)
Address: 405H
31
0
- 0 -
6
b0
a0
2
3
5
(3) Destination internal congestion
The
μ
PD98405 supports destination internal congestion in hardware. The destination internal congestion
supported by the
μ
PD98405 corresponds to destination operations 2-c and 2-d (see
Section 5.8.2
). Upon
detecting internal congestion at the destination, the
μ
PD98405 sets CI = 1 for the BRM cell and outputs
notification of congestion. Congestion of a receive FIFO is detected based on the number of cells
accumulated by that FIFO. The host can set, in cell units in the RFTH register, the number of FIFO-
accumulated cells that correspond to internal congestion. The format of the RFTH register is shown below.
The default for this register is 7FH which, when set, disables this function.
In order to use this function, set a value of up to 96 in this register (the size of the built-in receive FIFO of
the
μ
PD98405 is equal to 96 cells).
RFTH (Receive FIFO threshold register)
Address: 404H
31
0
rfifo_th
- 0 -
7
6