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CHAPTER 5 SAR FUNCTION
225
LCR register setting
Rate reduction, performed in response to internal congestion, is not applied to those VCs that are
transmitting at a rate less than that set in the LCR register. By setting a rate in this register, those VCs
that transmit at low rates can be excluded from the rate reduction performed as a result of internal
congestion control. When this register is set to the default value of 0, this function is disabled.
LCR (Internal congestion cell rate register)
Address: 502H
31
0
LCR
- 0 -
15
14
LCR: Set the internal congestion cell rate using the following rate expression.
Rate = 2
e
*(1 + m/512)*nz cells/seconds
14
0
m
9
8
nz
e
13
Using source internal congestion
This section explains the recommended methods of using the source internal congestion with the
μ
PD98405, and the values to be set in the registers.
Source internal congestion control is applied by initially setting the TTH0, TTH1, and LCR registers, as
well as the alpha parameter of the UBCR register then, when changing the band that can be used by
ABR, setting the band in the AUB register. The host can change the values of the TTH0, TTH1, LCR,
UBCR, and AUB registers at any time. The settings to be made for each register are explained below.
TTH0 register
The TTH0 register, in combination with the AUB register, controls internal congestion detection. To
enable the quickest possible detection of internal congestion, it is recommended that TTH0 be set to
0.
TTH1 register, UBCR register alpha parameter
The TTH1 register and UBCR register alpha parameter control the rate of ACR reduction, applied
after the detection of internal congestion. Although, upon the detection of internal congestion, the
μ
PD98405 performs ACR reduction as described above, it is also possible to reduce the rate by
setting the TTH1 register and the UBCR register alpha parameter. Basically, setting TTH1 = 7FFFH
eliminates the need to use rate reduction (when the band used by ABR is unusually small, or the
number of VCs is extremely large, the rate of ACR reduction performed upon internal congestion can
be increased by setting a small value for TTH1 and setting alpha to 1 (
α
= 1/2)).