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CHAPTER 5 SAR FUNCTION
224
Internal congestion is recognized upon the ACR total exceeding the band that can be used by the ABR
(AUB register setting), or upon the detection of the internal timer, that is used to manage cell transmission,
going out of synchronization. After the detection of internal congestion, the
μ
PD98405 reduces ACR,
based on the above expression, according to the difference between the ACR total and the value of the
AUB register.
When internal congestion does not occur, the Program timer and Service timer both count up in exactly the
same way. Upon the occurrence of internal congestion, the Service timer will lag behind the Program
timer. If internal congestion is averted, the operation of the Service timer will follow that of the Program
timer. Time_th0 is a threshold value used to detect internal congestion when a difference arises between
the Service Timer and the Program Timer. Time_th1 is a threshold value used to determine the rate of
reduction for ACR when internal congestion is detected.
The internal timer advances by 1 every 36 system clocks (this clock is input to the SCLK pin). Set
Time_th0 and Time_th1 using the value of this counter. For example, to set Time_th1 to 10, a time of 360
system clocks is shown. For every one count that this timer value is delayed, the
μ
PD98405 applies
internal control to introduce a delay of one cell time.
The formats of the registers used for detecting internal congestion, and for ACR reduction, are shown
below.
AUB (ABR, UBR band register)
Address: 501H
31
0
C_ABR+UBR
- 0 -
14
15
TTH0 (Time threshold register 0)
Address: 402H
31
0
Time_th0
- 0 -
19
18
TTH1 (Time threshold register 1)
Address: 403H
31
0
Time_th1
- 0 -
19
18
UBCR (UBR band control register)
Address: 500H
31
0
alpha
- 0 -
16
15
Nubr_th
11
beta1
beta2
2
3
6
5
12
8
7
- 0 -