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CHAPTER 5 SAR FUNCTION
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5.9 UBR FUNCTION
5.9.1 UBR Service Processing
The
μ
PD98405 supports the UBR service. To enable the use of the UBR service, it is necessary to perform
the following in addition to the VBR processing.
Setting a shaper
Setting the SCR register
(1) Setting a shaper
A VBR shaper (one of shapers 0 to 15) is used for the UBR service. The host must set the parameters for
a shaper to be used for the UBR service. The priority of this shaper must be the lowest.
(2) Setting the SCR register
The host must set the number of a shaper to be used for UBR service in the SCR register, and set the
enable bit of the SCR register to 1. The use of the shaper as a VBR shaper is thus disabled.
The format of the SCR register is as follows:
UEN
31
0
SHAPER NO. OF UBR
3
- 0 -
4
5
SHAPER NO. OF UBR:
Number of a shaper to be used for the UBR service
UEN:
Enable bit for the UBR service
5.9.2 UBR Bands
The UBR service has no mechanism for guaranteeing cell transfer. So, no band is assigned to it in advance.
If both VBR and UBR exist, any bands that are not assigned to the VBR are used for the UBR. If the VBR,
ABR, and UBR exist, any bands that are not assigned to the VBR are shared between the ABR and UBR. In
this case, a band may become unavailable to the UBR service, which has the lowest priority, because the ABR
service changes the rate dynamically. The
μ
PD98405 is provided with a function that can assign some bands
to the UBR even in such situations.
When the ABR and UBR exist, the UBCR register is used to assign bands to the UBR. The format of the
UBCR register is as follows: