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CHAPTER 5 SAR FUNCTION
192
The
μ
PD98405 is provided with the following two registers for the T1 timer function:
TSR register: 32-bit counter which counts up at the system clock cycle. The
μ
PD98405 references this
counter to obtain the current time. This counter starts counting immediately after a reset.
T1R register: Register in which the host sets the time allowed to receive a single packet (T1 time).
The time is defined using the high-order 16 bits of a 32-bit value. The time value is set in
units of 65,536 system clock cycles (see the description of the TSR register, above).
The low-order 16 bits are set to 0000H and are not used.
Note that the actual time differs even if the value set for T1R is the same, depending on the system clock of
the
μ
PD98405.
To monitor the T1 time, a link list created by using "FORWARD POINTER" and "BACKWARD POINTER"
at the seventh word of the VC table is used. These pointers store the "VC NUMBER" of the VC. Each time
the first cell of a new packet arrives, the
μ
PD98405 writes the contents of the TSR register to the "T1 TIME
STAMP" field of the VC table. In addition, the VC table is added to the end of the link list by correcting
"FORWARD POINTER" and "BACKWARD POINTER" of the VC table and the corresponding pointers of
the previously received VC. In other words, the link list starts with the "VC that started reception first" and
ends with the "VC that started reception most recently," and the list is enabled. Using this list, the
μ
PD98405 has only to check the "contents of the TSR register
≥
T ("T1 TIME STAMP")" of the "VC that
started reception first." Once the last cell of the packet has arrived, the
μ
PD98405 removes the VC table of
the VC from the link list.
For normal operation, the T1 time is set so that the packet can be received correctly.
Remark
The host can disable the T1 timer function for individual VCs separately. The T1 timer function
is disabled for a VC for which the T1D bit of the receive VC table is set to 1. The
μ
PD98405
does not monitor the T1 timer for this VC. Also, the VC is not linked to the T1 link list. The T1
timer disable function is useful mainly for VCs that use the ABR service, in which it may prove
impossible to estimate the T1 time when packet reception ends due to changes in the rate.