![](http://datasheet.mmic.net.cn/380000/-PD98405_datasheet_16745025/-PD98405_64.png)
CHAPTER 4 INTERFACES
64
(3) Burst size select function
If more than one burst size is enabled, the
μ
PD98405 checks the address field at the transfer destination,
and automatically selects the burst size to be used from the enabled burst sizes. This function is enabled
for both DMA read and write cycles that are performed by the
μ
PD98405. This function cannot be used
when 12-word burst is enabled.
Transfer destination address
Executable burst size
xxxxxxxx xxxxxxxx xxxxxxxx xx0000xx
16-
Note
, 8-, 4-, 2-, and 1-word burst
xxxxxxxx xxxxxxxx xxxxxxxx xxx000xx
8-, 4-, 2-, and 1-word burst
xxxxxxxx xxxxxxxx xxxxxxxx xxx100xx
4-, 2-, and 1-word burst
xxxxxxxx xxxxxxxx xxxxxxxx xxxx10xx
2- and 1-word burst
xxxxxxxx xxxxxxxx xxxxxxxx xxxxx1xx
1-word transfer
Note
16-word burst is assigned the highest priority only when transferring raw cell data.
Burst size
16
8
4
2
1
Priority
High -> Low
When performing DMA transfer, the
μ
PD98405 counts the number of zeros in the transfer destination
address field (AD2 to AD5), starting from the LSB. The size of the burst operation which can be executed
is determined from the number of zeros. Higher priorities are assigned in the order of 8-, 4-, 2-, and 1-word
bursts. The
μ
PD98405 selects that burst size which has been assigned the highest priority, from among
those executable and enabled, then executes burst transfer.
This function is enabled or disabled by using the "AD bit" of the GMR register.
Selects burst transfer mode.
0
When the
μ
PD98405 performs DMA transfer, the function to
check the transfer destination address field, and to select a
burst size from the enabled sizes according to the address,
is enabled.
1
When the
μ
PD98405 performs DMA transfer, the function to
check the transfer destination address field is disabled.
The
μ
PD98405 simply selects and uses the biggest burst
size from among those enabled.
AD bit
(GMR: bit 7)
Default = 0
Caution When the TBE bit is set to 1 to enable 12-word burst, the AD bit must always be set
to 1 to disable this function.
The following describes an example of the processing performed when this function is enabled (AD = 0) or
disabled (AD = 1).