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CHAPTER 4 INTERFACES
60
The
μ
PD98405 requests the external bus arbiter for the bus by driving the ATTN_B signal low. At the
same time, it indicates the direction of transfer by using DR/W_Bs and specifies the burst size with SIZE2
through SIZE0. The bus arbiter acknowledges the request by the ATTN_B signal by driving the GNT_B
signal low, and grants bus mastership to the
μ
PD98405. After driving the ATTN_B signal low, the
μ
PD98405 samples the GNT_B signal at the rising edge of the clock. When the
μ
PD98405 detects that
GNT_B has been driven low, it outputs an address at the rising edge of the clock, and starts sampling the
RDY_B signal. The
μ
PD98405 requires one clock cycle to output an address for both read and write. After
outputting the address, the
μ
PD98405 starts sampling the RDY_B signal at the rising edge of the clock.
During a DMA read operation, the
μ
PD98405 latches the data on AD31 through AD0 at the rising edge of
the clock whenever a low level is input to the RDY_B pin. Upon a DMA write operation being performed,
the
μ
PD98405 outputs data to AD31 through AD0 immediately after completion of the address cycle. At
μ
PD98405 outputs byte enable signals to BE3_B through BE0_B. The data is retained
until the level of the RDY_B signal goes low. The user can insert wait cycles by controlling the input of this
RDY_B signal. The
μ
PD98405 deactivates the ATTN_B signal at the first rising edge of the clock once all
data except one word has been transferred. The GNT_B signal may always be set to low.
The ABRT_B signal is used to abort the data transfer cycle. The
μ
PD98405 samples the ABRT_B signal,
as well as the RDY_B signal in the data transfer cycle after outputting the address. If a low level is input to
the ABRT_B pin in the data transfer cycle, the
μ
PD98405 aborts DMA transfer in that cycle and makes the
ATTN_B signal inactive. Subsequently, it makes the ATTN_B signal active again, then retries transfer of
the aborted data. If a low level is input to the ABRT_B and RDY_B pins at the same time, the
μ
PD98405
gives priority to the ABRT_B signal.
Figure 4-5 shows an example of the timing of the ABRT_B input signal.