SH7020 Datasheet, PDF (215/509 Pages) Renesas Technology Corp – SuperH™ RISC engine

with higher priority (2) will begin immediately. When channel 2 is also operating in the burst
mode, the channel 1 transfer will continue when the channel 2 transfer has completely finished.
When channel 2 is in the cycle steal mode, channel 1 will begin operating again after channel 2
completes the transfer of one transfer unit, but the bus will then switch between the two in the
order channel 1, channel 2, channel 1, channel 2. Since channel 1 is in burst mode, it will not give
the bus to the CPU. This example is illustrated in figure 9.12.
Bus
status
CPU DMAC DMAC DMAC DMAC DMAC DMAC DMAC CPU
ch1
ch1
ch2
ch1
ch2
ch1
ch1
ch2
ch1
ch2
CPU
DMAC ch1
Burst mode
DMAC ch1 and ch2
Cycle steal mode
DMAC ch1
Burst mode
CPU
Figure 9.12 Bus Handling when Multiple Channels Are Operating
9.3.5 Number of Bus Cycle States and DREQ Pin Sample Timing
Number of States in Bus Cycle: The number of states in the bus cycle when the DMAC is the
bus master is controlled by the bus state controller just as it is when the CPU is the bus master.
The bus cycle in the dual address mode is controlled by wait state control register 1 (WCR1) while
the single address mode bus cycle is controlled by wait state control register 2 (WCR2). For
details, see section 8.9, Wait State Control.
DREQ Pin Sampling Timing: Normally, when DREQ input is detected immediately prior to the
rise edge of the clock pulse (CK) in external request mode, a DMAC bus cycle will be generated
and the DMA transfer performed two states later at the earliest. The sampling timing after DREQ
input detection differs by bus mode, address mode and method of DREQ input detection.
• DREQ pin sampling timing in the cycle steal mode
In the cycle steal mode, the sampling timing is the same regardless of whether the DREQ is
detected by edge or level. When edge is being detected, however, once sampled it will not be
sampled again until the next edge detection. Once DREQ input is detected, the next sampling
is not performed until the first state, among those DMAC bus cycles thereby produced, in
which a DACK signal is output (including the detection state itself). The next sampling occurs
immediately prior to the rise edge of the clock pulse(CK) of the third state after the bus cycle
previous to the bus cycle in which the DACK signal is output.
198 RENESAS