Renesas H8SX/1650, Hardware Manual

Page: 236 / 692

Section 6 Bus Controller (BSC)
Rev.2.00 Jun. 28, 2007 Page 214 of 666
REJ09B0311-0200
6.14.2 Bus Transfer Timing
Even if a bus request is received from a bus master with a higher priority over that of the bus
master that has taken control of the bus and is currently operating, the bus is not necessarily
transferred immediately. There are specific timings at which each bus master can release the bus.
(1)
CPU
The CPU is the lowest-priority bus master, and if a bus request is received from the DTC, the bus
arbiter transfers the bus to the bus master that issued the request.
The timing for transfer of the bus is at the end of the bus cycle. In sleep mode, the bus is
transferred synchronously with the clock.
Note, however, that the bus cannot be transferred in the following cases.
•
The word or longword access is performed in some divisions.
•
Stack handling is performed in multiple bus cycles.
•
Transfer data read or write by memory transfer instructions, block transfer instructions, or TAS
instruction.
(In the block transfer instructions, the bus can be transferred in the write cycle and the
following transfer data read cycle.)
•
From the target read to write in the bit manipulation instructions or memory operation
instructions.
(In an instruction that performs no write operation according to the instruction condition, up to
a cycle corresponding the write cycle)
(2)
DTC
The DTC sends the internal bus arbiter a request for the bus when an activation request is
generated. When the DTC accesses an external bus space, the DTC first takes control of the bus
from the internal bus arbiter and then requests a bus to the external bus arbiter.
Once the DTC takes control of the bus, the DTC continues the transfer processing cycles. If a bus
master whose priority is higher than the DTC requests the bus, the DTC transfers the bus to the
higher priority bus master. If the IBCSS bit in BCR2 is set to 1, the DTC transfers the bus to the
CPU.