Datasheet

Section 6 Bus Controller (BSC)
Page 316 of 1408 R01UH0309EJ0500 Rev. 5.00
Sep 24, 2012
H8S/2456, H8S/2456R, H8S/2454 Group
6.13.2 Bus Transfer Timing
Even if a bus request is received from a bus master with a higher priority than that of the bus
master that has acquired the bus and is currently operating, the bus is not necessarily transferred
immediately. There are specific timings at which each bus master can relinquish the bus.
(1) CPU
The CPU is the lowest-priority bus master, and if a bus request is received from the DTC, DMAC,
or EXDMAC
*
, the bus arbiter transfers the bus to the bus master that issued the request. The
timing for transfer of the bus is as follows:
• The bus is transferred at a break between bus cycles. However, if a bus cycle is executed in
discrete operations, as in the case of a longword-size access, the bus is not transferred between
the component operations.
• With bit manipulation instructions such as BSET and BCLR, the sequence of operations is:
data read (read), relevant bit manipulation operation (modify), write-back (write). The bus is
not transferred during this read-modify-write cycle, which is executed as a series of bus cycles.
• If the CPU is in sleep mode, the bus is transferred immediately.
Note: * The EXDMAC is not supported by the H8S/2454 Group.
(2) DTC
The DTC sends the bus arbiter a request for the bus when an activation request is generated.
The DTC can release the bus after a vector read, a register information read (3 states), a single data
transfer, or a register information write (3 states). It does not release the bus during a register
information read (3 states), a single data transfer, or a register information write (3 states).