Renesas SH7781 - Manual (Page 480)

11. Local Bus State Controller (LBSC)
Rev.1.00 Jan. 10, 2008 Page 450 of 1658
REJ09B0261-0100
11.5.10 Master Mode
The processor in master mode holds the bus itself until it receives a bus request.
On receiving an assertion (low level) of the bus request signal (
BREQ) from the outside, the
master mode processor releases the bus and asserts (drives low) the bus use permission signal
(
BACK) as soon as the currently executing bus cycle ends. On receiving the BREQ negation (high
level) indicating that the slave has released the bus, the processor negates (drives high) the
BACK
signal and resumes use of the bus.
When the bus is released, all bus control output signals and input/output signals related to bus
interface enters a high-impedance state, except for
BACK for bus arbitration and DACK0 to
DACK3 for controlling DMA transfer.
The actual bus release sequence is as follows.
First, the bus use permission signal is asserted in synchronization with the rising edge of the clock.
The address bus and data bus are put in a high-impedance state in synchronous with the rising
edge of the clock next to the
BACK assertion. At the same time, the bus control signals (BS, CSn,
WEn, RD, R/W, CE2A, and CE2B) enters a high-impedance state. These bus control signals are
negated no later than one cycle before entering high-impedance. Bus request signal sampling is
performed on the rising edge of the clock.
The sequence for re-acquiring the bus from the slave is as follows.
As soon as
BREQ negation is detected on the rising edge of the clock, BACK is negated and bus
control signal driving is started. Driving of the address bus and data bus starts at the next rising
edge of an in-phase clock. The bus control signals are asserted and the bus cycle is actually
started, at the earliest, at the clock rising edge at which the address and data signals are driven.
In order to reacquire the bus and start execution of bus access, the
BREQ signal must be negated
for at least two cycles.