Operation - Hitachi H8/3062 Hardware Manual

6.6.1

Operation

CPU: The CPU is the lowest-priority bus master. If an external bus master requests the bus while
the CPU has the bus right, the bus arbiter transfers the bus right to the bus master that requested it.
The bus right is transferred at the following times:
• The bus right is transferred at the boundary of a bus cycle. If word data is accessed by two
consecutive byte accesses, however, the bus right is not transferred between the two byte
accesses.
• If another bus master requests the bus while the CPU is performing internal operations, such as
executing a multiply or divide instruction, the bus right is transferred immediately. The CPU
continues its internal operations.
• If another bus master requests the bus while the CPU is in sleep mode, the bus right is
transferred immediately.
External Bus Master: When the BRLE bit is set to 1 in BRCR, the bus can be released to an
external bus master. The external bus master has highest priority, and requests the bus right from
the bus arbiter driving the BREQ signal low. Once the external bus master acquires the bus, it
keeps the bus until the BREQ signal goes high. While the bus is released to an external bus
master, the H8/3062 Series chip holds the address bus, data bus, bus control signals (AS, RD,
HWR, and LWR), and chip select signals (CSn: n = 7 to 0) in the high-impedance state, and holds
the BACK pin in the low output state.
The bus arbiter samples the BREQ pin at the rise of the system clock (φ). If BREQ is low, the bus
is released to the external bus master at the appropriate opportunity. The BREQ signal should be
held low until the BACK signal goes low.
When the BREQ pin is high in two consecutive samples, the BACK pin is driven high to end the
bus-release cycle.
Figure 6.21 shows the timing when the bus right is requested by an external bus master during a
read cycle in a two-state access area. There is a minimum interval of three states from when the
BREQ signal goes low until the bus is released.
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