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DMAC: When the DMAC receives an activation request, it requests the bus right from the bus
arbiter. If the DMAC is bus master and the refresh controller or an external bus master requests
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the bus, the bus arbiter transfers the bus right from the DMAC to the bus master that requested the
bus. The bus right is transferred at the following times.
The bus right is transferred when the DMAC finishes transferring 1 byte or 1 word. A DMAC
transfer cycle consists of a read cycle and a write cycle. The bus right is not transferred between
the read cycle and the write cycle.
There is a priority order among the DMAC channels. For details see section 8.4.9, Multiple-
Channel Operation.
Refresh Controller: When a refresh cycle is requested, the refresh controller requests the bus
right from the bus arbiter. When the refresh cycle is completed, the refresh controller releases the
bus. For details see section 7, Refresh Controller.
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 by driving the BREQ signal low. Once the external bus master gets the bus, it
keeps the bus right until the BREQ signal goes high. While the bus is released to an external bus
master, the H8/3048 Series holds the address bus and data bus control signals (AS, RD, HWR,
and LWR) in the high-impedance state, holds the chip select signals high (CS
: n = 7 to 0), and
n
holds the BACK pin in the low output state.
The bus arbiter samples the BREQ pin at the rise of the system clock (ø). 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 signal is driven high to end
the bus-release cycle.
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