MSC8113 Reference Manual, Rev. 0
13-20
Freescale Semiconductor
System Bus
13.2.3.1 Address Arbitration
The arbitration configuration (external or internal) is chosen at system reset. For internal
arbitration, the MSC8113 provides arbitration for the 60x-compatible address bus and the system
is optimized for three external 60x-compatible bus masters besides the MSC8113. The bus
request (
BR
) for the external device is an external input to the arbiter. The bus grant signal for the
external device (
BG
) is an output to the external device. The
BG
signal is asserted by the
MSC8113 internal arbiter one clock after the current master on the bus asserts
AACK
. Therefore, it
is a qualified
BG
. Assuming that all potential masters deassert
ABB
one clock after receiving
AACK
,
the device receiving
BG
can start the address tenure by asserting
TS
one clock after receiving
BG
.
In addition to the external signals, there are internal request and grant signals for the MSC8113
internal devices.
Bus accesses are prioritized, with programmable priority. When an MSC8113 internal master
needs the system bus, it asserts the internal bus request along with the request level. The arbiter
asserts the internal 60x-compatible bus grant for the highest-priority request.
The MSC8113 supports address bus parking via the parked master bits in the arbiter
configuration register. The MSC8113 parks the address bus (asserts the address bus grant signal
in anticipation of an address bus request) to the external master or internal masters. When a
device is parked, the arbiter can hold
BG
asserted for a device even if that device has not
requested the bus. Therefore, when the parked device needs to perform a bus transaction, it skips
the bus request delay and assumes address bus mastership on the next cycle.
BR
is not asserted,
and the access latency is shortened by one cycle.
The MSC8113 and external device bus devices qualify
BG
by sampling
ARTRY
in the deasserted
state prior to taking address bus mastership. The deassertion of
ARTRY
during the address retry
window (one cycle after the assertion of
AACK)
indicates that no address retry is requested. If a
device detects
ARTRY
asserted, it cannot accept an address bus grant during the
ARTRY
cycle or
the cycle following. A device that asserts
ARTRY
asserts its bus request during the cycle after the
assertion of
ARTRY
and assumes bus mastership when it is given a bus grant.
The series of address transfers in Figure 13-5 shows the transfer protocol when the MSC8113
device is configured in 60x-compatible bus mode. In this example, the MSC8113 is initially
parked on the bus with
BG INT
asserted, which lets it start an address bus tenure by asserting
TS
.
Note that
BG INT
is an internal signal that is not reflected externally. During the same clock cycle,
the external master bus request is asserted to request access to the system bus, thereby causing the
deassertion of
BG INT
internally and the assertion of
BG
externally. Following MSC8113 address
tenure, the external master takes the bus and initiates its address transaction. The internal arbiter
samples
BR
during the clock cycle in which
AACK
is asserted; if
BR
is not asserted (no pending
request), it deasserts
BG
and asserts the parked bus grant (
BG_INT
in this example). The master can
assert
BR
and receive a qualified bus grant without subsequently using the bus. It can deassert
(cancel)
BR
before accepting a qualified bus grant.
Содержание MSC8113
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Страница 300: ...MSC8113 Reference Manual Rev 0 9 40 Freescale Semiconductor Extended Core ...
Страница 304: ...MSC8113 Reference Manual Rev 0 10 4 Freescale Semiconductor MQBus and M2 Memory ...
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