Datasheet

Chapter 18. Bus Operation 18-21
General Operation of External Master Transfers
If the MCF5407 is the only possible master, BG can be tied to GND—no arbiter is needed.
18.8.1 Bus Arbitration Signals
Bus arbitration signal timings in Table 18-7 are referenced to the system clock, which is not
considered a bus signal. Clock routing is expected to meet application requirements.
18.9 General Operation of External Master Transfers
An external master asserts its hold signal (such as HOLDREQ) when it executes a bus
cycle, driving BG
high and forcing the MCF5407 to hold all bus requests. During an
external master cycle, the MCF5407 can provide memory control signals (OE
, CS[7:0],
BE
/BWE[3:0], RAS[1:0], CAS[3:0]) and TA while the external master drives the address
and data bus and other required bus control signals. When the external master asserts TS
or
AS
to the MCF5407, the beginning of a bus cycle is identified and the MCF5407 starts
decoding the address driven.
Note the following regarding external master accesses:
Explicit
master
MCF5407 Driven Asserted The MCF5407 is explicit bus master when BG is asserted and at least
one bus cycle has been initiated. It asserts BD
and retains explicit
mastership until BG
is negated even if no active bus cycles are executed.
It releases the bus at the end of the current bus cycle, then negates BD
and three-states the bus signals.
External
master
External Not
driven
Negated An external device is bus master (BG
negated to MCF5407). The
MCF5407 can assert OE
, CS[7:0], BE/BWE[3:0], TA, and all DRAM
controller signals (RAS
[1:0], CAS[3:0], SRAS, SCAS, DRAMW, SCKE).
Table 18-7. ColdFire Bus Arbitration Signal Summary
Signal I/O Description
BR
O Bus request. Indicates to an external arbiter that the processor needs to become bus master. BR is
negated when the MCF5407 begins an access to the external bus with no other internal accesses
pending. BR
remains negated until another internal request occurs.
BG
I Bus grant. An external arbiter asserts BG to indicate that the MCF5407 can control the bus at the
next rising edge of CLKIN. When the arbiter negates BG
, the MCF5407 must release the bus as
soon as the current transfer completes. The external arbiter must not grant the bus to any other
device until both BD
and BG are negated.
BD
O Bus driven. The MCF5407 asserts BD to indicate it is current master and is driving the bus. If it loses
bus mastership during a transfer, it completes the last transfer of the current access, negates BD
,
and three-states all bus signals on the rising edge of CLKIN. If it loses mastership during an idle
clock cycle, it three-states all bus signals on the rising edge of CLKIN.
Table 18-6. MCF5407 Arbitration Protocol States (Continued)
State Master Bus BD Description