11.11. Bus Master Handover

Bus master handover occurs when a bus master, which is not currently granted the bus, becomes the new granted bus master.

A bus master becomes granted when AGNT is HIGH and BWAIT is LOW. AGNT HIGH indicates the bus master is currently the highest priority master requesting the bus and BWAIT LOW indicates the previous transfer has completed. Figure 11.7 shows the bus master handover process.

Figure 11.7. Bus Master Handover

  1. When AGNT is asserted, a bus master must drive the BTRAN signals during BCLK HIGH.

    This may continue for many cycles if the previous transfer is waited.

    Prior to handover, BTRAN must indicate an address-only cycle as the new bus master must commence with an address-only cycle to allow for bus turnaround.

  2. When the previous transfer completes, the new bus master is granted.

  3. In the last clock HIGH phase of the previous transfer, the address bus stops being driven by the previous bus master.

  4. The new bus master starts to drive the address bus and control signals during the clock LOW phase.

  5. The first transfer may then commence in the following bus cycle.

During a waited transfer, bus master handover may be delayed and it is possible that the AGNT to a particular bus master may be asserted and then negated, if another higher priority bus master then requests the bus before the current transfer has completed.

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