MPC8540 datasheet(588/1302 Pages) FREESCALE

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MPC8540 PowerQUICC III Integrated Host Processor Reference Manual, Rev. 1

13-84

Freescale Semiconductor

Local Bus Controller

13.5 Initialization/Application Information

13.5.1 Interfacing to Peripherals

13.5.1.1

Multiplexed Address/Data Bus and Unmultiplexed Address Signals

To save pins on the local bus, address and data are multiplexed onto the same 32 bit bus. An

external latch is needed to demultiplex and reconstruct the original address. No external

intelligence is needed, because the LALE signal provides the correct timing to control a standard

logic latch. The LAD pins can be directly connected to the data signals of the memory/peripheral.

Transactions on the local bus start with an address phase, where the LBC drives the transaction

address on the LAD signals and asserts the LALE signal. This can be used to latch the address and

then the LBC can continue with the data phase.

The LBC supports port sizes of 8,16, and 32 bit. For devices smaller than 32 bits, transactions must

be broken down. For this reason, LA[30:31] are driven unmultiplexed. For 8-bit devices,

LA[30:31] should be used and for 16-bit devices, LA[30] should be used. 32-bit devices use

neither of these signals.

In addition, the LBC supports burst transfers (not in the GPCM machine). LA[27:29] are the burst

addresses within a natural 32-byte burst. To minimize the amount of address phases needed on the

local bus and to optimize the throughput, those signals are driven separately and should be used

whenever a device requires the 5 least significant addresses. Those should not be used from

LAD[27:31].

All other addresses, A[0:26], must be reconstructed through the latch.

Figure 13-68. Multiplexed Address/Data Bus

13.5.1.2

Peripheral Hierarchy on the Local Bus

To achieve high bus speed interfaces for synchronous SRAMs or SDRAMs, a hierarchy of the

memories/peripherals connected to the local bus is suggested, as shown in Figure 13-69.

Muxed Address/Data

Unmuxed Address

LA[27:31]

LAD[0:31]

LALE

Local Bus Interface

Latch

A[0:26]

A[27:31]

D[0:31]

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