Epson S1D13504 manual Power PC Memory Read Cycle

Epson Research and Development	Page 9
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2.2.1 Normal (Non-Burst) Bus Transactions

A data transfer is initiated by the bus master by placing the memory address on address lines A0 through A31 and driving TS (Transfer Start) low for one clock cycle. Several control signals are also provided with the memory address:

•TSIZ[0:1] (Transfer Size) -- indicates whether the bus cycle is 8, 16, or 32-bit.

•RD/WR -- set high for read cycles and low for write cycles.

•AT[0:3] (Address Type Signals) -- provides more detail on the type of transfer being attempted.

When the peripheral device being accessed has completed the bus transfer, it asserts TA (Transfer Acknowledge) for one clock cycle to complete the bus transaction. Once TA has been asserted, the MPC821 will not start another bus cycle until TA has been de-asserted. The minimum length of a bus transaction is two bus clocks.

The following figure illustrates a typical memory read cycle on the Power PC system bus.

SYSCLK

TS

TA

A[0:31]

RD/WR

TSIZ[0:1], AT[0:3]

D[0:31]			Sampled when TA low
Transfer Start	Wait States	Transfer	Next Transfer
		Complete	Starts

Figure 2-1: Power PC Memory Read Cycle

Interfacing to the Motorola MPC821 Microprocessor	S1D13504
Issue Date: 01/02/02	X19A-G-010-05