PC87591L datasheet(70/437 Pages) NSC

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4.0 Embedded Controller Modules (Continued)

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Table 13. Bus Cycles of a 16-Bit Data Bus

Table 14. Bus Cycles of an 8-Bit Data Bus

On write cycles of a single byte, the remaining eight bits of the bus are floating.

On read cycles of a single byte, the remaining eight bits of the bus are ignored. There is no need for external pull-up resistors.

Clock and Bus Cycles

There are two types of bus cycles: data transfer and non-data transfer. Data transfer bus cycles cause transfer of data from

or to the memory device. Non-data transfer bus cycles (described in Section 4.1.9 on page 81) are used for observability of

internal bus transactions and do not involve data transfer from or to external devices.

There are four types of data transfer bus cycles:

●

Early write

●

Late write

●

Normal read

●

Fast read

The BIU uses EWR configuration bit in BCFG register to select the early or late write data transfer bus cycle. It uses FRE in

SZCFGn register (where “n” refers to zone 0, 1 or 2) to select normal read or fast read data transfer bus cycles.

The basic late write bus cycle takes two clock cycles. The basic early write bus cycle takes three clock cycles. When the BIU

uses the early write bus cycle, the RD signal is not required for interfacing with the memory device (with the exception of

flash). On reset, the early write bus cycle is configured.

The basic normal read bus cycle takes two clock cycles. Fast read bus cycle always takes one clock cycle. On reset, the

normal read bus cycle is configured.

Notes:

1. In the descriptions that follow, the “n” in SELn signal refers to two of the three available BIU select signals (numbered 0

or 1, corresponding to zone 0 or zone 1 respectively). The third signal is labelled SELIO.

2. For all timing diagrams, the value of BST0-2 depends on the type of core bus transaction.

3. In the following paragraphs, SZCFGn refers to three of the four BIU zone configuration registers (n = 0, 1 or 2) - the fourth

configuration register is labelled IOCFG.

Number of

Bytes

Transferred Core

Bus Bytes

Address (LSB)

Data Bus Pins

0-7

8-15

0-15

8-15

Then 0

0-7

Number of

Bytes

Transferred Core

Bus Bytes

Address (LSB)

Data Bus Pins

0-7

Then 1

0-71

1. Burst bus cycle, if burstable; otherwise, the core transaction is

broken into “basic” bus cycles.

0-7

Then 0

0-7

PC87591L Datasheet(PDF) 70 Page - National Semiconductor (TI)

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