ST90135 datasheet(85/190 Pages) STMICROELECTRONICS

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ST90158 - EXTERNAL MEMORY INTERFACE (EXTMI)

EXTERNAL MEMORY SIGNALS (Cont’d)

7.2.4 RW: Read/Write

(Alternate Function Output, Active low,

Tristate) identifies the type of memory cycle:

RW=”1” identifies a memory read cycle, RW=”0”

identifies a memory write cycle. It is defined at the

beginning of each memory cycle and it remains

stable until the following memory cycle. RW is re-

leased in high-impedance during bus acknowl-

edge cycle or under processor control by setting

the HIMP bit (MODER). RW is enabled via soft-

ware as the Alternate Function output of the asso-

ciated I/O port bit (refer to specific ST9 device to

identify the port and pin). Under Reset status, the

associated bit of the port is set into bidirectional

weak pull-up mode.

The behavior of this signal is affected by the MC,

ETO and BSZ bits in the EMR1 register. Refer to

the Register description.

7.2.5

BREQ,

BACK:

Bus

Request,

Bus

Acknowledge

Note: These pins are available only on some ST9

devices (see Pin description).

BREQ (Alternate Function Input, Active low) indi-

cates to the ST9 that a bus request has tried or is

trying to gain control of the memory bus. Once en-

abled by setting the BRQEN bit (MODER.1,

R235), BREQ is sampled with the falling edge of

the processor internal clock during phase T2.

Figure 46. External memory Read/Write sequence with external wait (WAIT pin)

SYST EM

AS (MC=0)

ALE (MC=1)

DS (MC=0)

MULTIPLEXED

RW (MC=0)

DS (MC=1)

RW (MC=1)

MULTIPLEXED

RW (MC=0)

DS (MC=1)

RW (MC=1)

WAI T

ADDRESS

ADD.

D.OUT

ADDRESS

D.OUT

ADD.

DATA OUT

D.IN

ADDRESS

CLOCK

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