ST90R158 datasheet(82/190 Pages) STMICROELECTRONICS

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ST90158 - EXTERNAL MEMORY INTERFACE (EXTMI)

7.2 EXTERNAL MEMORY SIGNALS

The access to external memory is made using the

AS, DS, DS2, RW, Port 0, Port1, and WAIT signals

described below.

Refer to Figure 45

7.2.1 AS: Address Strobe

AS (Output, Active low, Tristate) is active during

the System Clock high-level phase of each T1

memory cycle: an AS rising edge indicates that

Memory Address and Read/Write Memory control

signals are valid. AS is released in high-imped-

ance during the bus acknowledge cycle or under

the processor control by setting the HIMP bit

(MODER.0, R235). Depending on the device AS is

available as Alternate Function or as a dedicated

pin.

Under Reset, AS is held high with an internal weak

pull-up.

The behavior of this signal is affected by the MC,

ASAF, ETO, BSZ, LAS[1:0] and UAS[1:0] bits in

the EMR1 or EMR2 registers. Refer to the Regis-

ter description.

7.2.2 DS: Data Strobe

DS (Output, Active low, Tristate) is active during the

internal clock high-level phase of each T2 memory

cycle. During an external memory read cycle, the

data on Port 0 must be valid before the DS rising

edge. During an external memory write cycle, the

data on Port 0 are output on the falling edge of DS

and they are valid on the rising edge of DS. When

the internal memory is accessed DS is kept high

during the whole memory cycle. DS is released in

high-impedance during bus acknowledge cycle or

under processor control by setting the HIMP bit

(MODER.0, R235). Under Reset status, DS is held

high with an internal weak pull-up.

The behavior of this signal is affected by the MC,

DS2EN, and BSZ bits in the EMR1 register. Refer

to the Register description.

7.2.3 DS2: Data Strobe 2

This additional Data Strobe pin (Alternate Function

Output, Active low, Tristate) is available on some

ST9 devices only. It allows two external memories

to be connected to the ST9, the upper memory

block (A21=1 typically RAM) and the lower memo-

ry block (A21=0 typically ROM) without any exter-

nal logic. The selection between the upper and

lower memory blocks depends on the A21 address

pin value.

The upper memory block is controlled by the DS

pin while the lower memory block is controlled by

the DS2 pin. When the internal memory is ad-

dressed, DS2 is kept high during the whole mem-

ory cycle. DS2 is released in high-impedance dur-

ing bus acknowledge cycle or under processor

control by setting the HIMP bit (MODER.0, R235).

DS2 is enabled via software as the Alternate Func-

tion output of the associated I/O port bit (refer to

specific ST9 version to identify the specific port

and pin).

The behavior of this signal is affected by the

DS2EN, and BSZ bits in the EMR1 register. Refer

to the Register description.

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