ST92F124 datasheet(143/429 Pages) STMICROELECTRONICS

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ST92F124/F150/F250 - EXTERNAL MEMORY INTERFACE (EXTMI)

8.2 EXTERNAL MEMORY SIGNALS

The access to external memory is made using the

AS, DS, RW, Port 0, Port1, Port9, DS2 and WAIT

signals described below.

Refer to Figure 76.

8.2.1 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-impedance during the bus

acknowledge cycle or under the processor control

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

Under Reset, AS is held high with an internal weak

pull-up.

The behavior of this signal is also affected by the

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

EMR1 or EMR2 registers. Refer to the Register

description.

8.2.2 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 ac-

knowledge cycle or under processor control by set-

ting 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 also affected by the

LDS[2:0], UDS[2:0], DS2EN and MC bits in the

EMR1 or WCR register. Refer to the Register de-

scription.

8.2.3 RW: Read/Write

RW (Output, Active low, Tristate) identifies the

type of memory cycle: RW=”1” identifies a memory

read cycle, RW=”0” identifies a memory write cy-

cle. It is defined at the beginning of each memory

cycle and it remains stable until the following

memory cycle.

RW is released in high-impedance during bus ac-

knowledge cycle or under processor control by

setting the HIMP bit (MODER).

Under Reset status, RW is held high with an inter-

nal weak pull-up.

The behavior of this signal is affected by the MC

and ETO bits in the EMR1 register. Refer to the

8.2.4 DS2: Data Strobe 2

This additional Data Strobe pin (Alternate Function

Output, Active low, Tristate) allows two different

external memories to be connected to the ST9, the

upper memory block (A21=1 typically RAM) and

the lower memory block (A21=0 typically ROM)

without any external 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 enabled via software as the Alter-

nate Function output of the associated I/O port bit.

DS2 is released in high-impedance during bus ac-

knowledge cycle or under processor control by

setting the HIMP bit (MODER.0, r235).

The behavior of this signal is also affected by the

DS2EN bit in the EMR1 register. Refer to the Reg-

ister description.

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