ST92F124 datasheet(148/429 Pages) STMICROELECTRONICS

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

8.3 REGISTER DESCRIPTION

EXTERNAL MEMORY REGISTER 1 (EMR1)

R245 - Read/Write

Reset value: 1000 0000 (80h)

Bit 7 = Reserved.

Bit 6 = MC: Mode Control.

0: AS, DS and RW pins have the standard ST9 for-

mat.

1: AS pin becomes ALE, Address Load Enable.

This signal indicates to the external address

latch that a valid address is put on AD[7:0].

When ALE is high, the multiplexed address/data

bus AD[7:0] carries the LSBs of the memory ad-

dress, which must be latched on the falling edge

of this signal.

DS becomes OEN, Output ENable: When this

signal is low, the external memory should put

the data on the multiplexed address/data bus

AD[7:0]. The data is sampled by the microcon-

troller on the rising edge of the OEN signal.

RW pin becomes WEN, Write ENable: when this

signal is low, the multiplexed address/data bus

AD[7:0] carries the data to be written in the ex-

ternal memory. The external memory should

sample the data on the rising edge of the WEN

signal.

Bit 5 = DS2EN: Data Strobe 2 enable.

0: The DS pin is active for any external memory

access (lower and upper memory block).

The DS2 pin remains high.

1: If the lower memory block is addressed, the

DS2 pin outputs the standard DS signal, while

the DS pin stays high during the whole memory

cycle.

If the upper memory block is addressed, DS2 is

forced to “1” during the whole memory cycle.

Refer to Figure 77

Bit 4 = ASAF: Address Strobe as Alternate Func-

tion.

Depending on the device, AS can be either a ded-

icated pin or a port Alternate Function. This bit is

used only in the second case.

0: AS Alternate function disabled.

1: AS Alternate Function enabled.

Bit 3 = Reserved, must be kept cleared.

Bit 2 = ETO: External toggle.

0: The external memory interface pins (AS, DS,

DS2, RW, Port0, Port1, Port9) toggle only if an

access to external memory is performed.

1: When the internal memory protection is dis-

abled, the above pins (except DS which never

toggles during internal memory accesses) tog-

gle during both internal and external memory

accesses.

Bit 1 = BSZ: Bus size.

0: All outputs use the standard low-noise output

buffers.

1: P4[7:6], P6[5:4] use high-drive output buffers

Bit 0 = Reserved.

Caution: External memory must be correctly ad-

dressed before and after a write operation on the

EMR1 register. For example, if code is fetched

from external memory using the standard ST9 ex-

ternal memory interface configuration (MC=0),

setting the MC bit will cause the device to behave

unpredictably.

DS2EN

ASAF

ETO

BSZ

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