ST90R158 datasheet(87/190 Pages) STMICROELECTRONICS

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ST90158 - EXTERNAL MEMORY INTERFACE (EXTMI)

7.3 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 keep the ST9OLD mean-

ing.

1: AS pin becomes ALE, Address Load Enable

(AS inverted); Thus Memory Adress, Read/

Write signals are valid whenever a falling edge

of ALE occurs.

DS becomes OEN, Output ENable: it keeps the

ST9OLD meaning during external read opera-

tions, but is forced to “1” during external write

operations.

RW pin becomes WEN, Write ENable: it follows

the ST9OLD DS meaning during external write

operations, but is forced to “1” during external

read operations.

Bit 5 = DS2EN:

Data Strobe 2 enable.

0: The DS2 pin is forced to “1” during the whole

memory cycle.

1: If the lower memory block is addressed, the

DS2 pin follows the ST9OLD DS meaning (if

MC=0) or it becomes OEN (if MC=1). The DS

pin is forced to 1 during the whole memory cy-

cle.

If the upper memory block is used, DS2 is forced

to “1” during the whole memory cycle. The DS

pin behaviour is not modified.

Refer to Figure 44

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 2 = ETO:

External toggle.

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

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

to external memory is performed.

1: When the internal memory protection is dis-

abled (mask option available on some devices

only), the above pins (except DS and DS2 which

never toggle during internal memory accesses)

toggle during both internal and external memory

accesses.

Bit 1 = BSZ:

Bus size.

0: All the I/O ports including the external memory

interface pins use smaller, less noisy output

buffers. This may limit the operation frequency

of the device, unless the clock is slow enough or

sufficient wait states are inserted.

1: All the I/O ports including the external memory

interface pins (AS, DS, DS2, R/W, Port 0, 1) use

larger, more noisy output buffers .

Bit 0 = Reserved.

WARNING: External memory must be correctly

addressed before and after a write operation on

the EMR1 register. For example, if code is fetched

from external memory using the ST9OLD external

memory interface configuration (MC=0), setting

the MC bit will cause the device to behave unpre-

dictably.

DS2EN

ASAF

ETO

BSZ

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