ST90135 datasheet(89/190 Pages) STMICROELECTRONICS

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ST90158 - EXTERNAL MEMORY INTERFACE (EXTMI)

Bit 1:0 = UAS[1:0]:

Upper memory address strobe

stretch.

These two bits contain the number of wait cycles

(from 0 to 3) to add to the System Clock to stretch

AS during external upper memory block accesses

(MSB of 22-bit internal address=1). The reset val-

ue is 3.

WARNING: The EMR2 register cannot be written

during an interrupt service routine.

WAIT CONTROL REGISTER (WCR)

R252 - Read/Write

Reset Value: 0111 1111 (7Fh)

Bit 7 = Reserved, forced by hardware to 0.

Bit 6 = WDGEN:

Watchdog Enable.

For a description of this bit, refer to the Timer/

Watchdog chapter.

WARNING: Clearing this bit has the effect of set-

ting the Timer/Watchdog to Watchdog mode. Un-

less this is desired, it must be set to “1”.

Bit 5:3 = UDS[2:0]:

Upper memory data strobe

stretch.

These bits contain the number of INTCLK cycles

to be added automatically to DS for external upper

memory block accesses. UDS = 0 adds no addi-

tional wait cycles. UDS = 7 adds the maximum 7

INTCLK cycles (reset condition).

Bit 2:0 = LDS[2:0]:

Lower memory data strobe

stretch.

These bits contain the number of INTCLK cycles

to be added automatically to DS or DS2 (depend-

ing on the DS2EN bit of the EMR1 register) for ex-

ternal lower memory block accesses. LDS = 0

adds no additional wait cycles, LDS = 7 adds the

maximum 7 INTCLK cycles (reset condition).

Note 1: The number of clock cycles added refers

to INTCLK and NOT to CPUCLK.

Note 2: The distinction between the Upper memo-

ry block and the Lower memory block allows differ-

ent wait cycles between the first 2 Mbytes and the

second 2 Mbytes, and allows 2 different data

strobe signals to be used to access 2 different

memories.

Typically, the RAM will be located above address

0x200000

and

the

ROM

below

address

0x1FFFFF, with different access times. No extra

hardware is required as DS is used to access the

upper memory block and DS2 is used to access

the lower memory block.

WARNING: The reset value of the Wait Control

cles for external memory. To get optimum perfor-

mance from the ST9, the user should write the

UDS[2:0] and LDS[2:0] bits to 0, if the external ad-

dressed memories are fast enough.

WDGE N UDS2

UDS1

UDS0

LDS2

LDS1

LDS0

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