ST92F124 datasheet(149/429 Pages) STMICROELECTRONICS

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

EXTERNAL MEMORY INTERFACE REGISTERS (Cont’d)

EXTERNAL MEMORY REGISTER 2 (EMR2)

R246 - Read/Write

Reset value: 0001 1111 (1Fh)

Bit 7 = Reserved.

Bit 6 = ENCSR: Enable Code Segment Register.

This bit affects the ST9 CPU behavior whenever

an interrupt request is issued.

0: The CPU works in original ST9 compatibility

mode concerning stack frame during interrupts.

For the duration of the interrupt service routine,

ISR is used instead of CSR, and the interrupt

stack frame is identical to that of the original

ST9: only the PC and Flags are pushed. This

avoids saving the CSR on the stack in the event

of an interrupt, thus ensuring a faster interrupt

response time. The drawback is that it is not

possible for an interrupt service routine to per-

form inter-segment calls or jumps: these instruc-

tions would update the CSR, which, in this case,

is not used (ISR is used instead). The code seg-

ment size for all interrupt service routines is thus

limited to 64K bytes.

1: If ENCSR is set, ISR is only used to point to the

interrupt vector table and to initialize the CSR at

the beginning of the interrupt service routine: the

old CSR is pushed onto the stack together with

the PC and flags, and CSR is then loaded with

the contents of ISR. In this case, iret will also re-

store CSR from the stack. This approach allows

interrupt service routines to access the entire

4Mbytes of address space; the drawback is that

the interrupt response time is slightly increased,

because of the need to also save CSR on the

stack. Full compatibility with the original ST9 is

lost in this case, because the interrupt stack

frame is different; this difference, however,

should not affect the vast majority of programs.

Bit 5 = DPRREM: Data Page Registers remapping

0: The locations of the four MMU (Memory Man-

agement Unit) Data Page Registers (DPR0,

DPR1, DPR2 and DPR3) are in page 21.

1: The four MMU Data Page Registers are

swapped with that of the Data Registers of ports

0-3.

Refer to Figure 73

Bit 4 = MEMSEL: Memory Selection.

Warning: Must be kept at 1.

Bit 3:2 = LAS[1:0]: Lower 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 lower memory block accesses

(A21=”0”). The reset value is 3.

ENCSR DPRREM MEMSEL LAS1 LAS0 UAS1 UAS0

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