AT89LP51RB2 datasheet(23/254 Pages) ATMEL

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3722A–MICRO–10/11

AT89LP51RB2/RC2/IC2 Preliminary

3.5

Extended Stack

The AT89LP51RB2/RC2/IC2 provides an extended stack space for applications requiring addi-

tional stack memory. By default the stack is located in the 256-byte IDATA space of internal data

memory. The IDATA stack is referenced solely by the 8-bit Stack Pointer (SP: 81H). Setting the

XSTK bit in AUXR1 (see Table 5-6) enables the extended stack. The extended stack resides in

the EDATA space for up to 2KB of stack memory. The extended stack is referenced by an 11-bit

pointer formed from SP and the three LSBs of the Extended Stack Pointer (SPX: EFH) as shown

in Figure 3-16. SP is shared between both stacks. Note that the standard IDATA stack will not

overflow to the EDATA stack or vice versa. The stack and extended stack are mutually exclusive

and SPX is ignored when XTSK=0. An application choosing to switch between stacks by tog-

gling XSTK must maintain separate copies of SP for use with each stack space. Interrupts

should be disabled while swapping copies of SP in such an application to prevent illegal stack

accesses.

All interrupt calls and PUSH, POP, ACALL, LCALL, RET and RETI instructions will incur a one

or two-cycle penalty while the extended stack is enabled, depending on the number of stack

access in each instruction. The extended stack may only exist within the internal EDATA space;

it cannot be placed in XDATA. The stack will continue to use EDATA even if EDATA is disabled

by setting EXRRAM = 1.

Figure 3-16. Stack Configurations

00h

FFh

IDATA

(256)

00h

7FFh

EDATA

(2K)

SPX

XSTK = 0

XSTK = 1

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