DS87C550 datasheet(13/50 Pages) DALLAS

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DS87C550

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Stretch setting. When maximum speed is desired, software should select a Stretch value of 0. When using

very slow RAM or peripherals, the application software can select a larger Stretch value. Note that this

affects data memory accesses only and that there is no way to slow the accesses to program memory other

than to use a slower crystal (or external clock).

The specific timing of the variable speed Stretch MOVX is provided in the Electrical Specifications

section of this data sheet. Table 4 shows the resulting MOVX instruction timing and the read or write

strobe widths for each Stretch value.

DATA MEMORY CYCLE STRETCH VALUES Table 4

CKCON.2-0

MOVX MACHINE

OR WR STROBE WIDTH

CYCLES

IN MACHINE CYCLES

2 (forced internal)

0.5

3 (default external)

Dual Data Pointer With Inc/Dec

The DS87C550 contains several new, unique features that are associated with the Data Pointer register. In

the original 8051 architecture, the DPTR was a 16-bit value that was used to address off-chip data RAM

or peripherals.

To improve the efficiency of data moves, the DS87C550 contains two Data Pointer

registers (DPTR0 and DPTR1). By loading one DPTR with the source address and the other with the

destination address, block data moves can be made much more efficient. Since DPTR0 is located at the

same address as the single DPTR in the original 8051 architecture, code written for the original

architecture will operate normally on the DS87C550 with no modification necessary.

The second data pointer, DPTR1 is located at the next two register locations (up from DPTR0) and is

selected using the data pointer select bit SEL (DPS.0). If SEL = 0, then DPTR0 is the active data pointer.

Conversely, if SEL = 1, then DPTR1 is the active data pointer. Any instruction that reference the DPTR

(ex. MOVX A, @ DPTR) refers to the active data pointer as determined by the SEL bit. Since the bit

adjacent to SEL in the DPS register is not used, the fastest means of changing the SEL (and thereby

changing the active data pointer) is with an INC instruction. Each INC DPS Instruction will toggle the

active data pointer.

Unlike the standard 8051, the DS87C550 has the ability to decrement as well as increment the data

pointers without additional instructions. When the INC DPTR instruction is executed, the active DPTR is

incremented or decremented according to the ID1, ID0 (DPS.7-6), and SEL (DPS.0) bits as shown. The

inactive DPTR is not affected.

ID1

ID0

SEL

RESULT OF INC DPTR

INCREMENT DPTR0

DECREMENT DPTR0

INCREMENT DPTR1

DECREMENT DPTR1

DS87C550 Datasheet(PDF) 13 Page - Dallas Semiconductor

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