ST92F124 datasheet(38/426 Pages) STMICROELECTRONICS

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ST92F124/F150/F250 - DEVICE ARCHITECTURE

SYSTEM REGISTERS (Cont’d)

2.3.4 Paged Registers

Up to 64 pages, each containing 16 registers, may

be mapped to Group F. These paged registers

hold data and control information relating to the

on-chip peripherals, each peripheral always being

associated with the same pages and registers to

ensure code compatibility between ST9 devices.

The number of these registers depends on the pe-

ripherals present in the specific ST9 device. In oth-

er words, pages only exist if the relevant peripher-

al is present.

The paged registers are addressed using the nor-

mal register addressing modes, in conjunction with

the Page Pointer register, R234, which is one of

the System registers. This register selects the

page to be mapped to Group F and, once set,

does not need to be changed if two or more regis-

ters on the same page are to be addressed in suc-

cession.

Thus the instructions:

spp #5

ld R242, r4

will load the contents of working register r4 into the

third register of page 5 (R242).

Warning: During an interrupt, the PPR register is

not saved automatically in the stack. If needed, it

should be saved/restored by the user within the in-

terrupt routine.

PAGE POINTER REGISTER (PPR)

R234 - Read/Write

Reset value: xxxx xx00 (xxh)

Bits 7:2 = PP[5:0]: Page Pointer.

These bits contain the number (in the range 0 to

63) of the page specified in the spp instruction.

Once the page pointer has been set, there is no

need to refresh it unless a different page is re-

quired.

Bits 1:0: Reserved. Forced by hardware to 0.

2.3.5 Mode Register

The Mode Register allows control of the following

operating parameters:

– Selection of internal or external System and User

Stack areas,

– Management of the clock frequency,

– Enabling of Bus request and Wait signals when

interfacing to external memory.

MODE REGISTER (MODER)

R235 - Read/Write

Reset value: 1110 0000 (E0h)

Bit 7 = SSP: System Stack Pointer.

This bit selects an internal or external System

Stack area.

0: External system stack area, in memory space.

1: Internal system stack area, in the Register File

(reset state).

Bit 6 = USP: User Stack Pointer.

This bit selects an internal or external User Stack

area.

0: External user stack area, in memory space.

1: Internal user stack area, in the Register File (re-

set state).

Bit 5 = DIV2: Crystal Oscillator Clock Divided by 2.

This bit controls the divide-by-2 circuit operating

on the crystal oscillator clock (CLOCK1).

0: Clock divided by 1

1: Clock divided by 2

Bits 4:2 = PRS[2:0]: CPUCLK Prescaler.

These bits load the prescaler division factor for the

internal clock (INTCLK). The prescaler factor se-

lects the internal clock frequency, which can be di-

vided by a factor from 1 to 8. Refer to the Reset

and Clock Control chapter for further information.

Bit 1 = BRQEN: Bus Request Enable.

0: External Memory Bus Request disabled

1: External Memory Bus Request enabled on

BREQ pin (where available).

Note: Disregard this bit if BREQ pin is not availa-

ble.

Bit 0 = HIMP: High Impedance Enable.

When a port is programmed as Address and Data

lines to interface external Memory, these lines and

the Memory interface control lines (AS, DS, R/W)

can be forced into the High Impedance state.

0: External memory interface lines in normal state

1: High Impedance state.

PP5

PP4

PP3

PP2

PP1

PP0

SSP

USP

DIV2

PRS2

PRS1

PRS0 BRQEN HIMP

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