Precautions - ZiLOG Z8 PLUS User Manual

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The upper nibble of the register file address,

provided by the register pointer, specifies

the active working-register group.

Precautions

Registers in the Standard Register File must be correctly used or certain conditions produce inconsistent

results.

The watch-dog timer can only be disabled via software if the first instruction out of RESET performs this

function. During the execution of the first instruction after the Z8

the TCTLHI register can be written. After the first instruction, hardware does not allow the upper five bits

of this register to be written.

Some control registers, including the port inputs and timer count registers, may be updated by hardware.

Writing these registers from software always overrides the hardware update from the same cycle, but with

unpredictable results. For example, writing into the count value register of a running timer can cause

R7 R6 R5 R4 R3 R2 R1 R0

Working Register Group F

Specified Working Register Group

Working Register Group 1

Working Register

Group 0

Figure 1-5. Register Pointer

The lower nibble specifies the

current page of RAM.

R253

(Register Pointer)

The lower nibble

of the register

file address,

provided by the

instruction, points

to the specified

R15

LUS

leaves RESET, the upper five bits of

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