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CHAPTER 5 DATA MEMORY

RPC-2300 Page 13

backup RAM when pow er is off. Battery life will

depend upon RAM size, type, temperature, and time the

SBS-2300 has power applied to it. You can expect the

battery to last between 5 to 10 years for operation at

25°C. At 50°C, life is about 1/2 as much.

To install a DS-1213D, r emove the RAM chip in U2,

install the DS-1213D, and install the RAM chip on top of

the module.

STORING VARIABLES IN RAM

The term "var iables" in this context includes numb ers,

strings, arrays, recipes, and formulas as applied to your

application.

Programs and CA MBASIC II variables reside in

segment 0. Your var iables are generally stored in

segment 1 and higher. 32K of RAM is available for

your program and variable storage in segment 0. The

program and basic variables (A, B(15), C $, etc.) always

reside in segment 0 and are cleared on reset. Variables

you peek and poke to usually reside in segment 1 and

above. Var iables referenced by peek and poke

statements. Each segment has an address range from 0

to 65535.

PEEK and POKE commands store and retrieve values

from memory. For example:

20 POKE 12,A,1

puts the value of A into segment 1, address 12.

Use the PEEK statement to retrieve the variable:

50 B = PEEK(12, 1)

You can store and retrieve arrays, strings, and variables

in this way. There ar e many variations of PEE K and

POKE statements. Refer to the CAMBA SIC II

Programming Manual for additional information and

examples. A list of comm ands appea rs at the end of this

chapter.

CORRUPTED VARIABLES

RAM may be battery backed using a Dallas

Semiconductor DS-1213D Smartsocket. W hen your

application must rely on the accuracy of data after power

up, corr upted variables becomes a possibility.

The nature of RAM is it is easily written to. Any

POKE' d data is susceptible to corruption. This is

especially true when the board is powered down. The

DS-1213C monitors the supply voltage and turns off

writing when it is below about 4.65 volts. However,

when POKE ing long data, such as strings and floating

point numbers, a power down could interrupt a saving

process. The result is information is corrupted. A

scenario is explained below.

A program is running and POKEing data into RAM. At

the same time it is poking, a reset occurs. A reset can

occur due to power loss, someone pushing the reset

button, or a wa tchdog time r time out.

If the program was POKEing a string (POKE $), floating

point number (FPOKE), double byte (DPOKE ), or arr ay

while the reset occurred, the data became corrupted.

This is because the complete value was not saved.

Since it is impossible to predict or delay a reset, a work

around is to duplicate or triplicate POKEd values. That

is, you would have to save the same information in two

or three different places. F or purposes of discussion,

POKEd variables are called sets because data can consist

of a mixture of variables, strings and arrays.

On power up, your program would compare values from

one set to the other one or two. If the two (or three)

agreed, then there was no corruption and the program

can reliably use the values. In practice, you would read

information from set 1, but would save data to all two or

three.

The use of duplicate or triplicate sets depends upon what

the system must or can do if data is corrupted. When

using a duplicate set, a corrupted set indicates that

default values (from the program) should be used, since

it is uncertain if the first or second set is corrupted.

Both data sets would then be re-initialized.

A triplicate set is used to recover the last set or indicate

that the data in the first set is valid. The pr ocedure and

logic is as follows.

Data is written to each element in a set in a specific and

consistent order (data to an entire set does not have to be

written to, just that element). For example, a calibration

constant is saved (POKE' d) in three different places.

Assume that the constant was assigned address 0, 100,

and 200 in segment 1. The data is POKEd to address 0

first, then 100, then 200.

1 2 ... 10 11 12 13 14 15 16 17 18 19 20 ... 35 36

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