AN897 datasheet(6/16 Pages) MICROCHIP

6 / 16 page

AN897

DS00897B-page 6

 2004 Microchip Technology Inc.

FLOWCHARTS

The second design’s flowchart is shown in Figure 15. It

is very similar to the first design program, with the

exception that it has added a PGA hysteresis routine.

The firmware is available in the 00897.zip file. The

firmware was written in relocatable assembly code.

The main.asm file controls the overall program flow.

The PGA routines are in pga.inc and pga.asm. The

thermistor routines are in Therm_PGA2.inc and

Therm_PGA2.asm

FIGURE 15:

Flowchart for Second

Design.

The Signal Analysis PC Program commands the

PIC16F684 firmware to perform a real-time sample.

The firmware reads the ADC value and passes it to the

PGA hysteresis routine. Figure 16 shows the detail of

the PGA hysteresis routine. The routine checks to see

what PGA gain is set (the variable “PGAgain”). Based

on PGAgain, the ADC value is tested for end-point

(trip) values. If the ADC value is beyond the trip point

value, PGAgain is set to the next higher or lower gain

setting. Upon exiting the PGA hysteresis routine, the

firmware checks if PGAgain was changed. If there was

no change (Return 0), the program continues. If there

was a change (Return 1), the firmware re-reads the

ADC.

Once the PGA gain and ADC value are known, both

values are passed to the piece-wise linear interpolation

routine. Based on the PGA gain setting, the correct

look-up table is referenced. The PwLI routine converts

the 10-bit ADC value into a 16-bit fixed decimal point

degrees Celsius value. The fixed decimal point format

reports degrees Celsius in tenths of a degree.

Performing the piece-wise linear interpolation in tenths

of a degree provides better resolution of degrees

Celsius. Finally, the 16-bit degrees Celsius value is

sent to the Signal Analysis PC Program for display on

the real-time strip chart graph.

In the final design, the designer can elect to receive

reports in tenths of a degree or that they be rounded up

into whole degrees.

FIGURE 16:

Flowchart for Second Design’s PGA Hysteresis Subroutine.

Read ADC

Perform PwLI

Send °C to Strip Chart

End

PGA Hysteresis

Yes

Was PGAgain

Changed?

Start

Get Real Time Sample

PGA Hysteresis

PGAgain

= 1

Set PGAgain = 8

PGAgain

= 8

PGAadc

< 113

PGAadc

< 226

PGAadc

> 960

PGAgain

= 32

PGAadc

> 960

Set PGAgain = 32

Set PGAgain = 1

Set PGAgain = 8

Return 0

Return 1

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