4

AVR401

0953C–AVR–02/03

The transistor should be connected to a pin as long away from the comparator inputs as

possible. When a pin is switched, a noise spike appear at the adjacent pins. This will

cause problems when measuring low voltages, as the noise spike might trigger the com-

parator before the capacitor voltage has reached the measured voltage.

Figure 3 shows measured linearity for a 4 MHz clocked application using the component

values calculated in the above example.

Figure 3. Measured Linearity

Implementation

The software consist of several subroutines. The routines “reference” and “convert” han-

dles the charging and timing. After they complete execution, the main program must

perform the calculation needed. This is done using two subroutines that performs divi-

sion and multiplication, “div17u” and “mul9”. There are also two delays used by the other

routines and the main program. They are used to discharge the capacitor completely

and to generate a delay between each conversion.

“reference” Subroutine –

Measures the Reference

Voltage

The routine discharges the capacitor, turns on the transistor and charges the capacitor

until the capacitor voltage is equal to the reference voltage. The time from the beginning

of the charging and until the voltages are equal is measured. The capacitor is then dis-

charged again. The charging time is used together with the charging time from the

“convert” routine to calculate the input voltage.

This routine does not have to be called every time a conversion is performed, depending

on variations in ambient temperature. Especially the parameter h

quite temperature dependent, so if the ambient temperature is varying, the subroutine

will have to be executed frequently. In the example program, the “reference” routine is

called each time a conversion is performed.

LINEARITY

0

50

100

150

200

250

01

23

45

INPUT VOLTAGE

O

U

T

P

U

T

V

A

L

U

E

Table 1. “reference” Subroutine Performance Figures

Parameter

Value

Code Size

24 words

Execution Cycles

Depends on the reference voltage.

Register Usage

Low Registers

High Registers

Global

:None

:2

:1