AN929 datasheet(2/22 Pages) MICROCHIP

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AN929

DS00929A-page 2

 2004 Microchip Technology Inc.

Frequency Output

Oscillators provide a frequency output proportional to

temperature that can be interfaced to a microcontroller,

as shown in Figure 2. While a Resistor-Capacitor (RC)

operational amplifier (op amp) oscillator can accurately

measure the resistance of an RTD, this circuit is

typically not used with a thermistor because of the large

logarithmic change in the sensor’s resistance over

temperature. The main advantage of a frequency

output is that an ADC is not required. A frequency

output is also useful in applications where the sensor

conditioning circuitry is combined with a remote sensor.

A logic-level output signal is less sensitive to noise than

an analog output signal that transmits information to

the microcontroller.

FIGURE 2:

Block Diagram of a

Frequency Output Sensor.

The accuracy of the frequency measurement is directly

related to the quality of the microcontroller’s clock

signal. Precision high-frequency microcontroller clock

oscillators are readily available. However, they are

relatively expensive. The two options available to

measure frequency are the fixed time or fixed cycle

methods. The microcontroller resources required for

determining frequency varies depending upon the

processor

bandwidth,

available

peripherals

and

desired measurement accuracy.

FIXED TIME METHOD

The fixed time method, shown in Figure 3, consists of

counting the number of pulses within a specific time

window, such as 100 ms. The frequency is then calcu-

lated by multiplying the count by the integer required to

correlate the number of pulses in one second. This

measurement approach inherently minimizes the effect

of error sources (such as jitter) by averaging many

oscillator pulses in the time window. The fixed time

method utilizes a firmware delay or hardware delay

routine. The firmware can poll for input edges, though

this consumes processor bandwidth. A more common

implementation uses a hardware timer/counter to count

the input cycles during a firmware delay. If a second

timer is available, the delay can be generated using this

timer, thus requiring minimal processor bandwidth.

FIGURE 3:

Frequency Output - Fixed

Time Method.

FIXED CYCLE METHOD

In the fixed cycle method, shown in Figure 4, the

number

cycles

measured

fixed

and

the

measurement

time

variable.

This

approach

measures the elapsed time for a fixed number of

cycles. The number of cycles is chosen by the designer

based on the desired accuracy, input frequency,

measurement rate and the microcontroller clock

frequency (FOSC). FOSC determines the minimum time

an edge can be resolved. The measurement error will

then be proportional to the total amount of time versus

FOSC. Increasing the number of cycles measured will

increase the measurement time and reduce the error.

Increasing FOSC will also decrease the minimum time

to resolve an edge, thereby reducing the error.

FIGURE 4:

Frequency Output - Fixed

Cycle Method.

The fixed cycle method can utilize firmware to both

measure time and poll the input edges. This, however,

is processor-intensive and has accuracy limitations.

For example, a more common implementation is to uti-

lize the Capture/Compare/PWM (CCP) module of a

PICmicro® microcontroller configured in the Capture

mode. This approach uses the 16-bit timer TMR1

peripheral and has excellent accuracy and range.

RC op amp

Oscillator

PICmicro®

Microcontroller

RTD

Clock

Algorithm:

Count the number of clock pulses in a time window.

Oscillator

Signal

Time Window

Example: Measure the number of oscillation pulses in a

100 msec. window and multiply by 10 to

determine the frequency.

Algorithm:

Determine the time between a fixed number of oscillation

Oscillator

Signal

Time

Example: Measure time between four rising edges of the

oscillation signal.

pulses.

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