CN-0164

Circuit Note

Rev. A | Page 2 of 5

The ADuC7060 precision analog microcontroller has a low

power ARM7 core as well as a myriad of precision analog

functions. The onboard multiplexer, digitally programmable

gain amplifier (PGA), voltage reference, programmable current

sources, and 24-bit sigma-delta ADC allow almost any temper-

ature and bridge sensors to be directly connected. In this case, a

4-wire Pt100 (100 Ω platinum RTD) temperature sensor was

chosen. Further details on the measuring circuit can be found

in the AN-0970 Application Note.

The wireless band chosen for this application is the sub-GHz,

license-free ISM (industrial, scientific, medical) band. The

ADF7020 transceiver, which supports bands in the 431 MHz to

478 MHz as well as the 862 MHz to 956 MHz frequency ranges,

is, therefore, a natural choice. This low power transceiver

requires very few external components, is easily connected to

the ADuC7060 precision analog microcontroller, and offers

excellent performance.

The ADP121 voltage regulator provides the 2.5 V supply from

two 1.5 V batteries. The very low quiescent current of this

voltage regulator (11µA at no load) is paramount in maxi-

mizing battery lifetime.

CIRCUIT DESCRIPTION

Two buses connect the ADF7020 ISM transceiver with the

ADuC7060 precision microcontroller. Both buses are serial and

bidirectional. One of these buses configures the transceiver, and

it requires four microprocessor ports. The second bus is the

data bus, which enables the data transaction between controller

and transceiver. This bus requires at least three microprocessor

ports. In this particular application, two ports are used instead

of one bidirectional port with two interrupts. This simplifies the

software but necessitates the use of an extra diode and resistor

to separate incoming and outgoing data streams. A parallel

combination of two Schottky diodes ensures a logic low, which

is less than 200 mV. The BAT54C has two diodes in the same

package (connecting Pin 1 and Pin 2 together for a parallel

configuration). All digital ports on the ADuC7060 have

programmable pull-up resistors; however, an external pull-up

resistor is also required. With a data rate of 10 kbps, a 4.7 kΩ

resistor works well.

Three factors determine the overall current drawn by the

circuit: the requirement of the individual components in both

sleep and active modes), the amount of time the system is

active, and the amount of time the transceiver itself is active.

The first factor is addressed by choosing low power components

such as the ADuC7060 and the ADF7020. The second factor,

minimizing the activity of the system, is achieved by keeping

the system inactive as long as possible. It is worth considering

the tradeoff between integer versus floating point arithmetic—

in many cases, integer is sufficient, has a shorter execution time,

and, thus, provides greater savings. The final factor, reducing air

time, is achieved in part by using a protocol with minimum

overhead, but also to a large extent by using the ADF7020,

which has very high receiver sensitivity and good out-of-band

rejection, thus maximizing the probability that the data package

contains correct data.

Code Description—General

The system spends the majority of time in deep sleep mode,

with a current consumption of 50 µA to 60 µA (depending on

ambient temperature). Timer 2 wakes the system every second.

Every 60 seconds, an ADC measurement is executed, linearized,

and transmitted. Timer 2 can wake the system from deep sleep;

the other three timers cannot. Timer 2 is 16-bit, meaning that it

wakes every second when running from a 32 kHz clock (in sleep

mode). After the ADC is started, the system goes into pause

mode (see the ADuC7060 data sheet for more information). This

is a reduced power mode, albeit not as reduced as deep sleep.

The ADC wakes the system when finished. A temperature value

is calculated from the ADC results and is packaged and

transmitted.

Packaging essentially means placing appropriate data in a

buffer. In this case, the data consists of a 4-byte floating point

temperature value and a 2-byte CRC (cyclic redundancy check).

In a more complex system, a header with node address, received

signal strength, and other information precedes this data.

Before sending this buffer to the ADF7020 transceiver, an

8-byte preamb to help synchronize the receiving node and a

3-byte synchronization word, or sync word, are sent. This is a

unique 3-byte number that is checked for a match at the receiver

node before a package can be received.

The hardware is very similar on the receiving side; an ADF7020

transceiver is configured to listen for the unique sync word.

After the sync word is received, the data package follows. The

data is sent to the PC via the UART.

Flowcharts for the main loops of both the measurement node

and the base receiving node are displayed in Figure 2.

Source code for this circuit can be found at this address:

www.analog.com/CN0164_Source_Code.