AN-1014
Application Note
Using the Atmel® picoPower™ AVR® Microcontroller
with Cymbet™ EnerChips™
A Demonstration Vehicle for Cymbet Solid State Thin Film Rechargeable Batteries
Consideration needs to be given to the minimum voltage needed
by the system to run off backup power, on how to switch between
main power and backup power, and on how to signal the
microcontroller that it will be running off backup power. Special
algorithms may be used so that the microcontroller sheds loads
and puts itself into a low-power sleep mode during backup.
The power-fail sensing circuit must not feed power back into the
main power bus when the system switches over to the EnerChip
for backup power. In the Demo Board, a 4.7 MOhm resistor is
used to limit current on the VREF line to ≈500 nA. A diode would
have eliminated all current leakage and have been an even better
choice.
Figure 1. Cymbet Atmel picoPower AVR Demo Board.
Introduction
The Cymbet EnerChip and Atmel picoPower AVR are combined
in a demonstration board designed to show the advantages of
Cymbet EnerChip rechargeable batteries for embedded processor
applications. Cymbet EnerChips provide virtually unlimited
system life. They are smaller, lighter and environmentally
friendlier than primary lithium coin cells for backup power. The
demo board consists of an Atmel Atmega169P picoPower AVR
microcontroller with integrated LCD controller, Cymbet EnerChip
cells, and a Cymbet charge control circuit.
System Considerations
Cymbet EnerChips are ideal for applications requiring a high
number of charge/discharge cycles, such as backing up
microcontrollers during intermittent power outages. Such systems
will need a charge controller to keep the EnerChip at a full state of
charge without overcharging the battery and thereby reducing its
useful life. The charge circuit will need a source of voltage higher
than the EnerChip charging voltage of 4.1V. This may require a
charge pump. The charge circuit must regulate the charge voltage
and have a means of isolating itself from the EnerChip (current
blocking) to prevent the EnerChip from discharging through the
charge circuit when system power is off. Isolation from the main
power supply and voltage reduction from the battery output to the
load may also be necessary.
Figure 2. Charge Controller Block Diagram.
Charge Circuit Description
The charge circuit in Figure 4 meets all key system requirements.
It is powered by two lithium coin cells, so that no boost charge
pump is needed. Charge voltage regulation utilizes a Zetex
ZR40401F41TA shunt voltage regulator, but could also have
used a low IQ LDO, such as a TI TPS715XX regulator.
Current blocking to prevent the EnerChip from being discharged
when main power is off is implemented with transistors Q2, Q3,
and Q5. Transistors Q3 and Q5 are dual packaged devices, with
the two transistors wired in series so that the off-state leakage
current is negligible for a µAh-rated EnerChip.
PNP transistor Q2 is used as a comparator to shut off the charge
circuit when the input voltage falls below ≈ 4.8V (4.1V plus the
VBE of the transistor). When Q2 turns off, N-channel FET Q3 is
turned off, which then turns off P-channel FET Q5.
© Cymbet Corporation • 18326 Joplin Street, Elk River, MN 55330 • 763-633-1780 • www.cymbet.com
DOC-111014 RevA
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AN-1014: Atmel picoPower AVR Demo Board
Demonstration Description
Software Description
Two on-board coin cells are used as the primary power source in
lieu of an external power supply. Place the two coin cells in series
in the coin cell holder, with the negative terminal toward the board
for both cells. Position the battery-select jumper to select the
correct EnerChip. Close power switch SW1 for charging. Also
press pushbutton S1 (at bottom left of board) to turn off the LCD
display and minimize current drain. The EnerChip will be 80%
charged within 30 minutes and 100% charged after 45 minutes.
See AN-1003 for more details on battery charging times.
The software uses an interrupt driven control loop. The processor
is normally in the “powersave” mode to conserve power. Timer2
is clocked off the external 32.768 KHz crystal and generates an
interrupt at 1 sec intervals. This wakes the processor, which will
then update the display as needed, then go back to sleep.
After the battery is fully charged, slide the power switch to Off
and back to On. This will turn on the LCD display. The indicator
caption will be START. The board will wait for an operator input.
Display ready for
timing.
Display while timing Display stopped
after pressing S2.
after pressing S2.
Press pushbutton S2 (at bottom right of board) to start the timer
function. The LCD will show elapsed minutes and seconds. The
indicator caption will be STOP. Press S2 again to stop timing. The
indicator caption will be CLEAR. Press S2 once more to clear the
display and reset the timer.
Turn off power switch SW1 or remove the coin cells to demonstrate Demo Board operation from the EnerChip alone.
Press pushbutton S1 (at bottom left of board) to toggle the LCD
display on and off. With the LCD off, current draw on the
EnerChip will be ≈ 2 µA. A 50 µAh CBC050 EnerChip will then
provide backup for 25 hours and a CBC012 EnerChip for 6 hours.
With the LCD on, current draw will be ≈ 14 µA, and the above
times will be reduced to 3.6 hours and 51 minutes, respectively.
To measure current draw, insert a micro-Ammeter in place of the
battery-select jumper.
Timer Initialization Routine
void timer2_init(void)
{
#asm("cli")
// clear global interrupt flag
TIMSK2 = 0;
// disable OCIE2A and TOIE2
ASSR=0x08;
// use external 32 crystal
TCNT2=0x00;
// clear counter
TCCR2A = 0x05;
// divide by 8
while ((ASSR & 1) | (ASSR & 4));
// wait for tcn2ub and tcr2ub to clear
TIFR2 = 0x7F;
// set overflow limit
TIMSK2 = 0x01;
// set overflow interrupt enable bit
#asm("sei")
// reset global interrupt flag
}
LCD Setup Registers
LCDCRA=0x80;
LCDCRB=0xD7;
LCDCCR= 0x0F;
LCDFRR=0x02;
// normal waveform; buffers on
// now set for external clock 32.768
// contrast control
// clock divider
Other LCDs might require different settings. Further power
reductions can be obtained by turning off the buffers and using
the ATmega169 “Low Power” LCD waveform; however, this
may reduce the contrast and sharpness of the display. Consult
Atmel, www.atmel.com, for further details and latest information
on the ATmega169P AVR microcontroller.
The 3-1/2 digit LCD display will first show minutes and seconds
in MM:SS format. After 19:59 (19 min: 59 sec), it switches to
HH:MM format. After 19:59 (19 hrs: 59 min), the unit stops
counting.
When power switch SW1 is in the Off position, the display is
automatically blanked and the EnerChip is being discharged.
When finished with the evaluation, remove the battery select
jumper to prevent the EnerChip from becoming fully discharged,
possibly reducing its life.
© Cymbet Corporation • 18326 Joplin Street, Elk River, MN 55330 • 763-633-1780 • www.cymbet.com
DOC-111014 RevA
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AN-1014: Atmel picoPower AVR Demo Board
Figure 3. Schematic Diagram, Atmel ATmega169P Microcontroller.
© Cymbet Corporation • 18326 Joplin Street, Elk River, MN 55330 • 763-633-1780 • www.cymbet.com
DOC-111014 RevA
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AN-1014: Atmel picoPower AVR Demo Board
Figure 4. Charge Control Circuit.
Note: Atmel® and AVR® are registered trademarks of Atmel Corporation and picoPower™ is a trademark of Atmel Corporation. All other trademarks
are the property of their respective owners.
© Cymbet Corporation • 18326 Joplin Street, Elk River, MN 55330 • 763-633-1780 • www.cymbet.com
DOC-111014 RevA
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