User guide

44
ATmega162/V
2513E–AVR–09/03
Minimizing Power
Consumption
There are several issues to consider when trying to minimize the power consumption in
an AVR controlled system. In general, sleep modes should be used as much as possi-
ble, and the sleep mode should be selected so that as few as possible of the device’s
functions are operating. All functions not needed should be disabled. In particular, the
following modules may need special consideration when trying to achieve the lowest
possible power consumption.
Analog Comparator When entering Idle mode, the Analog Comparator should be disabled if not needed. In
the other sleep modes, the Analog Comparator is automatically disabled. However, if
the Analog Comparator is set up to use the Internal Voltage Reference as input, the
Analog Comparator should be disabled in all sleep modes. Otherwise, the Internal Volt-
age Reference will be enabled, independent of sleep mode. Refer to “Analog
Comparator” on page 194 for details on how to configure the Analog Comparator.
Brown-out Detector If the Brown-out Detector is not needed in the application, this module should be turned
off. If the Brown-out Detector is enabled by the BODLEVEL Fuses, it will be enabled in
all sleep modes, and hence, always consume power. In the deeper sleep modes, this
will contribute significantly to the total current consumption. Refer to “Brown-out Detec-
tion” on page 49 for details on how to configure the Brown-out Detector.
Internal Voltage Reference The Internal Voltage Reference will be enabled when needed by the Brown-out Detector
or the Analog Comparator. If these modules are disabled as described in the sections
above, the internal voltage reference will be disabled and it will not be consuming
power. When turned on again, the user must allow the reference to start up before the
output is used. If the reference is kept on in sleep mode, the output can be used imme-
diately. Refer to “Internal Voltage Reference” on page 51 for details on the start-up time.
Watchdog Timer If the Watchdog Timer is not needed in the application, this module should be turned off.
If the Watchdog Timer is enabled, it will be enabled in all sleep modes, and hence,
always consume power. In the deeper sleep modes, this will contribute significantly to
the total current consumption. Refer to “Watchdog Timer” on page 51 for details on how
to configure the Watchdog Timer.
Port Pins When entering a sleep mode, all port pins should be configured to use minimum power.
The most important thing is to ensure that no pins drive resistive loads. In sleep modes
where the I/O clock (clk
I/O
) is stopped, the input buffers of the device will be disabled.
This ensures that no power is consumed by the input logic when not needed. In some
cases, the input logic is needed for detecting wake-up conditions, and it will then be
enabled. Refer to the section “Digital Input Enable and Sleep Modes” on page 66 for
details on which pins are enabled. If the input buffer is enabled and the input signal is
left floating or have an analog signal level close to V
CC
/2, the input buffer will use exces-
sive power.
JTAG Interface and
On-chip Debug System
If the On-chip debug system is enabled by the OCDEN Fuse and the chip enter Power
down or Power save sleep mode, the main clock source remains enabled. In these
sleep modes, this will contribute significantly to the total current consumption. There are
three alternative ways to avoid this:
• Disable OCDEN Fuse.
• Disable JTAGEN Fuse.
• Write one to the JTD bit in MCUCSR.
The TDO pin is left floating when the JTAG interface is enabled while the JTAG TAP
controller is not shifting data. If the hardware connected to the TDO pin does not pull up