Reducing Power Consumption
User
Presses SW1
MCU
Controls LEDs
HC08 CPU
HC08 CPU
Echo
Driver
Machester
encodes
msg,
transmits
using Echo
decodes msg
and removes
Manchester
encoding
CPU reads msg
from ram buffer
CPU writes msg
to ram buffer
Echo
Decoded msg
To mcu via SPI
interface
sendsOOK
modulated
message
Echo
driver
Ram
buffer
Ram
Echo
driver
Echo
Echo
buffer
Driver writes
msg to buffer
Driver reads
Msg from buffer
Figure 2. Data Flow and Message Encoding Steps in Message Transfer
5
Reducing Power Consumption
Echo can be placed into a standby mode to reduce power consumption. Controlling how Echo enters and
wakes from standby mode can be achieved in two ways: using the internal strobe oscillator or external
control via an I/O pin.
When Echo is in receive mode and the strobe oscillator is enabled (#define ECHO_SOE_VALUE 1in the
driver’s header file), the internal strobe oscillator uses the values RONand ROFFin the RXONOFFregister to
control how long Echo is awake and asleep, respectively. These values may be set in the Echo header file.
If Echo’s strobe pin is connected to the MCU, it can control on/off periods directly by toggling this pin.
When strobe is high, Echo will turn on and stay on. When strobe is low, Echo will turn off. If Echo
recognizes an ID during an on period, it will stay on regardless of the strobe pin and internal strobe
oscillator.
NOTE
Echo takes little time to wake up out of standby before it is possible to
receive a message. This time is typically 1.2 ms.
More detailed information on the operation of the strobe oscillator and strobe pin can be found in the Echo
data sheet. In particular, the section titled ‘Receiver On/Off Control’ explains detailed timing information.
The state machine descriptions show the interaction of the strobe oscillator and strobe pin on Echo’s state.
5.1
Consequences on Message Format
If the power-saving features of Echo are used when receiving a message, a message can begin while Echo
is in standby mode. To avoid this, the ID must be repeated long enough to cover at least two of Echo’s ON
periods. Both message formats described in Section 3, “Message Format,” can repeat the ID over a period
of time to allow Echo time to wake up. Figure 3 illustrates the required formats.
Software Drivers for MC33696, Rev. 1
Freescale Semiconductor
5
FREESCALE [ Freescale ]
