www.ezurio.com
DSH_BT024-00200_1v4 © Ezurio Ltd 2006
13
8.
Low Power Modes
The current drain from the Vcc power input line is dependent on various factors. The three most significant factors
are the voltage level at Vcc, UART baud rate and the operating mode.
The hardware specification for the Embedded Intelligent
Bluetooth
Module allows for a voltage range of 3.3 to
7.0v at Vcc. Tests have shown that there is no significant difference in current draw when Vcc is 5 or 6V.
The UART baud rate has a bearing on power drain because as is normal for digital electronics, the power
requirements increase linearly with increasing clocking frequencies. Hence higher baud rates result in a higher
current drain.
Finally with regards to operating mode the significant modes are; idle, waiting for a connection, inquiring,
initiating a connection and connected. With connected mode, it is also relevant to differentiate between no data
being transferred and when data is being transferred at the maximum rate possible.
The operating mode can best be described by stating the AT commands required to enter that mode. In addition,
there are certain S Registers which have a direct impact on power consumption, which are described next.
The Embedded Intelligent
Bluetooth
Module has provision for an LED which can be configured to display
connection status. Tests have shown that this LED can consume up to 5.3mA which is more than double the
current draw when in Idle mode. S Register 534 can be used to completely disable this indicator.
S Registers 508 to 511, which specify the page and inquiry scan intervals and windows, can be used to adjust the
average current drain when in discoverable and or connectable modes. Registers 508 and 509 specify the interval
and window for page scans and registers 510 and 511 specify the interval and window for inquiry scans. Register
pairs 508/509 and 510/511 describe duty cycles when the module goes into scan modes. It is while scanning that
the highest current draw occurs. The average current draw is determined by simple arithmetic using the values
stored in the 508/509 and 510/511 register pairs.
The current drain while waiting for a connection or discoverable mode is about 30 times higher than in idle mode.
This is when the page/inquiry scan duty cycle is 100%. These modes give the quickest response to a page or
inquiry request from a remote peer.
It is possible to reduce the duty cycle down to as low as 0.5% at the expense of response time. The response
time can be specified via S Registers 508 and 510 for page and inquiry respectively, where the worst case
response time can be as high as 2.5 seconds. Then the duty cycle can be varied by changing the value of S
Registers 509 and 511 appropriately.
For example, if S Register 508 and 510 are both set to 1000ms and S Register 509 and 511 are both set to 11ms
then the duty cycle is reduced to 1%, this means that average current drain at 5.0v will be 2% of 65mA plus the
normal idle mode current, that is, it is as low as 2.75mA. However, in this case, it can take up to 1 second to
establish a connection.
The connected state current consumption while a master or slave can be considerably reduced by enabling Sniff
mode, described in detail in the next section.
8.1
Low Power Modes using Sniff
Bluetooth
connections are master/slave in nature. A master sends packets and a slave has to acknowledge that
packet in the next timeslot. Timeslots in
Bluetooth
are 625 microseconds wide. This implies that a master will
always know when packets will be sent and received, which further means it is able to optimise power usage by
switching on power hungry circuitry only when needed.
A slave on the other hand does NOT have prior knowledge of when a packet will be received and has to assume
that a packet will be received from a master on every receive slot. This means that it has to leave its receiving
circuitry on for most of the receive slot duration. The result of this is high power consumption, where a slave with
no data transmission still consumes around 31mA whereas a master consumes only 6mA.
This problem was identified very early in the evolution of
Bluetooth
(especially since headsets spend all their time
as a slave in a
Bluetooth
connection) and it was solved by having a mode called Sniff, with appropriate lower
layer negotiating protocol.
Sniff mode during connection is basically an agreement between the slave and its master that data packets will
only be exchanged for N timeslots every M slots. The slave can then assume that it will never be contacted during
mA
3.3V
3.20
Current per LED (when fitted)
5.0V
5.30