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.
5.4 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 it’s
receiving circuitry on for most of the receive slot duration. The result of
this is high power consumption as illustrated in the power table above,
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
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