BTM410/411
Bluetooth
®
AT Data Module
45
www.lairdtech.com
Laird Technologies
AT COMMAND SET
REFERENCE
6.
Page Scan / Inquiry Scan Interval and Window
Page scanning means being connectable. Inquiry scanning means being discoverable. With the
following S registers the power consumption of the BTM can be influenced. However, lower power
consumption means longer connection establishment time and longer time until a BTM is
discovered by other devices.
The page scan window defines the time for the module to look out for incoming connection
requests (paging). The inquiry scan window defines the time for the module to look out for
incoming inquiry requests (device discovery). If the module is both connectable and discoverable
(512=4 or AT+BTP issued), it will mutually do page scanning and inquiry scanning as shown in Figure
5. If connectable only, the module will perform page scanning only (repeatedly) and if discoverable
only, then the module will perform page scanning only.
S register 508 defines the page scan interval in ms, range is [11..2250].
S register 509 defines the page scan window in ms, range is [11..2250].
7.
Sniff Mode
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 optimize 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 on slave side. In general, a slave draws about 5 times the current of a master.
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 M-N slots, and so can switch its power hungry circuitry off. The
specification goes further by also specifying a third parameter called ‘timeout’ (T) which specifies
‘extra’ timeslots that the slave will agree to listen for after receiving a valid data packet. Put another
way, if a data packet is received by the slave, then it knows that it MUST carry on listening for at
least T more slots. If within that T slot time period another data packet is received, then the timer is
restarted. This mechanism ensures low power consumption when there is no data transfer – at the
expense of latency. When there is a lot of data to be transferred, it acts as if sniff mode were not
enabled.
Figure 5: Page and Inquiry Scan Intervals and Windows
Summary of Contents for BTM410
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