DVK-BTM521 Laird Technologies, DVK-BTM521 Datasheet - Page 94

• Manufacturer Part Number: DVK-BTM521
• Description: BT MM DEV KIT
• Manufacturer: Laird Technologies
• Type: Transceiver, Bluetoothr

Specifications of DVK-BTM521

• Frequency: 2.4GHz
• Interface Type: RS-232
• Processor Series: BTM521
• Silicon Manufacturer: Laird Technologies
• Kit Application Type: Communication & Networking
• Application Sub Type: Bluetooth
• Kit Contents: Development Board & Software Tools
• Features: Supports CSR
• Rohs Compliant: Yes
• For Use With/related Products: BTM521
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant
• Other names: DVK-BTM521

BTM520/521
Bluetooth
AT COMMAND SET
REFERENCE
94 www.lairdtech.com
®
Multimedia Plus Module
9. Sniff Mode
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].
S register 510 defines the inquiry scan interval in ms, range is [11..2250].
S register 511 defines the inquiry scan window in ms, range is [11..2250].
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 five 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 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.
It is stated above that during sniff mode, a slave listens for N slots every M slots. The Bluetooth
specification states that a master can have up to 7 slaves attached to it with all slaves having requested
varying sniff parameters. It may therefore be impossible to guarantee that each slave gets the M
parameter it requested. In light of this, the protocol for enabling sniff mode specifies that a requesting
peer specify the M parameter as a minimum and maximum value. This will allow the master to
interleave the sniff modes for all slaves attached.
For this reason, the sniff parameters are specified in the BTM module via four S registers. S Register
561 is used to specify ‘N’, S Register 562 is used to specify ‘T’ and S Registers 563/564 are used to
specify minimum ‘M’ and maximum ‘M’ respectively. Although the specification defines these
parameters in terms of timeslots, the S register values have to be specified in units of milliseconds
and the firmware does the necessary translation to timeslots.
Figure 3.28: Page and Inquiry Scan Intervals and Windows
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