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DSH_BT024-00200_1v4 © Ezurio Ltd 2006
14
N-M 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.
This is illustrated in the diagram below.
For this reason, the sniff parameters are specified in the Embedded Intelligent
Bluetooth
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.
D
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High Power Consumption
Low Power Consumption
M
Slots (Negotiated)
M
Slots (Negotiated)
N
Slots
N
Slots
N
Slots
T
Slots
T
Slots
T
Slots
T
Slots
T
Slots
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D
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D
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9.
Application Examples
9.1
RS232 Modem Signals
Just as a telephony modem has control and status lines, the module also provides for 6 control and status lines as
per the table below. The direction column is as seen from the module’s viewpoint.
Direction
Function
IN or OUT *
CI also known as RI (Ring Indicate)
IN or OUT *
DCD (Data Carrier Detect)
IN
DSR (Data Set ready)
OUT
DTR (Data Terminal Ready)
IN
CTS (Clear to Send)
OUT
RTS (Request to Send)
* configurable with S register 552