FXTH870xD
Sensors
Freescale Semiconductor, Inc.
115
13.3.1
Initial Time Interval
When generating an initial time interval the MCU loads the RFM interval generator variables and then goes into the STOP1 mode.
When the initial time interval ends the data in the RFM data buffer is automatically sent and the MCU will wake at the end of the
transmission. The initial time interval is made up of two components:
Eqn. 10
where:
t
INIT
= Total time interval before first frame is transmitted in ms
t
BASE
= Base time in ms;
5 ms not recommended
t
RAND
= Pseudo-random time in ms based on a Galois 7-bit LFSR
The components of this time are described in the following sections.
13.3.2
Interframe Time Intervals
When generating an interframe time interval the MCU loads the RFM interval generator variables and then goes to the STOP1
mode. When the interframe time interval ends the data in the RFM data buffer is automatically sent and the MCU will wake at the
end of the transmission. The interframe time interval is made up of three components:
Eqn. 11
where:
t
IFRM
= Total time interval between each transmitted frame in ms
t
BASE
= Base time in ms;
5 ms not recommended
t
FN
= Time adder in ms for frame number
t
RAND
= Pseudo-random time in ms based on a Galois 7-bit LFSR
The components of this time are described in the following sections.
13.3.3
Base Time Interval
The base time interval, t
BASE
, is used in the initial time interval and in datagram transmissions with two or more frames. The
programmable frame space interval is based on a simple 8-bit, count-down timer as described by the RFBT[7:0] control bits in
the RFCR4 register. This time interval is forced to zero when the RFBT[7:0] are all clear.
The range of the base time must be set to 0 or between 5 and 255 ms using a clock generated from the MFO divided by 125.
13.3.4
Pseudo-Random Time Interval
The pseudo-random time interval, t
RAND
, is used both in the initial and the interframe time intervals if the LFSR[6;0] bits are set
to something other than all zeros. When the ISPC bit is set the pseudo-random initial time interval before the first data frame will
be 40 times the value of t
RAND
.
When the LFSR[6:0] bits are used the t
RAND
time will vary based on a pseudo-random generated binary number using a Galois
linear feedback shift register (LFSR) implemented using the primitive polynomial for a 7-stage register as shown in
Figure 93
.
This LFSR creates a sequence of 127 binary numbers including $01 through $3F which are each repeated only once in each
sequence of 127 clocks of the shift register. The LFSR is initialized to $40 during power up of the device. When a random interval
is to be determined the contents of the LFSR are sampled as the “random number” for calculating the required interval time.
Following the use of the random interval the LFSR is clocked once to advance it to the next pseudo-random number.
The range of the pseudo-random time is 1 to 127 ms using a clock generated from the MFO divided by 125. The current value
of the LFSR can be changed and/or read by the LFSR[6:0] bits in the RFCR5 register.
t
INIT
t
BASE
40
t
RAND
+
=
t
IFRM
t
BASE
t
RAND
t
FN
+
+
=
Содержание FXTH870 D Series
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