RFXCOM RFXtrx433 Especificaciones descargar pdf (Pagina 50)

Chapter 4. Implementation 43

result, not all data can be obtained and a compromise must be made, whether weak or strong

signals are preferred. In this work, the settings were adjusted for medium range, allowing sense

devices up to 50 cm to the gateway as well as enabling long ranges up to 10 m (indoor). Devices

that are switched within this 50 cm range can be detected, but reliable detection is not guaranteed.

Another disadvantage of the dynamic reference voltage is the ambient noise during intermissions.

The integrator then lowers the reference voltage to its detection threshold, which is only slightly

above the noise level of the input. As a result, the output starts rustling and causes a permanent

processor load, as the processing algorithm is called continuously.

lowpass

gain

integrator

comp

rising time

falling time

Figure 4.8.: The simpliﬁed internal structure of the RFM22B in receiving mode: Incoming signals

are ampliﬁed at ﬁrst and then ﬁltered by a low-pass ﬁlter. A comparator compares the

signal with a reference voltage from an integrator and outputs a binary data stream.

The integrator circuitry is also called automatic gain control (AGC) as it generates a

dynamic reference voltage to increase the overall sensitivity.

The second part of the signal chain is the deglitching algorithm: It is an advanced ﬁltering

algorithm for removing errors in the data stream provided by the RFM22B. Figure 4.9 on the next

page shows a part of the data stream of a HE307EU switch.

The deglitching algorithm makes use of one peculiarity of noise: its pulses are shorter than the

ones of our data stream. Therefore, the deglitching algorithm ﬁlters out all pulses that are shorter

than a deﬁned minimum value. Listing 4.1 on the following page shows its code. The deglitching

function is realized as a pin-change interrupt and is called at every rising edge of the rx-clock

signal. At ﬁrst, it fetches the actual value of the RECEIVE_PIN and shifts it in the variable

stream. The number of 1 in a row is counted and if a 0 interrupts this sequel, count is evaluated:

If count is smaller than a minimum number, the pulse was too short and therefore the 1 sequel

is replaced with 0s. Otherwise, the pulse was long enough to be not ﬁltered out and therefore

remains unchanged. The most signiﬁcant bit of stream is monitored for changes, which represent

edges in received signal and outputted to the processing block. The deglitching function is called

approximately 38,000 times per seconds, therefore it is optimized for speed and kept very short.

Nevertheless, each call takes almost 30 CPU cycles, which equals a 4.5% continuous processor

load.

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RFXCOM RFXtrx433 Especificaciones Pagina 50