Theory Of Operation; Glass Break Detector Theory Of Operation; Analog Front End Circuit Of Glass Break Detector - Renesas RL78/I1D User Manual

DETECT-IT RL78/I1D detector boards kit
6.

Theory of operation

Glass Break detector theory of operation

6.1.1

Analog Front End circuit of Glass Break detector

The Analog Front End circuit consists of a microphone, low pass filter and high pass filter. To implement a glass break
sensor an Electret microphone monitors ambient sound so that sound amplitude, frequency band and sound duration are
analyzed to distinguish between ordinary sharp/loud sounds versus actual glass break events. Any sudden change in
sound level will have some low frequency (envelope) content or "thud" event, even if the event mainly has high
frequency components like breaking glass. Generally a true glass break event will have high frequency sound
components greater than 1 KHz, and at least 100ms duration.
• Low pass filter with comparator interrupt
At MCU power startup, the low pass filter operational amplifiers are initialized and biased at VDD/2, where VDD
depends on the CR123 lithium battery voltage (2.7V to ~3.1V). With no sound events, both low pass operational
amplifier stage outputs will be at VDD/2 (1.35V to 1.55V). Even though this audio filter is referred to as "Low
Pass/Low Frequency" there is AC coupling between the microphone and first stage of low pass filter, and AC coupling
between the first stage output (Operational amplifier Ch1) and the second stage input (Operational amplifier Ch0).
Therefore any DC bias variations are eliminated between operational amplifier analog front end stages, and besides, the
microphone will not reproduce sounds much below 25Hz frequency.
The output of low pass second stage (Operational amplifier Ch0) is DC coupled into the Comparator input and ADC
input of the MCU (since they connected internally on the same MCU pin 39). For a sufficiently loud sound event, the
RL78/I1D MCU internal comparator triggers an MCU interrupt (when IVREF voltage threshold is exceeded), which
wakes up the MCU out of STOP mode.
If a Low Frequency amplitude output from AMP0O triggers the Comparator 0 window interrupt, then the AMP2 and
AMP3 operational amplifiers are turned on in High Speed mode. At this point, further Operational amplifier AMP0
output and Comparator 0 interrupts are ignored. After 2ms stabilization wait time, Comparator 1 windowed interrupt is
enabled and a 200ms time interval is started. If the Comparator 1 window interrupt occurs before 200ms timeout, then
the Glass Break alarm is sounded. If no Comparator 1 window interrupt occurs before 200ms, then no Glass Break is
sounded and the MCU configures the AMP1 and AMP0 operational amplifiers back to Low Power consumption mode
and turns off AMP2 and AMP3 off. Then MCU returns to STOP mode, waiting for next loud sound event.
Note: All four RL78/I1D operational amplifiers must either be placed in low power or high speed mode together.
Therefore, the low pass filter (Operational amplifier channels Ch0 and Ch1) are ignored when the high pass
filter is enabled.
• High Pass Filter:
Since the high pass filter needs both high gain and high frequency response up to ~15kHz range, the high speed
operational amplifier mode is needed (with GBW = 1.7MHz). The high pass filter is not powered on until after a loud
sound event occurs, since high speed operational amplifier mode draws ~260µA for 2 operational amplifier channels.
After enabling the 2 operational amplifiers channels in high pass filter, approximately 1ms of stabilization time is
required. The high pass filter second stage output (Operational amplifier Ch3) is also DC coupled into a second ADC
input channel and that ADC input is sampled for 100ms after first low frequency sound event is detected. If a minimum
of 100ms of high frequency activity is detected, the glass break sensor alarm is sounded for a minimum of 2 seconds.
After sampling, the high pass filter and 100ms have elapsed with < 100ms high frequency detection the high pass filter
is turned off and the low pass filter operational amplifiers are turned on again. Otherwise if the >100ms threshold for
high frequency content is exceeded (and alarm enabled), the high pass filter can be turned off and low pass filter turned
back on (operational amplifier in low speed mode), to look for a new sound event.
Note: all four RL78/I1D operational amplifiers must either be placed in low power or high speed mode together.
Therefore, the low pass filter outputs are ignored when the high pass filter stages are enabled.
As summary, here is the method how the Glass Break detector works generally:
UM-YDETECT-IT-RL78 V1.30
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