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

DETECT-IT RL78/I1D detector boards kit
• Medium speed On-Chip Oscillator (4 MHz MOCO) and special Low Power CPU run mode:
The usage of this Low Power mode and this 4MHz MOCO oscillator allow both a fast (4µs typical) wake-up time and
low current drain (134µA @ 1MHz). Since CPU performance is usually not critical for sensor applications, a lower
speed native clock at 1MHz/2MHz/4MHz can be used instead of the High Speed On-Chip Oscillator (24MHz HOCO).
However, the present demonstration uses Low Speed MCU mode @ 8MHZ system clock speed.
• PCLBUZ1 buzzer output:
When CO alarm threshold is achieved, the PCLBUZ1 output is enabled

Motion detector theory of operation

6.3.1

Analog Front End circuit of Motion Detector

The Analog Front end circuit consists of the Pyroelectric Infrared (PIR) sensor, and 2 stages of AC-coupled amplifier.
• PIR sensor
The muRata part number IRA-E700STO PIR sensor has typical peak-to-peak output levels in the 1mV-10mV range for
detected IR sources. To make these small AC voltage changes strong enough to be measured by the MCU, while
rejecting ambient Infrared light levels, a large gain is needed to amplify the 1mV-10mV typical pulses up to a 2-3Vpeak
to peak voltage range. Amplification is accomplished in 2 stages of operational amplifiers, as follows.
• First stage AC-coupled amplifier (Ch1 operational amplifier)
At MCU power startup, Ch0 and Ch1 operational amplifiers are initialized in Low Power mode. The first stage of AC-
coupled amplifier is non-inverting and amplifies the PIR output pulse approximately x101. In low power mode, this
operational amplifier draws ~2.25µA quiescent current typically. For example, a +5mV peak pulse from the PIR sensor
output will result in a ~+0.505V pulse at the output of AMP1O. The timing of the first stage pulse is governed by the
R65 (10kΩ) and C56 (22µF) time constant, coupled thru the R91 (1MΩ) and C57 (10nF) RC network.
• Second stage AC-coupled amplifier (Ch0 operational amplifier)
The second stage has an adjustable gain from x6.7 to x33.9 and is inverting. Since the second stage is AC-coupled, it is
un-affected by any DC bias variation in the PIR sensor or the first stage amplifier. However, the second stage positive
(+) input is biased at VDD/2, at the midpoint of power supply voltage, so the AMP0O output is also VDD/2 at
quiescent condition with no PIR output change (no pulsing). This bias network is composed of two 4.75MΩ resistors
(R64 and R68), and therefore only adds about 0.35µA current drain to the battery load.
With the example of a +5mV PIR sensor pulse, and +0.505V pulse coming from first stage, the second stage output
should go to the VSS (0V) rail and momentarily saturate there. Even a short pulse from the PIR sensor will set up a low
and high-going excursion on Ch0 (AMP0O) output lasting for about 1 second. This ~1second settling time is due to RC
time constants on both the first stage and the second stage. After receiving an input pulse, the timing of second stage
settling is governed by the R89 (343kΩ) and C55 (22µF) time constant, coupled thru the JP9 potentiometer (0Ω to
900kΩ) + R90 (220kΩ) and C57 (22nF) RC network.
• Comparator input ICOMP0
There are two comparator reference voltages tied to both comparator pairs – IVREF0 for the high reference and
IVREF1 for the low reference. ICOMP0 is tied to a comparator pair. This configuration allows the ICOMP0 input to
trigger an MCU (wakeup) interrupt on either :
 single threshold ICOMP0 > IVREF0 only,
 or (2) single threshold ICOMP0 < IVREF1 only,
 or (3) the "window" mode which triggers on either threshold condition (1) or (2).
The Motion sensor takes advantage of the window mode, where the MCU interrupt is triggered at ~0.67 x VDD for
positive going threshold and ~0.3x VDD for negative going threshold. These threshold values are set using 2
potentiometer settings (JP11 and JP10). When the AMP0O output has excursions triggering these threshold values, the
Motion sensor alarm will sound as long as the ICOMP0 interrupts keep occurring. ICOMP1 has an identical function
on 2 additional comparators, although ICOMP1 comparator pair is not used in the Motion sensor design. The
comparators in Low Speed mode with MCU voltage regulator @1.8V draws 2.2µA typical in Window interrupt mode.
• Comparator IVREF0 and IVREF1 voltage reference
There is a single resistor bias chain that sets both IVREF0 and IVREF1. The total resistance of this chain is about
2.3MΩ, so additional current consumed by this resistor chain is only ~1.4µA @3.3V supply. Setting of IVREF0 and
IVREF1 is explained in section.
Motion Sensor board setup/calibration below.
UM-YDETECT-IT-RL78 V1.30
Kit User Manual
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