Mcu/Digital System And Maintain Low Current Drain For Long Battery Life; Smoke Detector Theory Of Operation; Analog Front End Circuit Of Smoke Detector - Renesas RL78/I1D User Manual

DETECT-IT RL78/I1D detector boards kit
When using VDD = 3.4V from E1 emulator, please use following values:
IVREF0 = ~ 0.86V (0.25*VDD), IVREF1 = ~ 2.1V (0.62V*VDD)
6.3.2

MCU/digital system and maintain low current drain for long battery life

• Low power interval timers like Real Time Counter, 12-bit Interval Timer, 2 units of 8-bit Interval Timer:
Real Time Counter interrupt at 1 second rate is used to wake up MCU, and count 1sec intervals for alarm minimum
timing.
• ADC conversion with internal VBGR = 1.45V (nominal) reference voltage:
ADC unit is not used in the Motion sensor design since the Comparator Window trigger mode is used.
• 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 the Motion sensor threshold is triggered the PCLBUZ1 output is enabled for multiple durations of 1second ON
and 1second OFF, until the Ch0 (AMP0O) output is no longer triggering Comparator Ch0 interrupts.

Smoke detector theory of operation

6.4.1

Analog Front End circuit of Smoke detector

The Analog Front End Circuit consists of the Infrared emitter (LED), IR Photodiode detector, and 2 stages of DC-
coupled amplification
• Infrared emitter LED:
An ordinary Infrared LED, as typically used in an Infrared Remote Controller, is driven at about 8mA forward current
thru an NPN transistor controlled by the MCU. The high level block diagram- show at the beginning of this user manual
- omits the transistor driver for simplicity. This infrared LED is only enabled for ~200ms every 3 seconds sample
interval, so as to keep the average battery current low. The output angle of the infrared emitted beam is relatively
narrow at +/- 17.5 degrees and pointed away from the IR photodiode detector.
• Infrared photodiode (detector):
An Infrared Photodiode (and Infrared Emitter) is mounted inside an Optical smoke chamber, where the chamber is
made of non-reflective, black plastic with baffles to allow smoke/smoke particles to enter the chamber, but largely
blocks most external, ambient light from entering. The Infrared LED output is off axis from the Infrared photodiode.
Both the Infrared LED and Infrared photodiode are pointed towards the center of the Optical smoke chamber's circle,
with ~120 degrees angle between the emitter main beam and photodiode detector surface. With no reflecting smoke
particles, little of the IR emitted light is captured by the photodiode. With reflecting smoke particles the IR Photodiode
is able to "see" enough infrared light energy to provide avalanche current thru the reverse biased photodiode, and that
increased current is amplified by operational amplifier Ch1 (AMP1O). The position of the IR emitter and IR photodiode
is quite sensitive to allow a good operation of the detection.
• First stage DC-coupled trans- impedance amplifier (Ch1 operational amplifier):
At MCU power startup, Ch0 and Ch1 operational amplifiers are left disabled to conserve power. Every 3 seconds, the
MCU enables both channels in high speed mode and waits for the operational amplifiers to stabilize. Please refer to
timing diagram and waveforms in the figure below. At timeframe "A", Ch1 (and Ch0) operational amplifier is turned
on and stabilizes in less than 125µs. Stage 1 is a trans-impedance amplifier with a 1MΩ feedback resistor - configured
to amplify a small current, such as 0.1µA to 2µA to a proportional output voltage, by a factor of 1Million. For example,
a 0.2µA input would result in a 0.2V DC voltage increase on the operational amplifier Ch1 output.
UM-YDETECT-IT-RL78 V1.30
Kit User Manual
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