2 Demo Hardware

The block diagram for a demo board based on the TPS8802 and LP-MSPM0L1306 is shown in Figure 1-1. This demo board includes a coin cell CO sensor, a basic 3D printed smoke chamber and a 3 terminal piezo element for alarm notification as well as commonly found functions such as status LED’s and push-to-test / silence button. The hardware board also includes an LMT84 for ambient air temperature measurements which can be used in combination with the internal temperature sensor of the MSPM0L1306 on the launchpad for rudimentary heat detection. In the simplest use case of the demo hardware, power is applied using 2 AA batteries and the launchpad is plugged into a PC using a USB cable for data capture or display using the GUI described in this report.

Although the basic 3D printed smoke chamber is designed for multi-wavelength forward scatter only measurements, the PCB includes alternative locations for LED’s and PD’s as shown in Figure 2-1. These alternate locations allow, but is not limited to the following chamber designs: forward scatter only (single or multi-wavelength), multi-angle (single or multi-wavelength) with 3 LED’s and 1 PD or 2 LED’s + 2 PD’s. In this way, customers can use this hardware together with their own chamber and optical design or evaluate multiple designs.

Figure 2-1 PCB Locations and Angular Information for Alternate LED and PD Configurations

Figure 2-2 and Figure 2-3 show the details of the photoelectric front-end implementation for the LED driver and the 2-stage trans-impedance amplifier receiver, respectively. The demo software and GUI allow modifications on the fly for the LED drive strength and temperature coefficient setting. The LED drive strength is controlled with an 8-bit DAC using the PDACx registers in the TPS8802. The CSA and CSB pins are connected to ADC channels on the MCU with the results shown in the GUI. Using this information together with the sliders found in the GUI for the LED DAC settings, instant feedback for the actual LED drive strength based on the temperature coefficient setting, external current setting resistor, and the programmed LED DAC value is received. Similarly, for the trans-impedance amplifier, the PGAIN register can be modified on the fly with the GUI. Separate PGAIN settings are provided for each LED so that the receiver gain is different depending on which LED is being flashed. This is necessary to tune the response for different wavelength LED’s or LED’s with different scattering angle configuration. The input amplifier stage of the TPS8802 is an ultra-low input offset (voltage and current), wide bandwidth design, designed for amplifying low-level photo diode currents. With the photoelectric front-end fully configurable with software, different configurations and optimizations in hardware can be measured and analyzed with ease. Lastly, since the TPS8802 does not include the ADC, the analog to digital conversion takes place in the MCU where signal processing can take place or be immediately stored instead of using an additional serial interface and delays to get the data from the TPS8802. This saves both power and cost for the sub-system. In this demo, the photoelectric receiver is sampled 5 times by the ADC for each LED flash. These ADC samples are averaged to reduce noise of the measurement.

Figure 2-2 TPS8802 LED Driver

Figure 2-3 TPS8802 Photoelectric Receiver Trans-impedance Amplifier

Additionally, the TPS8802 integrates a boost converter, analog supply LDO, microcontroller supply LDO, photoelectric chamber analog front end (AFE), carbon monoxide sensor AFE, interconnect driver, piezo horn driver, analog multiplexer, and digital core. The high integration greatly reduces component count in smoke alarms and carbon monoxide alarms. The TPS8802 can be powered from a variety of sources:

• 9-V battery
• 3-V battery
• 2-V to 15-V DC supply
• DC supply with battery backup

The two LED drivers have highly configurable temperature compensation to support IR and blue LEDs over a wide range of currents. The wide bandwidth of the photo-amplifier saves power due to reduced LED on-time. The CO amplifier has integrated gain resistors. The horn driver is compatible with two-terminal or three-terminal piezo horns, and the three-terminal self-resonant mode is tunable to maximize piezo loudness. The wired interconnection driver allows multiple smoke alarm units to communicate alarm conditions. Each block is highly configurable with the digital core I2C interface, supporting on-the-fly adjustment of amplifier gains, regulator voltages, and driver currents. Digital features such as sleep mode, under-voltage boost enabling, and one-time boost charging are designed to reduce power consumption for the 10-year battery alarms. Configurable status and interrupt signal registers alert the MCU of fault conditions such as under-voltage, over-temperature, and interconnection alerts.

The MSPM0 microcontrollers are part of MSP's highly-integrated, ultra-low-power 32-bit MSPM0 MCU family based on the enhanced Arm® Cortex®-M0+ core platform operating at up to 32-MHz frequency. These cost-optimized MCUs offer high-performance analog peripheral integration, support extended temperature ranges from -40°C to 125°C, and operate with supply voltages ranging from 1.62 V to 3.6 V. The MSPM0L devices provide up to 64KB embedded flash program memory with up to 4KB SRAM. These MCUs incorporate a high-speed on-chip oscillator with an accuracy up to ±1.2%, eliminating the need for an external crystal. Additional features include a 3-channel DMA, 16- and 32-bit CRC accelerator, and a variety of high-performance analog peripherals important for use in smoke detector applications such as:

• One 12-bit 1.68-MSPS ADC with configurable internal voltage reference (1.4V or 2.5V)
• One high-speed comparator with built-in reference DAC
• Two zero-drift zero-crossover operational amplifiers with programmable gain
• One general-purpose amplifier
• An on-chip temperature sensor for use as part of heat detection or for compensation of LED current strength and photo diode temperature drift.

These devices also offer intelligent digital peripherals such as four 16-bit general purpose timers, one windowed watchdog timer, and a variety of communication peripherals including two UARTs, one SPI, and two I²Cs.