Smart meters in a smart grid have the ability to measure, control and communicate in one single device. For those battery powered smart devices like flow meters, electrical energy is precious to keeping them “smart”. Having a smarter design to lower power consumption can keep them alive for a longer time.
The extended scan interface (ESI) is a module for the ultra-low-power MSP430FR69X microcontroller (MCU) which consists of multiple sub-modules, such as analog front ends and different state machines. Those modules automate sensor measurements without CPU intervention to minimize power consumption of the system.
Sensor circuit design also takes a big role to reduce system power consumption. Many sensors have an enable pin for low power operation. The enable pin is typically controlled by the MCU. If the measurement requires high sample rate, the MCU needs to wake up often, which increases power consumption.
The excitation circuit and the sampling and hold circuit of the ESI have the characteristic that the ESICHx pins or the ESICOM pin can be configured to connect to the ground during measurements. The connection is controlled by the timing state machine of the ESI. The ESICHx pins or the ESICOM pin can be used as enable pins to activate the sensors without MCU intervention. Therefore, the MCU hands-off the measurement process completely to the ESI and goes into sleep mode for saving power.
Figure 1 - Sensor signal path and the pins that can be act as enable pins
The TI Water Meter Reference Design for GMR Sensors, using extended scan interface (ESI) (TIDM-GMR-WATERMTR) shows an example to reduce the power consumption of the GMR sensor circuit. GMR sensors are typically connected to the power supply and the ground of the system. This connection causes a constant current flow that wastes energy as the measurement only takes a very short period of time. By connecting the GMR sensor to the ESICOM instead of system ground, the ESICOM acts as an enable pin. The ESICOM connects to the ground to turn on the GMR sensor only when the measurement is in process. When the system is idle, the ESICOM disconnects the ground so that the GMR sensor consumes zero power.
The TI Water Meter Reference Design for Optical Sensors, Using Extended Scan Interface (ESI) (TIDM-OPTICALWATERMTR) shows an example to use ESICHx pins as enable pins. This configuration enables multiple sensors to turn on individually during the measurement. If higher current is required to drive the sensor, a simple transistor circuit can be used to support the sensor.
Figure 2 - EN signal path
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