Milestone 5 has been accomplished!
OpenEPT revision 1 PCB has been prepared and dispatched for manufacturing. In this blog post, we aim to outline implementation details and present overall hardware functionalities.
Figure 1 - OpenEPT revision 1 hardware 3d representation
OpenEPT hardware summary of features:
Board designed for LiPo one-cell battery use
Energy profiling of MCU simulated load (up to 3.3 A)
Energy profiling of external 5 V load (up to 1 A)
Energy profiling of external 3.3V load (up to 1 A)
Energy profiling of external LiPo battery voltage load (up to 3.3 A)
Overvoltage protection (over 4.5 V)
Overcurrent protection (current consumption over 3.3 A)
Reverse polarity protection
Battery indication circuit
Overvoltage indication
Overcurrent indication
Reverse Polarity indication
User RGB led
Shut down unused circuits through configurable jumpers, or MCU`s GPIOs
Hardware description
The picture below represents a block diagram of the OpenEPT rev 1 board. The board is designed to fit into EEZ DIB chassis, which specifications can be found here. Besides that, the board is designed to be used as a standalone solution. The overall hardware block design is illustrated in Figure 2.
Figure 2 - Overall hardware block diagram
Power supply circuit
Input Block provides a board with LiPo battery voltage, as well as User input from where the user can manually disable/enable some parts of the circuit that are not meant to be used for design. For example, the user can disable hardware blocks for 3V3 and 5V output voltage if those voltages are not meant to be used. User inputs can be controlled manually through jumpers, or with MCU through GPIO`s.
Necessary voltages which will be used in the design are taken from EEZ DIB BUS, with OpenEPT rev 1 board only voltages of 12 V, 5 V, and GND are used. The picture below represents a block diagram that converts voltages from EEZ DIB BUS to power supply voltages which the board will use. A negative voltage is gained through the charge pump circuit and is necessary because not all components are rail to rail.
Figure 3 - Power supply logic
Protection circuit
OpenEPT rev1 board supports board and load protection with reverse polarity, overvoltage, and overcurrent protection. The signal for overcurrent protection is generated from the current sensing circuit, where the current limit through the shunt is set to 3.3 A. An overvoltage protection circuit will trigger if the input voltage (VBAT) is more than 4.5 V. Protection circuits do not leak any significant current when conditions for protection are not met. Each type of protection will signal its activation through red LEDs, where there is also a green LED for battery present indication. The block scheme of protection and indication circuits is shown in the Figure 4.
Figure 4 - Protection circuit logic
Sensing circuit
Current consumption is measured through a shunt resistor and a high-side current sense amplifier circuit. The design takes into consideration that this current will be bidirectional in the future, and for that purpose, INA296A1 current sense amplifier is used in a configuration where if there is no current, the output voltage of the amplifier will be 1.65 V. Voltage which represents current consumption, and battery voltage are then fed to MCU`s ADC. The block diagram which shows these features is shown in the picture below.
Figure 5 - Sensing logic
Output voltages for MCU platforms
OpenEPT rev1 board also provides necessary output voltages that external devices can use. The board supports outputs of 5 V, 3.3 V, and VBAT where the user can attach loads that need energy profiling. Users can also shut down voltages that are not intended to be used, either through MCU GPIO`s or by setting User Inputs (jumpers) correctly. The block diagram for output voltages is shown in Figure 6.
Figure 6 - Output voltages