The design for this custom PCB was initially loosely based on the previous semester’s design. However, due to changes in power allocation and functionality of the PCB, the design has reverted to a state more like the Fall 2017 Sojourner’s design. This portion of the design handles only with the control of the motors. It includes 2 dual motor H-bridges (DRV8847), a GPIO Expander (TCA6408APWR), and an 8:1 Multiplexor (TCA9548APWR). Due to the limited number of analog pins on the top shield of the 3DoT board, I²C Communication Protocols were used to send and receive data from every IC used in the design. The previous design had used the DRV 8830 motor drivers, which utilized I²C addresses by setting pins A0 and A1 to high, low, or open.

Initially, the PCB was designed to run off the battery power supplied from the 3DoT board. Unfortunately, since the battery only supplied a max of about 3.7 V, a complicated boost circuit would have been necessary for our design. To circumvent the issue of implementing a boost, an external power source (2S 45C LiPo battery) was chosen instead. The new issue with the battery was the variance of the voltages; it could vary from 6.4V to 8.4V. The DRV 8830 motor drivers utilized a small range of voltages, around 3 to 6 V, which the driver could accept as inputs. Because of this, the DRV 8847 were implemented, rather than the DRV 8830. The advantage of using the new drivers is its wide range of voltage inputs, from 2.8V to 18 V. This is desirable for our new design, because it takes a lot of power to run all 6 motors and all 4 servos at once.

One of the disadvantages for using the DRV 8847 its need for analog inputs for each motor. While it’s compatible with I²C communication, it’s used for the PWM control of the motor, not the control for the direction. Because of this, the GPIO Expander was implemented. This allows us to control the motor driver inputs through I²C still.

While the control for each motor had been settled, there was still the issue of reading the hall-effect sensors for each  motor. Since there wasn’t enough pins to read 6 motors, there was a need to find a way to read the information. An 8:1 multiplexor was picked as the best option to streamline all the data into a single pin. To further reduce the number of pins needed, the use of I²C was taken advantage of once again; this granted the 3DoT the ability to control each motor and read data from each encoder through two signal lines from the I²C lines.