By Chelsea Mediavilla (Electronics and Control)

This trade-off study compares the performance and features of two microcontrollers. The cost, power and communicative capabilities of each MCU is considered.

Introduction

The main objective of the Spring 2016 SPARCCS project is to design a printed circuit board containing a dosimeter and single event upset (SEU) monitor. The microcontroller (MCU) chosen for this mission must be cable of fulfilling this objective while satisfying the requirements outlined by the CubeSat Project. In this study microcontrollers will be compared in order to determine a suitable choice for the satellite. The microcontrollers of interest are the ATmega328P and ATmega32U4.

Requirements

The most pressing requirements to consider are the cost, power consumption, and communication capabilities of the microcontroller. Due to the SPARCCS team’s budget of $250, it is essential that the SPARCCS team be as cost efficient as possible. A cheaper MCU would be preferable provided that it satisfies the project’s other needs. The chosen controller must be able to run off of the 5 volts the board receives through the PC 104. Hardware and software UART must also be supported by the chip in order to allow communication with the SEU monitor and the Command and Data Handling unit of the satellite.Though not required, the final printed circuit board would ideally be capable of uploading Arduino sketches via USB. USB connections allows for easier access to the code, as opposed to the less common DB-9 ports.

ATmega328P

Atmel’s ATmega328P microcontroller is well-known in Do It Yourself projects. The chip can be found in various Arduinos, the Fall 2015 SPARCCS project, and Spark Fun’s own dosimeter, the Spark Fun Geiger Counter. This microcontroller has gained popularity due to its numerous features, low power, and low cost. The ATmega328P requires 1.8 to 5.5V in order to operate. This allows for operation at the 5 volts allocated by the CubeSat Project. According to the DigiKey website, a single chip is priced at about $3.38; well within our budget. Additionally, UART is supported in this controller, allowing necessary communication with other units in the satellite. The chip, however, does have more pins than necessary for a Geiger tube and SEU monitor. With a pin count of 32, a large portion of the controller will go unused. The ATmega328P also does not support USB connection and thus requires additional circuitry or the purchase of a USB interface.

ATmega32U4

The second microcontroller was suggested by the CubeSat Project and functions in a similar manner to the ATmega328p. The ATmega32U4 supports UART and the 2.7 to 5.5 operating voltage ensures functionality at 5 volts. It is worth noting, however, that this microcontroller has a higher minimum voltage for operation. Low power states and power saving opportunities are thus limited when compared to the ATmega328P. Other differences from the 328P include a higher cost and an increase in pin count. The 44 pin ATmega32U4 is priced at $6.50; over twice as much as the 328P. However, this more expensive microcontroller also contains a built in USB interface, thus eliminating the need for extra hardware for USB connection.

Conclusion

The Spring 2016 SPARCCS team requires a microcontroller capable of controlling two instruments, operating at 5 volts, and communicating through UART and USB. Both microcontrollers satisfy the power and UART requirements, however only the ATmega32U4 includes a built in USB interface. Using the ATmega 328P would require an extra USB chip to connect the USB port to the microcontroller. Because the ATmega 32U4 contains all of the capabilities of the 328P and allows USB connection, the increase in cost is negligible. Therefore, the ATmega 32U4 shall be used in the Spring 2016 SPARCCS project.

 

Requirements Table

	ATmega 32U4	ATmega 328P
Cost	$6.50	$3.70
Operating voltage	2.7-55 volts	1.8-5.5 volts
USB compatible (for programming sensors)	Yes	USB interface required
Supports UART	Yes	Yes