HMR3100
Advanced Information SENSOR PRODUCTS
Solid State Electronics Center • www.magneticsensors.com • (800) 323-8295 • Page 3
Circuit Description
The HMR3100 Digital Compass Solution circuit board includes the basic magnetic sensors and electronics to provide
a digital indication of heading. The HMR3100 has a Honeywell HMC1022 two-axis magnetic sensor on board. The
HMR3100 allows uses to derive compassing (heading) measurements when the board is in a horizontal (flat)
position.
The HMR3100 circuit starts with the HMC1022 two-axis magnetic sensors providing X and Y axis magnetic sensing
of the earth’s field. These sensors are supplied power by a switching transistor to conserve power with battery
operated products. The sensor output voltages are provided to a dual operational amplifier and then to analog to
digital converters (ADC) onboard a microcontroller (uC) integrated circuit. The microcontroller integrated circuit
periodically samples the amplified sensor voltages, performs the offset corrections, and computes the heading. This
microcontroller also performs the external serial data interface and other housekeeping functions such as the
calibration routine.
The power supply for the HMR3100 circuit board is to be about a +3 to +5 volt range allowing the user to provide a
single lithium battery to logic level supply voltages. The power supply architecture is a single ground system for
single ended supply sources (+ and ground return)
USART Communication Protocol
HMR3100 module communicates through binary data and ASCII characters at four selectable baud rates of 2400,
4800, 9600, or 19200. The default data bit format is USART 9600.N.8.1. The baud rate selection is determined by
the position of jumpers J1 and J3. These jumpers are zero ohm SMT resistors (jumpers) and are normally high (logic
1) when removed, and grounded (logic 0) when in place. At 2400 baud, no jumpers are present for a 1,1 logic
presentation. At 4800 baud J3 is present for a 1,0 logic presentation. The factory default setting of 9600 baud is
created by a jumper present on J1 for a 0,1 logic presentation. With J1 and J3 jumpers present for a 0,0 logic
presentation, the compass module works at 19200 baud. See Figure 5 for jumper locations. Jumper J2 is for factory
testing, and J4 is for X-axis inverse should the end-item mount the HMR3100 module upside down.
The HMR3100 is ready to output heading information when the board is powered up. Note that when the device is
first powered up, it may need calibration. The HMR3100 has three modes of operation described as normal,
continuous, and calibration.
