Hmr3100 – Honeywell HMR3100 User Manual

Page 4

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Solid State Electronics Center (800) 323-8295 Page 4

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 1 for jumper locations. Jumper J2 is for factory
testing, and J4 is for Y-axis inversion should the end-item mount of the HMR3100 module be upside down (pins up).

The HMR3100 sends data via the TXD line (Pin 5) in standard serial bus form at logic levels, but uses the RTS (Pin 3)
and RXD (Pin 6) to select the three active modes of operation. Normally RTS and RXD input lines are left high until
data or hard-iron calibration is needed from the HMR3100. The RXD line is left high unless a calibration is requested.
The RTS line will be either be pulsed low or held low to initiate an active mode. Otherwise a low-power sleep mode is
the default state. The RXD and RTS data inputs are passively pulled high via the microcontroller if left open.

Normal Mode

When the host processor (external to the HMR3100), sends a RTS low pulse to the RTS pin, the HMR3100 will send
status/heading data via the TXD pin. The host shall hold the RXD pin high during this mode. The RTS shall be held
high when not pulsed. The HMR3100 will return to sleep mode when RTS is left high after the three-byte
status/heading data packet is sent. Up to 20 heading queries per second can be accomplished given fast enough
baud rates. A caution is advised that average current draw is proportional to supply voltage and amount of queries
handled. At the 20 Hz rate, 1 to 5 milliamperes of current is consumed with lesser query rates taking advantage of the
less than one-microampere sleep mode current draw between queries. Figure 2 shows the normal mode timing

Figure 2

Normal Mode Timing Diagram

Figure 2

Normal Mode Timing Diagram