HMC5883
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C COMMUNICATION PROTOCOL
The HMC5883 communicates via a two-wire I2C bus system as a slave device. The HMC5883 uses a simple protocol
with the interface protocol defined by the I2C bus specification, and by this document. The data rate is at the standard-
mode 100kbps or 400kbps rates as defined in the I2C Bus Specifications. The bus bit format is an 8-bit Data/Address
send and a 1-bit acknowledge bit. The format of the data bytes (payload) shall be case sensitive ASCII characters or
binary data to the HMC5883 slave, and binary data returned. Negative binary values will be in two’s complement form.
The default (factory) HMC5883 7-bit slave address is 0x3C for write operations, or 0x3D for read operations.
The HMC5883 Serial Clock (SCL) and Serial Data (SDA) lines have optional internal pull-up resistors, but require resistive
pull-ups (Rp) between the master device (usually a host microprocessor) and the HMC5883. Pull-up resistance values of
about 10k ohms are recommended with a nominal 1.8-volt digital supply voltage (DVDD). Other values may be used as
defined in the I2C Bus Specifications or with the internal 50k ohm pull-up resistors (SDAP, SCLP) that can be tied to
DVDD.
The SCL and SDA lines in this bus specification can be connected to a host of devices. The bus can be a single master to
multiple slaves, or it can be a multiple master configuration. All data transfers are initiated by the master device which is
responsible for generating the clock signal, and the data transfers are 8 bit long. All devices are addressed by I2C’s
unique 7 bit address. After each 8-bit transfer, the master device generates a 9 th clock pulse, and releases the SDA line.
The receiving device (addressed slave) will pull the SDA line low to acknowledge (ACK) the successful transfer or leave
the SDA high to negative acknowledge (NACK).
Per the I2C spec, all transitions in the SDA line must occur when SCL is low. This requirement leads to two unique
conditions on the bus associated with the SDA transitions when SCL is high. Master device pulling the SDA line low while
the SCL line is high indicates the Start (S) condition, and the Stop (P) condition is when the SDA line is pulled high while
the SCL line is high. The I2C protocol also allows for the Restart condition in which the master device issues a second
start condition without issuing a stop.
All bus transactions begin with the master device issuing the start sequence followed by the slave address byte. The
address byte contains the slave address; the upper 7 bits (bits7-1), and the Least Significant bit (LSb). The LSb of the
address byte designates if the operation is a read (LSb=1) or a write (LSb=0). At the 9 th clock pulse, the receiving slave
device will issue the ACK (or NACK). Following these bus events, the master will send data bytes for a write operation, or
the slave will clock out data with a read operation. All bus transactions are terminated with the master issuing a stop
sequence.
I2C bus control can be implemented with either hardware logic or in software. Typical hardware designs will release the
SDA and SCL lines as appropriate to allow the slave device to manipulate these lines. In a software implementation, care
must be taken to perform these tasks in code.
OPERATIONAL EXAMPLES
The HMC5883 has a fairly quick stabilization time from no voltage to stable and ready for data retrieval. The nominal 8.3
milli-seconds with the factory default single measurement mode means that the six bytes of magnetic data registers
(DXRA, DXRB, DZRA, DZRB, DYRA, and DYRB) are filled with a valid first measurement.
To change the measurement mode to continuous measurement mode, after the 8.3 milli-second power-up time send the
three bytes:
0x3C 0x02 0x00
This writes the 00 into the second register or mode register to switch from single to continuous measurement mode
setting. With the data rate at the factory default of 15Hz updates, a 67 milli-second typical delay should be allowed by the
I2C master before querying the HMC5843 data registers for new measurements. To clock out the new data, send:
0x3D, and clock out DXRA, DXRB, DZRA, DZRB, DYRA, and DYRB located in registers 3 through 8. The HMC5883 will
automatically re-point back to register 3 for the next 0x3D query. All six data registers must be read properly before new
data can be placed in any of these data registers.
