Project:HM55B

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Revision as of 05:23, 12 December 2010 by 93.97.176.250 (talk) (= Abridged command set)
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About the module

$20-25 compass module for embedded systems, this comes in a convenient DIL package.

The SMD will probably be cheaper, though I have not managed to source one yet.

Datasheet

Bus Pirate pinout

From http://dangerousprototypes.com/docs/Bus_Pirate_101_tutorial

Pin name Description (Bus Pirate is the master)
MOSI 	Master data out, slave in (SPI, JTAG), Serial data (1-Wire, I2C, KB), TX* (UART)
CLK 	Clock signal (I2C, SPI, JTAG, KB)
MISO 	Master data in, slave out (SPI, JTAG) RX (UART)
CS* 	Chip select (SPI), TMS (JTAG)
AUX 	Auxiliary IO, frequency probe, pulse-width modulator
ADC 	Voltage measurement probe (max 6volts)
Vpu 	Voltage input for on-board pull-up resistors (0-5volts).
+3.3v 	+3.3volt switchable power supply
+5.0v 	+5volt switchable power supply
GND 	Ground, connect to ground of test circuit 

Buspirate.jpg

In the photo the pins on right connector, from top left:

GND  +3.3V
+5V  ADC
Vpu  AUX
CLK  MOSI
CS   MISO

Connecting

Wiring

HM55B   Bus Pirate

Din(1)  MOSI
Dout(2) MISO
VSS(3)  GND
CLK(4)  CLK
/EN(5)  CS
Vdd(6)  +5V

Starting a terminal session

Plug in the bus pirate then (assuming it's the only USB device plugged in a debian based os):

$ screen /dev/ttyUSB0 115200 

Testing with the BusPirate

Once in the terminal session, apply these settings:

  • M (mode)
  • 8 (3-wire raw)
  • 4 (400KHz)
  • 2 (3.3V high)
  • W (turn on 5V output)

Checkpoint

There should now be +5v measured between HM55B pins 3 and 6.

Abridged menu options

To cover this use case:

? help  
     
/\ Clock H/L ~ CLK pin

-/_ Data H/L ~ MOSI pin

! Read bit   ~ MISO pin

[] CS L/H    ~ CS pin  

&/% Delay us/ms

: Repeat 

Reset HM55B

Reading in plain english, "Set clock pin to low, set CS pin to low, shift out %0000 on the rising edge of the clock then set CS pin to high".

Noting that CS maps to active low /EN pin.

\[/_\/_\/_\/_\]

Bus Pirate output:

CLOCK, 0
CS ENABLED
CLOCK, 1
DATA OUTPUT, 0
CLOCK, 0
CLOCK, 1
DATA OUTPUT, 0
CLOCK, 0
CLOCK, 1
DATA OUTPUT, 0
CLOCK, 0
CLOCK, 1
DATA OUTPUT, 0
CLOCK, 0
CS DISABLED

Start measurement

Start a measurement by taking /EN low again, then shift-out %1000. Leave /EN low until checking the measurement status.

\[/-\/_\/_\/_\

Bus Pirate output:

CLOCK, 0
CS ENABLED
CLOCK, 1
DATA OUTPUT, 1
CLOCK, 0
CLOCK, 1
DATA OUTPUT, 0
CLOCK, 0
CLOCK, 1
DATA OUTPUT, 0
CLOCK, 0
CLOCK, 1
DATA OUTPUT, 0
CLOCK, 0

Checking measurement status

To check the measurement status, start by sending a positive pulse to /EN. Then, shift-out %1100.

][/-\/-\/_\/_\

Bus Pirate output:

CS DISABLED
CS ENABLED
CLOCK, 1
DATA OUTPUT, 1
CLOCK, 0
CLOCK, 1
DATA OUTPUT, 1
CLOCK, 0
CLOCK, 1
DATA OUTPUT, 0
CLOCK, 0
CLOCK, 1
DATA OUTPUT, 0
CLOCK, 0

Polling

Shift in the status flag, upon receipt of %1100, discontinue polling.

!!!!

Bus Pirate output:

READ BIT: 1
READ BIT: 1
READ BIT: 0
READ BIT: 0

Take sample

Leave /EN low, and move on to shifting-in the x and y-axis values.

Shifting-in the x and y-axis values is a simple matter of shifting-in 11 bits for the x-axis measurement followed by 11 more bits for the y-axis measurement. After completing the y-axis shift-in, set the /EN pin high again.

!!!!!!!!!!!!!!!!!!!!!!]

Bus Pirate output:

READ BIT: 1
READ BIT: 1
READ BIT: 1
READ BIT: 1
READ BIT: 1
READ BIT: 1
READ BIT: 0
READ BIT: 1
READ BIT: 1
READ BIT: 0
READ BIT: 1
READ BIT: 1
READ BIT: 1
READ BIT: 1
READ BIT: 1
READ BIT: 1
READ BIT: 1
READ BIT: 0
READ BIT: 1
READ BIT: 1
READ BIT: 1
READ BIT: 1
CS DISABLED

Converting the reading to an angle

The x-axis reports (field strength) × cos(θ), and the y-axis reports the (field strength) × sin(θ). To resolve θ into a clockwise angle from north, use arctan(-y/x)

angle θ = arctan(-y/x)

In the example:

x = 0b11111101101 y = 0b11111101111

The Hitachi HM55B chip on the Compass Module reports its x and y axis measurements in terms of microteslas (μT) in 11-bit signed values.

Since both most significant bits equal one, x and y are negative and we take the two's complement:

11111101101 00000010010 00000010011

x = -19

11111101111 00000010000 00000010001

y = -17

angle θ = arctan(17/-19)

angle θ = -41.82

Sample readings:

0,-6

-1,-11

-1,-19


Combining all commands

A delay is required between starting the measurement and shifting in a measurement completed status flag:

\[/_\/_\/_\/_\][/-\/_\/_\/_\][/-\/-\/_\/_\&&&&&&!!!!!!!!!!!!!!!!!!!!!!!!!!]

Note that &&&&&& works but &:6 does not, even though the nominal value in both cases is the same, presumably because BusPirate parser adds a huge delay. This means that we don't know how fast this module can be clocked, because each ! will be > 50ms apart.

The BusPirate wiki suggests this will work (since we've determined that it still provides the 22 sample bits even if the reading is incomplete):

\[0b0000 &]
[0b1000 && ]&[& 0b1100 r:4]

Exiting

ctrl-a \y

Improvements

  • Script all of the commands above.
  • Script to include a conversion algorithm returning an actual reading in degrees.
  • Add a compass calibration procedure.