Project:HM55B

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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. [http://www.parallax.com/dl/docs/prod/compshop/HM55BModDocs.pdf Datasheet] Italics in this article are straight quotes from datasheet. == Bus Pirate pinout == From http://dangerousprototypes.com/docs/Bus_Pirate_101_tutorial <pre> 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 </pre> [[File:Buspirate.jpg|200px]] In the photo the pins on right connector, from top left: GND +3.3V +5V ADC Vpu AUX CLK MOSI CS MISO == Connecting == === Wiring === <pre> HM55B Bus Pirate Din(1) MOSI Dout(2) MISO VSS(3) GND CLK(4) CLK /EN(5) CS Vdd(6) +5V </pre> [[File:HM55B.jpg|200px]] [[File:BusPirateConnector.jpg|200px]] === Starting a terminal session === Plug in the bus pirate then (assuming it's the only USB device plugged in a debian based os): <pre> $ screen /dev/ttyUSB0 115200 </pre> == 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) NB The numbering might be different depending on firmware version. === Checkpoint === There should now be +5v measured between HM55B pins 3 and 6. === Abridged menu options === To cover this use case: <pre> ? help /\ Clock H/L ~ CLK pin -/_ Data H/L ~ MOSI pin ! Read bit ~ MISO pin [] CS L/H ~ CS pin &/% Delay us/ms : Repeat </pre> === 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 - noting output formatting is configurable(TODO check options): <pre> 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 </pre> === 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: <pre> 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 </pre> === Checking measurement status === ''To check the measurement status, start by sending a positive pulse to /EN. Then, shift-out %1100.'' ][/-\/-\/_\/_\ Bus Pirate output: <pre> 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 </pre> === Polling === ''Shift in the status flag, upon receipt of %1100, discontinue polling.'' !!!! Bus Pirate output: <pre> READ BIT: 1 READ BIT: 1 READ BIT: 0 READ BIT: 0 </pre> === 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: <pre> 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 </pre> === Sample readings === 0,-6 -1,-11 -1,-19 === 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: <pre> x = 0b11111101101 y = 0b11111101111 </pre> ''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: <pre> 11111101101 00000010010 00000010011 </pre> x = -19 <pre> 11111101111 00000010000 00000010001 </pre> y = -17 angle θ = arctan(17/-19) angle θ = -41.82 The range is plus and minus 180. Positive values sweeping east, negative sweeping west. The reading above indicates north west. === Combining all commands === A delay is required between starting the measurement and shifting in the status flag: \[/_\/_\/_\/_\][/-\/_\/_\/_\][/-\/-\/_\/_\&&&&&&&&!!!!!!!!!!!!!!!!!!!!!!!!!!] Note that &&&&&&&& works but &:8 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 minimum required delay was 6us, 7us and 8us in three modules tested so, in production, as per data sheet, it would be a question of starting a measurement and polling status flag till 0b1100 was received. The [http://dangerousprototypes.com/docs/SPI 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 == * <s>Script all of the commands above.</s> * <s>Script to include a conversion algorithm returning an actual reading in degrees.</s> * <s>Add a compass calibration procedure (vital), though the proof of concept works so might not be practical to do this at the bit level.</s> == Source code == All improvements implemented in C. Code: [https://github.com/dsikar/at90usb162_HM55B github.com].

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