Build HAT Serial Protocol
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build-date: 2021-10-15
build-version: githash: 61115e7-dirty
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Legal Disclaimer Notice 1
Build HAT Serial Protocol
Introduction
The BuildHAT is a board that provides an interface between a Raspberry Pi host and up to four LEGO LPF2 (LEGO Power
Functions version 2) devices. Supported LPF2 devices include a wide range of actuators and sensors. Firmware running
on the HAT deals with the hard real-time requirements of the LPF2 devices, including monitoring for connection and
disconnection events and interrogating devices to determine their capabilities and properties.
The HAT communicates with the Raspberry Pi host over the 'command port', a 115200 baud serial interface with eight
bits per character, no parity and one stop bit ('8N1'). There is no flow control. The command port protocol is entirely in
plain text, and it is perfectly possible to simply run a terminal emulator on the host and interact manually with the HAT.
When experimenting the HAT in this way it is convenient to enable echo mode so that you can see what you are typing:
see the description of the echo command below. See also the plimit command, which must be sent before many
operations will work correctly.
This document describes the commands available over that interface.
NOTE
A Python library is provided that provides a higher-level interface to the functions of the HAT: in most cases it will be
preferable to use that library rather than the lower-level commands described here.
Introduction 2
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Port and Device Basics
The firmware numbers the ports from 0 to 3. There is the notion of the 'current port', set using the port command. Many
commands implicitly address the current port.
Each port may have a device connected to it. A device may be 'active', which means that it communicates with the HAT
using a serial interface, or it may be 'passive'. Passive devices include some lights and motors, although most types of
motor are active devices. Active devices can offer some feedback to the HAT: for example, an active motor might contain
position or speed sensors to allow it to be controlled precisely.
An active device has up to sixteen 'modes'. A mode can be thought of as a small memory buffer in the device, much like
the concept of a 'characteristic' in Bluetooth terminology. Some modes are intended to be written to, to control the device;
some are intended to be read from, to extract sensor readings for example.
When a device is plugged in to the HAT, the HAT emits a 'connected' message followed by the information it has about
the device. For a passive device this will just be an identifying code number. For an active device this will be an identifying
code number followed by other information including the connection baud rate, software and hardware version numbers
and a list of the available modes. See the list command below for more detail.
When a device is unplugged from the HAT, a 'disconnected' message is emitted.
Port and Device Basics 3
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Device Power
The LPF2 connector supplies power to a connected device in two ways. The first of these is a normal digital logic power
supply at +3.3V which is always present. If a device attempts to draw too much current from this supply then a 'Port
power fault' message is emitted for as long as the fault persists.
The second power source is at about +7.2V and intended for driving motors and other relatively high-power devices. This
is supplied by a separate dedicated driver IC for each port. If a fault is detected on this supply then a 'Motor power fault'
message is emitted repeatedly. This fault condition is latched in hardware and must be cleared explicitly after the cause
of the fault has been resolved: see the clear_faults command below.
The motor driver ICs can output PWM waveforms of either polarity to allow speed and direction control of motors.
Device Power 4
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On-board Controllers
The firmware implements an independent controller for the motor power output on each port of the HAT. The controller
can be in one of two modes: 'direct PWM' (the default) and 'PID'. In addition, each controller has an associated setpoint,
which can be constant or varying: see the set command below.
In direct PWM mode the setpoint, which must be in the range from –1 to +1, directly controls the power output. This is
useful in simple motor applications, for driving lights, and for certain devices that use 'motor power' for other purposes.
Such devices usually need to have power enabled very shortly after connection is established, and usually need to be
powered in reverse: i.e., set –1.
In PID mode a proportional-integral-differential controller reads a value from a sensor: typically this will be a speed or
angle sensor on a motor being controlled. This value is called the 'process variable'. The PID controller adjusts the output
power to attempt to have the process variable track the setpoint closely. Using this you can, for example, attempt to run a
motor at constant speed under varying load, or move a motor to a given position. See the pid command below for more
details.
On-board Controllers 5
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Command Summary
Any command can be abbreviated to its shortest unique prefix. Multiple commands can be given on one line separated by
semicolons.
help, ?
Prints a synopsis of the available commands.
echo <0|1>
Disables (default) or enables echo of characters received over the command port.
version
Prints the version string for the currently-running firmware.
port <port>
Sets the current port, used implicitly by many other commands.
vin
Prints the voltage present on the input power jack.
ledmode <ledmode>
Sets the behaviour of the HAT’s LEDs.
ledmode
–1 LEDs lit depend on the voltage on the input power jack (default)
0 LEDs off
1 orange
2 green
Effect
3 orange and green together
list
Prints a list of all the information known about the LPF2 devices connected to the HAT. Typical output is as follows.
Ê P0: connected to active ID 40
Ê type 40
help, ? 6
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Ê nmodes =3
Ê nview =3
Ê baud =115200
Ê hwver =00000004
Ê swver =11000000
Ê M0 LEV O SI = PCT
Ê format count=1 type=0 chars=1 dp=0
Ê RAW: 00000000 00000009 PCT: 00000000 00000064 SI: 00000000 00000009
Ê M1 COL O SI = PCT
Ê format count=1 type=0 chars=2 dp=0
Ê RAW: 00000000 0000000A PCT: 00000000 00000064 SI: 00000000 0000000A
Ê M2 PIX O SI =
Ê format count=9 type=0 chars=3 dp=0
Ê RAW: 00000000 000000AA PCT: 00000000 00000064 SI: 00000000 000000AA
Ê M3 TRANS SI =
Ê format count=1 type=0 chars=1 dp=0
Ê RAW: 00000000 00000002 PCT: 00000000 00000064 SI: 00000000 00000002
Ê speed PID: 00000000 00000000 00000000 00000000
Ê position PID: 00000000 00000000 00000000 00000000
Ê P1: no device detected
Ê P2: connected to passive ID 8
Ê P3: connected to active ID 30
Ê type 30
Ê nmodes =5
Ê nview =3
Ê baud =115200
Ê hwver =00000004
Ê swver =10000000
Ê M0 POWER SI = PCT
Ê format count=1 type=0 chars=4 dp=0
Ê RAW: 00000000 00000064 PCT: 00000000 00000064 SI: 00000000 00000064
Ê M1 SPEED SI = PCT
Ê format count=1 type=0 chars=4 dp=0
Ê RAW: 00000000 00000064 PCT: 00000000 00000064 SI: 00000000 00000064
Ê M2 POS SI = DEG
Ê format count=1 type=2 chars=11 dp=0
Ê RAW: 00000000 00000168 PCT: 00000000 00000064 SI: 00000000 00000168
Ê M3 APOS SI = DEG
Ê format count=1 type=1 chars=3 dp=0
Ê RAW: 00000000 000000B3 PCT: 00000000 000000C8 SI: 00000000 000000B3
Ê M4 CALIB SI = CAL
Ê format count=2 type=1 chars=5 dp=0
Ê RAW: 00000000 00000E10 PCT: 00000000 00000064 SI: 00000000 00000E10
Ê M5 STATS SI = MIN
Ê format count=14 type=1 chars=5 dp=0
Ê RAW: 00000000 0000FFFF PCT: 00000000 00000064 SI: 00000000 0000FFFF
Ê C0: M1+M2+M3
Ê speed PID: 00000BB8 00000064 00002328 00000438
Ê position PID: 00002EE0 000003E8 00013880 00000000
list
For each port the HAT ports the type and ID of any device connected. For active devices it also gives the number of
'modes' and 'views', the baud rate of the HAT’s connection to the device, and the device’s hardware and software version
numbers. Next comes a list of the available modes, any possible combi modes, and any recommended PID parameters.
The first line for each mode gives its name (such as LEV) and unit, such as PCT or DEG. The second line gives the number of
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data items in the mode (usually 1) , its type (0=signed char, 1=signed short, 2=signed int, 3=float), a hint as to a suitable
number of characters to use to display its value, and a suitable number of decimal places. The third line gives minimum
and maximum values for the data in that mode in various units.
clear_faults
Clears any latched motor power fault.
coast
Switches the motor driver on the current port to 'coast' mode, that is, with both outputs floating.
pwm
Switches the controller on the current port to direct PWM mode.
off
Same as pwm; set 0.
on
Same as pwm; set 1.
pid <pidparams>
Switches the controller on the current port to PID mode. The pidparams specify from where the process variable is to be
fetched and the gain coefficients for the controller itself. They are, in order, as follows.
Name Meaning
pvport
pvmode
pvoffset
pvformat process variable format: u1=unsigned byte, s1=signed byte, u2=unsigned
port to fetch process variable from
mode to fetch process variable from
process variable byte offset into mode
short, s2=signed short, u4=unsigned int, s4=signed int, f4=float
clear_faults
pvscale
pvunwrap
Kp
Ki
Kd
windup
The PID controller fetches the process variable from the mode specified by the pvport, pvmode, pvoffset, pvformat
parameters. Note that a suitable select command is required to ensure that this mode’s data are available.
process variable multiplicative scale factor
0=no unwrapping; otherwise modulo for process variable phase unwrap
proportional gain
integral gain
differential gain
integral windup limit
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It then multiplies the value from the mode by pvscale.
The pvunwrap parameter allows 'phase unwrapping' of the process variable. The commonest use of this is with an angular
sensor that outputs a value from –180° to +179° depending on its absolute position. When the sensor is turned
continuously, the reading will jump from +179° to –180° once per revolution. Setting the pvunwrap parameter to 360 will
cause the unwrapper to add or subtract 360° at each discontinuity to make its output continuous (and in principle infinite
in range). More precisely, the output of the unwrapper is guaranteed equal to its input modulo the pvunwrap parameter,
with the smallest possible change between one sample and the next.
Kp, Ki and Kd are the standard PID controller parameters. The implied unit of time in the integrator and differentiator is one
second.
The output of the error integrator is clamped in absolute value to the windup parameter.
set <setpoint>
Configures the setpoint for the controller on the current port. The setpoint can be a (floating-point) constant or a
waveform specification.
A waveform specification is one of square, sine, triangle, pulse and ramp, followed by four floating-point parameters.
For square, sine and triangle waveforms these parameters are the minimum value, the maximum value, the period in
seconds, and the initial phase (from 0 to 1). For a pulse waveform the first three parameters are the setpoint value during
the pulse, the setpoint value after the pulse, and the duration of the pulse; the fourth parameter is ignored. For a ramp
waveform the first three parameters are the setpoint value at the start of the ramp, the setpoint value at the end of the
ramp, and the duration of the ramp; the fourth parameter is again ignored.
When a pulse or ramp is completed, a message pulse done or ramp done is emitted.
bias <bias>
Sets a bias value for the current port which is added to positive motor drive values and subtracted from negative motor
drive values. This can be used to compensate for the fact that most DC motors require a certain amount of drive before
they will turn at all.
plimit <limit>
Sets a global limit to the motor drive power on all ports. For safety when experimenting the default value is 0.1; this will
usually need to be increased.
select
set <setpoint>
Deselects any previously-selected mode on the current port.
select <selmode>
Selects the specified mode on the current port and repeatedly outputs that mode’s data as raw hexadecimal.
select <selmode> <offset> <format>
Selects the specified mode on the current port. Repeatedly extracts a value from that mode starting at the specified
offset, interpreting it according to the specified format (u1=unsigned byte, s1=signed byte, u2=unsigned short, s2=signed
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short, u4=unsigned int, s4=signed int, f4=float) and outputs that value.
selonce
As select but outputs data once rather than repeatedly.
combi <index>
Deconfigures any previously-configured combi mode on the current port at the specified index.
combi <index> <clist>
Configures a combi mode on the current port at the specified index. clist is a list of pairs of numbers, each pair giving a
mode and an offset into that mode. Note that the offset is a 'dataset' offset, i.e., is multiplied by the size of the data
elements in that mode (as given by the list command) to obtain a byte offset.
write1 <hexbyte>*
Writes the given hexadecimal bytes to the current port, the first byte being a header byte. The message is padded if
necessary, and length and checksum fields are automatically populated.
write2 <hexbyte>*
Writes the given hexadecimal bytes to the current port, the first two bytes being header bytes. The message is padded if
necessary, and length and checksum fields are automatically populated.
debug <debugcode>
Not required for normal use. Note that some debug modes can generate output faster than the serial port can handle at
its standard speed.
signature
Not required for normal use.
selonce
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Appendix A: Examples
The following are some simple examples to illustrate how to use the above commands.
Using a motor from the SPIKE Prime set
Plug the motor into port 0. Then send
port 0
to address port 0,
plimit 1
to remove the power limit, and
set triangle 0 1 10 0
to generate a triangle wave setpoint. The motor will accelerate to full speed and decelerate back to zero continuously with
a period of ten seconds.
Now try
port 0
combi 0 1 0 2 0 3 0
to select a combi mode with index zero containing data from modes 1, 2 and 3,
select 0
to select this combi mode, and
plimit 1 ; bias .4
to remove the power limit and set a reasonable bias value for the motor. You can now set up a position PID controller that
reads a 2-byte value from offset 5 in this combi mode, which is the absolute position of the motor in degrees, from –180°
to +179°. We scale this by 1/360=0.0027777778 to get a position in revolutions from –0.5 to +0.5, unwrap the phase with
a modulo of 1:
pid 0 0 5 s2 0.0027777778 1 5 0 .1 3
Using a motor from the SPIKE Prime set 11
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Here the PID parameters are Kp=5, Ki=0 and Kd=0.1. The integral windup limit is set arbitrarily at 3.
Now if you send
set square 0 1 3 0
the motor will alternately rotate one revolution clockwise and one revolution anticlockwise with a period of three seconds.
Using the colour sensor from the SPIKE Prime set
port 0 ; plimit 1 ; set -1 ; select 0
will turn on the sensor’s light and continuously report a number corresponding to the colour detected. The sensor has
many other modes.
Using the ultrasonic distance sensor from the SPIKE Prime set
port 0 ; plimit 1 ; set -1 ; select 1
will power up the sensor and continuously report a number corresponding to the distance to the object in front of the
sensor in millimetres.
To control the four LEDs on the sensor use the following sequence
select 5 ; write1 c5 pp qq rr ss
where pp, qq, rr and ss are four hexadecimal numbers from 0 to 0x64 (100 decimal) that control the brightnesses of the
individual LEDs.
Using the force sensor from the SPIKE Prime set
port 0 ; select 0
will continuously report a number corresponding to the force detected.
Using the 3x3 colour light matrix from the SPIKE Essential set
port 0 ; plimit 1 ; set -1
will turn on the light matrix. If the device does not receive power shortly after connection is established it will disconnect.
Using the colour sensor from the SPIKE Prime set 12
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select 0 ; write1 c0 p
where p is a number from 0 to 9, will light the matrix in bar-graph style according to the value of p.
select 1 ; write1 c1 p
where p is a hexadecimal number from 0 to 0x0a will light the matrix in a solid colour according to the value of p: 0=off;
1=red; 2=magenta; 3=blue; 4=cyan; 5=pale green; 6=green; 7=yellow; 8=orange; 9=red, 0a=white.
The most flexible mode is mode 2, where you can specify the colour of each LED individually.
select 2 ; write1 c2 12 23 34 45 12 23 34 45 12
gives a random pattern of dull colours and
write1 c2 67 72 78 82 89 92 9a a4 aa
gives some random bright ones. The first hex digit (from 0 to a) specifies the brightness and the second hex digit (from 0
to a) the basic colour. So for example
write1 c2 1a 2a 3a 4a 5a 6a 7a 8a 9a
gives shades of white and
write1 c2 a1 a2 a3 a4 a5 a6 a7 a8 a9
gives all the basic colours at full brightness.
Mode 3 allows you to specify transitions.
write1 c3 1
enables row-by-row animated transitions, while
write1 c3 2
enables a fade to black and fade back up.
Using the 3x3 colour light matrix from the SPIKE Essential set 13
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Appendix B: Passive ID Codes
ID (decimal) Device
1 System medium motor
2 System train motor
3 System turntable motor
4 general PWM/third party
5 button/touch sensor
6 Technic large motor (some have active ID)
7 Technic XL motor (some have active ID)
8 simple lights
9 Future lights 1
10 Future lights 2
11 System future actuator (train points)
Appendix B: Passive ID Codes 14
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Appendix C: Active ID Codes
ID (hex) Device
25 colour and distance sensor
26 Medium linear motor
2E Technic large motor
2F Technic XL motor
30 SPIKE Prime medium motor
31 SPIKE Prime large motor
3D SPIKE Prime colour sensor
3E SPIKE Prime ultrasonic distance sensor
3F SPIKE Prime force sensor
40 SPIKE Essential 3x3 colour light matrix
41 SPIKE Essential small angular motor
Appendix C: Active ID Codes 15
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