Compiling libhid for Raspbian Linux on a Raspberry Pi

My son and I are working on a project using a Raspberry Pi and I needed to be able to talk to a USB HID device. This requires a software library called libhid but unfortunately it is not available as a package on Raspbian linux. I downloaded the source and attempted to compile it but ran into an error:

lshid.c:32:87: error: parameter ‘len’ set but not used [-Werror=unused-but-set-parameter]
cc1: all warnings being treated as errors
make[2]: *** [lshid.o] Error 1
make[2]: Leaving directory `/root/libhid-0.2.16/test'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/root/libhid-0.2.16'
make: *** [all] Error 2

After some googling I found a couple others on the Raspberry Pi forums that ran into the same problem. One of the commenters came up with a simple fix that requires a quick edit of the source code. In ~/libhid-0.2.16/test you need to edit the file lshid.c

Here is the code before making the edit:

39 /* only here to prevent the unused warning */
40 /* TODO remove */
41 len = *((unsigned long*)custom);
42
43 /* Obtain the device's full path */


Here is the code after the edit.
You need to comment out line 41 and then add len = len; and custom = custom;

39 /* only here to prevent the unused warning */
40 /* TODO remove */
41 //len = *((unsigned long*)custom);
42 len = len;
43 custom = custom;
44
45 /* Obtain the device's full path */

After editing the file simply run configure, make and make install like normal. The library will be put into /usr/local. Make sure you run sudo ldconfig before trying to compile any software that uses libhid. Thanks Raspberry Pi forums!

pyMCU and DS18B20 temperature sensors

As part of my Garage Monitor project I am using a Maxim DS18B20 digital temperature sensor with a pyMCU microcontroller. I found lots of pages and sample code for using the DS18B20 with Arduino boards and various other microcontrollers but nothing for the pyMCU. The pyMCU uses a Python library published by the board manufacturer to control it. The DS18B20 uses the 1-Wire communication bus and the pyMCU Python library includes functions for 1-Wire communication called owWrite and owRead. The tricky part is figuring out the command sequence to coax a temperature reading out of the sensor. The communication with the sensor involves writing several hex values to it and then reading a hex temperature value. That hex value is then converted to a decimal value.

It took me the better part of three evenings to figure out the sequence of writes and reads. The other thing that was a bit difficult was figuring when to send reset pulses.

The pyMCU 1-Wire function uses this format:

owWrite(pinNum, mode, writeData)

The mode value determines if a reset pulse is sent before or after the data. Here is how I wired it:

And is the code that I wrote:

#!/usr/bin/python
#
# Written by Matthew McMillan
# matthew.mcmillan at gmail dot com
#
# This code reads a temperature value from
# a DS18B20 sensor using a pyMCU board. It only
# reads from a single sensor.
#
import pymcu           # Import the pymcu module

# Function to read value from DS18B20 temperature sensor
# Need to pass digital pin number and flag for F or C units
#
# 0x33  READ ROM. Read single device address.
# 0xCC  SKIP ROM. Address all devices on the bus
# 0x44  CONVERT T. Initiate temperature conversion
# 0xBE  READ SCRATCHPAD. Initiate read temp stored in the scratchpad.

def readtemp(pin,ForC):
    mb.owWrite(pin,1,0x33)
    ReadVal = mb.owRead(pin,0,8)
    mb.owWrite(pin,1,0xCC)
    mb.owWrite(pin,0,0x44)
    mb.owWrite(pin,1,0xCC)
    mb.owWrite(pin,0,0xBE)
    ReadVal = mb.owRead(pin,0,12)
    HexVal1 = hex(ReadVal[1])
    HexVal2 = hex(ReadVal[0])
    HexVal1 = HexVal1[2:4]
    HexVal2 = HexVal2[2:4]
    HexVal = '0x'+HexVal1+HexVal2
    DecVal = int(HexVal, 0)
    TempC = (DecVal * 0.0625)
    if ForC:
        TempF = ((TempC*9)/5)+32
        TempFR = round(TempF, 1)
        return TempFR
    else:
        TempCR = round(TempC, 1)
        return TempCR

################
# Main program
################

# Initialize mb (My Board) with mcuModule Class Object
mb = pymcu.mcuModule() 

#  Need to pass digital pin number and flag for ForC
tempout = readtemp(7,1)

print 'Temp: ' + str(tempout)

Please leave a comment if you found this helpful. I found this page on hackaday.com very helpful in figuring this out.

Raspberry Pi Heat Sinks

A friend of mine noticed that his Raspberry Pi got quite warm while playing 1080p video.  I am planning on using my Raspberry Pi in the garage which gets pretty dang hot here in Texas so we were both interested in adding additional cooling to the Pi. He found a post talking about making custom heat sinks out of old CPU heat sinks. One of the commenters on that post suggested using Zalman VGA Ram Heatsinks. Fry's Electronics has the silver colored version of these heat sinks for only $4.99. They come in a pack of eight and you only need three for each Raspberry Pi. 

The Raspberry Pi CPU (or SoC to be completely accurate) is 12mm x 12mm and so are the Zalman VGA Ram Heatsinks. So the CPU is easy enough to add a heat sink. You can just peel off the backing of the thermal tape and stick it on the CPU. There are two other components that would benefit from cooling as well. One is the USB/Ethernet controller and the other is the voltage regulator. For these two chips I had to cut down the heat sinks so they would fit without touching other components on the board. I used my Dremel tool with a cutoff wheel to trim them down to size and then a sanding drum to clean up the edges. The thermal tape that was on the heat sinks got pretty messed up during the cutting but the pack of eight came with two extra pieces of thermal tape so I trimmed down those extra pieces. You could also use thermal compound to stick them on if you have some of that.

Location of the various chips that may need cooling.

I just eye balled the closest fins that would fit the chip and marked it
 with an ultra fine point Sharpie marker.

After cutting the heat sinks with my Dremel tool.

Heat sinks installed on my Raspberry Pi.

Measuring light levels and temperature with a pyMCU board

My pyMCU board running a light sensor and a Maxim DS18B20 digital temperature sensor.

Using an LCD Display with my pyMCU board

I connected a 2x16 LCD display to my pyMCU

First test of my pyMCU board