Tag-Connect - A Smaller, Better Debug Connector

I design circuit boards (PCBs) with programmable devices - things like MCUs, PLDs and FPGAs - for a living. It's almost always necessary to provide a connector so that an external programmer, debugger or something as simple as a serial port can be hooked up to a board.

Programming, Debug and Serial Interface Hardware

In the early stages, this connector enables development and debug. Later on, it allows programming and configuration in the production process. The connector is often a simple header with one or two rows of pins on a 0.1 inch (2.54mm) pitch. Device manufacturers specify various pinouts to match their own programming and debug hardware. So far, so not very exciting...

Recently, I needed to design a board with an Atmel AVR MCU. It had to be no larger than 60mm x 14mm. There was no way I could fit Atmel's standard 2x3 header, so I started looking at placing small test pads and producing a companion board with pogo pins to break these out to a full size header. This is often a good way to go - the companion board can take on other duties, and maybe evolve into a full blow test jig - but it's another design task, and I was short of time and trying to keep costs down.

Enter Tag-Connect with their own clever take on this problem:

10 way Tag-Connect to IDC cable ("no legs" version)

Tag-Connect make cables with various standard connectors at one end, wired to a "plug of nails" at the other - a small array of pogo pins, together with rigid alignment pins.

6-way and 10-way Tag-Connect plugs, and a retaining clip

You place a corresponding array of round pads on your PCB, and locating holes to match the alignment pins. The "plug of nails" can then be pushed into the board with no danger of misalignment or rotation, allowing the relevant signals to be brought out to a programmer or whatever.

If you need to hold the plug in place for a while, there are a couple of options. For short periods of time, a clip (shown in the above image) can be used to hold the alignment pins.

10 way Tag-Connect attached to target board and retained by clip

If you want to work for longer, a more secure option is to use a version of the plug with "legs" that clip into additional holes on the board.

Tag-Connect version with legs for longer term attachment

Be careful about your PCB layout - Tag-Connect provide advice on this subject. And remember that the socket pins on the retaining clip may touch down on the back surface of your PCB; keep tracks and vias out of these areas to avoid mysterious and intermittent short circuits!

TC2050 land pattern with leg holes, showing escape routing

So the Tag-Connect approach saves space as well as the cost of purchasing and fitting a physical connector. And because not everybody has a Tag-Connect cable, or knows your pin-out, it can also add a little bit of security-through-obscurity.

Thanks to Chris Monslow at Kielowatts for pointing me at Tag-Connect.

Switching Mains Loads with Relays

I want to control the external lights around my home from a small computer to provide a bit of automation. The plan is to use an Arduino, and have it switch the lights on and off via relays. There are plenty of low cost relay boards to choose from. Most seem to be clones of the same basic designs which offer either 2, 4 or 8 relays. Here's a typical 4 way board:

4 Way Relay Board

This board has a 2.54mm pitch pin header with 6 pins - ground, +5VDC power and 4 inputs that control the coils of the 4 relays. Each relay is provided with an individual 3 way screw terminal block that is wired to its common, normally open and normally closed contacts. Note that these terminals are big enough for 1-1.5mm solid wire and the relays are rated for up to 10 Amps. My advice is to stay way below this.

The inputs are protected from the "heavy lifting" of energising the relay coils by opto-isolators. Grounding an input pin allows current to flow from the +5VDC supply through a current limiting resistor, a surface mount red LED, and then the LED within the opto-isolator. The forward current is about 2mA in this situation so it's fine to directly connect these inputs to digital I/Os on an Arduino. The relay coils are wired to the +5VDC supply and the opto-isolator phototransistor collectors, and each is provided with a usual flyback diode. (Note that the +5VDC supply might get in the way of using one of these boards with a Raspberry Pi; these use 3.3V digital I/Os and could be damaged by higher voltages.)

So far I've built a box with an 8 way relay board and connected it to 8 external lamps. A small AC-DC module provides +5VDC locally; I will locate the control system elsewhere, with the link to the relay box containing just relay control lines and ground in a multi-core cable.

8 Way Relay Board - Driving Lights

Working on electrical installations can be dangerous and is heavily regulated by law. Consult a qualified electrician!

12 Volt DC PIRs for Outdoor Use

I've wanted to automate the operation of some external lights in my home for a long time, and PIR motion sensors are a must. I already have several as part of an intruder alarm system, but they are not rated for outdoor use; electronics that is typically uses components with wider than normal operating temperatures (e.g. -40 to +85 degrees Celsius instead of 0 to +70), and conformal coating is often used to protect circuitry from moisture (condensation).

When I first looked for external PIRs (several years ago), I found that almost every one of them was designed to be powered by the mains, and to switch a mains load (a lamp). But I need to drive the inputs of a microprocessor based control system from the PIR, so I would prefer a contact closure (volt free) output. I did find one or two low voltage DC units, but they were horrendously expensive (nearly 100 GBP).

A more recent search turned up a much more reasonable solution:

12V DC Outdoor PIR Motion Sensor

This unit retails for under 25 GBP from Solar and Wind Power Systems. It is IP44 rated, runs from a 12 Volt (AC or DC) power supply and has a relay that can switch up to 10 Amps. It allows adjustments to on-time, range (sensitivity) and ambient light.

Note that the unit is supplied with a 4 way terminal block that is quite large (appropriate for 10 Amps). I swapped this for a smaller one. Also, the knock-outs molded into the ABS plastic housing are quite difficult to remove - I resorted to drilling them out.

Here's a video of a quick test of one. In this setup, the ambient light adjustment is at maximum (to allow daylight operation), the range/sensitivity is low, and the on-time is at a minimum - corresponding to about 5 seconds. The power supply is 12VDC, and the output is wired to ground the cathode of a blue LED - the anode is wired to 12V via a 470R resistor.

Notice that the unit's output remains on while the PIR is detecting motion, and stays on afterwards for 5 seconds.

I checked the range quickly in this setup, and detection worked reliably at 4 meters in daylight.

Arduino 16x2 LCD/Keypad Shield

Many embedded projects benefit from a user interface, and shields with a 16x2 LCD and a 5 button keypad are popular with Arduino users. Most are almost exactly like this one. As with many Arduino peripherals, they are very inexpensive - less than 6 GBP at the time of writing for the one I chose with first class postage within the UK, less still if you can wait for one from China.

Arduino LCD/keypad Shield

Boards such as this are often referred to as "1602A" type, with various manufacturers adding various prefixes and suffixes. They all seem to be based on the Hitachi HD44780 LCD controller (or a compatible clone). And most seem to use a 4-bit parallel interface to this controller (which can also support 8-bit operation). The keypad buttons are wired with a resistor ladder between the power rails so that a unique voltage corresponding to each button is delivered into one of the MCU's ADC inputs.

There are various examples and drivers available for 1602A display/keypad boards running on the Arduino, but I wanted to understand it from first principles, so I coded my own simple driver in C after studying the controller datasheet and the board schematic. This allows you to clear and write strings to the display, and read debounced key presses. I wrote a quick demo program in C using Atmel Studio 7 for this which you can download here.

Note that the documentation is not reliable when it comes to the character set burned into the controller's ROM. The upper 128 character codes in my module did not match the data sheet, so - for example - I had to create a user defined character to get a "degree" symbol for the temperature reading shown in the above picture.

Here's a video showing the demo program running:

LED Brightness Control from an Arduino with PWM

I wanted to be able to control the brightness of an LED backlight in some pushbuttons. The AVR microcontroller on an Arduino has hardware timers with a Pulse Width Modulation (PWM) feature that allows you to send a regular stream of pulses out of a pin with control of the ratio of on time to off time (the mark/space ratio). This is a well established way of controlling LED brightness.

After setting up the AVR timer correctly, you write a value to its output comparison register (OCR) to control the mark/space ratio. A minor catch is that the ratio can be varied between 1:255 and 256:0 (for the 8-bit case) so the output can be fully on, but not quite fully off - indeed you see a perceptible but faint glow from the LED when the OCR is written with zero. My fix to this was to adjust things so the mark/space ratio can be varied between 0:256 and 255:1; the fact that "fully on" is no longer quite possible is imperceptible. This is accomplished by writing the brightness level minus one to the OCR, and treating zero as a special case - by turning off the output pin (via its Data Direction Register), relying on an external pulldown to keep the pin low.

I built a very simple driver circuit on a breadboard with a 2N7000 MOSFET; the Arduino drives its gate, which is pulled down with a 10k resistor. The source is grounded and the drain connects to the LED cathode. The LED anode connects to +12VDC via a 470R resistor which sets the forward current at about 20mA (the forward voltage is about 3V).

I wrote a quick test in C using Atmel Studio 7 - you can download it from here. And here's a video of the result, showing a fade up, a fade down, a sharp on transition, and a sharp off transition:

The fading effect looks much better to me.

Arduino Real Time Clock

I needed an accurate real time clock for an Arduino based home automation project; many are I2C controlled and based on Maxim's DS3231 which is achieves an accuracy of about 1 minute per year (2ppm) through temperature compensation - a potentially useful side effect of this is that the part provides a temperature reading. Most boards also include an I2C EEPROM. I found a seller on eBay offering 3 units for under 5 GBP, including next day postage:

Real Time Clock and EEPROM board

The board provides a header with 6 pins:

• GND - ground
• VCC - power (3.3V or 5V)
• SCL - I2C clock
• SDA - I2C data
• SQW - programmable interrupt/square wave output
• 32K - free running 32kHz clock output

There's a holder for a CR2032 coin cell on the reverse, to provide battery backup. Note that these are typically not included with the boards (there are international restrictions on shipping lithium batteries).

I hooked it up to my Arduino Mega 2560 with male-female flying leads: I wired GND and VCC to GND and 5V on the power header, SCL and SDA to their counterparts on the communication header, and the SQW output to the INT3 input.

I then wrote a quick program in to test the board (I prefer to code in C using Atmel's Studio for AVR microcontroller development, for more on this see here.) This used the nice, stable and simple I2C master driver written by Peter Fleury. Note that I ignored the board's EEPROM.

The program prints the current time, day, date and temperature once a second:

       
RTC test
time: 12:14.44   day: TUE   date: 08/03/2016   temperature: +24.00
time: 12:14.45   day: TUE   date: 08/03/2016   temperature: +24.00
time: 12:14.46   day: TUE   date: 08/03/2016   temperature: +24.00

The program uses the Arduino's USB serial port for console I/O, based on Mika Tuupola's recipe here. I use TeraTerm on a desktop Windows PC for this, connected to the Arduino's COM port at 115200 baud.

You can download a ZIP of the Atmel Studio 7 project from here.

Raspbian Jessie Lite

A new lightweight version of Raspbian Jessie has been released. It looks promising for “headless” applications (no monitor, keyboard or mouse). I fired it up on my Raspberry Pi Model B.

You can get Raspbian Jessie Lite from here. At the time of writing, the version was dated 2016-02-09 and the kernel version was 4.1. The ZIP download weighed in at 365MB. This contained a similarly sized compressed IMG file. Next, I had to write this image to SD card.

Note that choosing a reliable SD card is something of a science, with guidance available from various sources. For example, see here. I chose an 8GByte Lexar micro SD card that came bundled with an SD card adapter. I bought it from Amazon because of their no-fuss returns policy:

Lexas 8GB microSDHC card with SD adapter

The procedure for writing to image to SD cards can be found  here. I followed the Windows recipe, first extracting the IMG from the ZIP, and then running Win32DiskImager to write the IMG to my SD card:

Win32DiskImager application

It took a few minutes to get these steps done. I then inserted the freshly written SD card in the Pi, connected it to the HDMI input of my monitor, and plugged in an ethernet cable. Boot messages appeared on the monitor within a couple of seconds of power on. The login prompt appeared after maybe 20-30 seconds:

First Boot

I then switched my monitor back to displaying the output from my desktop PC, aiming to use an ssh client to interact with the now fully headless Pi. There are plenty to choose from; my favourite is PuTTY.

To make life easier on my LAN, I use static IP addresses for various devices, such as my NAS, and including my Raspberry Pi. To do this, I reserve an address in my DHCP server’s configuration, linking the device's MAC address to a fixed IP address. For the Pi I chose 192.168.1.9, as shown on the PuTTY configuration dialog:

PuTTY Launch Window

After double-clicking the raspberrypi entry (to Open it), I blew past the ensuing security warning by clicking Yes. A terminal window then appeared, and I was then able to log in with the usual default credentials (username = pi, password = raspberry):

PuTTY Terminal Window

Many images assume a 2GB SD card and size things accordingly, meaning wasted space on larger cards. I checked, and this was indeed the case:

       
pi@raspberrypi:~ $ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/root       1.3G  912M  281M  77% /
devtmpfs        214M     0  214M   0% /dev
tmpfs           218M     0  218M   0% /dev/shm
tmpfs           218M  4.4M  214M   3% /run
tmpfs           5.0M  4.0K  5.0M   1% /run/lock
tmpfs           218M     0  218M   0% /sys/fs/cgroup
/dev/mmcblk0p1   60M   20M   41M  34% /boot

raspi-config can be used to fix this:

       
pi@raspberrypi:~ $ sudo raspi-config

You simply highlight option 1 and hit Enter...

raspi-config in action

...and a few seconds later you can reboot and the job is done. I did this, and checked the outcome:

       
pi@raspberrypi:~ $ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/root       7.3G  913M  6.1G  13% /
devtmpfs        214M     0  214M   0% /dev
tmpfs           218M     0  218M   0% /dev/shm
tmpfs           218M  4.4M  214M   3% /run
tmpfs           5.0M  4.0K  5.0M   1% /run/lock
tmpfs           218M     0  218M   0% /sys/fs/cgroup
/dev/mmcblk0p1   60M   20M   41M  34% /boot

Notice that /dev/root now has 6.1G available.

I will be trying to use the resulting setup for a few embedded projects.

AVR Studio 7 and the Arduino Mega 2560

I've heard of the Arduino family of microcontroller boards but have not worked with one until today. I decided to take a closer look because I do know the Atmel AVR, in particular the 8-bit mega family, and an Arduino is a reasonably cheap way to get one to experiment with. I chose an Arduino Mega 2560 board.

My AVR experience is based on various editions of Atmel's Studio IDE - so I wanted to find out how to use it with the Arduino hardware, ignoring the Arduino software as much as possible.

Here are the steps I followed to run a simple C program. My starting point was a fresh install of Atmel Studio (version 7.0.783) and the Arduino software (version 1.6.7) on a desktop PC running Windows 10 - and (of course) an Arduino Mega 2560 connected to the PC via USB, with it's green power LED on.

(Note that you can click on an image to see a full size / full resolution version.)

• on the Start Page, select New Project...

  

  Atmel Studio 7 - Start Page

• in the New Project dialog, select GCC C Executable Project, specify a Name and a Location then click OK
  

  

  Atmel Studio 7 - New Project dialog

• in the Device Selection dialog, select ATmega2560 (the type of AVR microcontroller used on the Arduino board) then click OK
  

  

  Atmel Studio 7 - Device Selection dialog

• a minimal, template project is created, ready for you to enter code...
  

  

  Atmel Studio 7 - Virgin Project

• choose test Properties (assuming your project is called test) from the Project menu, then select Toolchain, and under AVR/GNU C Compiler select Symbols
  

  

  Atmel Studio 7 - Project Properties tab

• in the Defined Symbols (-D) box, click the Add Item button and enter F_CPU=16000000UL (this specifies the processor clock frequency in Hertz; for the Arduino Mega 2560 this is 16MHz - the "UL" suffix means Unsigned Long)
  

  

  Atmel Studio 7 - adding a defined symbol

• select the main.c tab and enter enough C code to see life signs - mine pulses the "L" LED once then twice, over and over
  

  

  Atmel Studio 7 - entering a simple C test program

       
#include <avr/io.h>
#include <util/delay.h>

int main(void)
{
 DDRB |= (1 << PORTB7); // PORTB7 direction = out
 
 while (1)
 {
  
  // 1 pulse
  PORTB |= (1 << PORTB7); // PORTB7 hi = LED L on
  _delay_ms(500); // 0.5 sec
  PORTB &= ~(1 << PORTB7); // PORTB7 hi = LED L off
  
  // delay
  _delay_ms(2000); // 2 sec

  // 2 pulses
  PORTB |= (1 << PORTB7);
  _delay_ms(500);
  PORTB &= ~(1 << PORTB7);
  _delay_ms(500);
  PORTB |= (1 << PORTB7);
  _delay_ms(500);
  PORTB &= ~(1 << PORTB7);

  // delay
  _delay_ms(2000);
 }
}

• now the interesting part - adding support for the Arduino's programming mechanism, which requires the use of the avrdude utility that is included in the Arduino software installation


• first, identify the COM port that the Arduino is connected to by running the Device Manger - in my case it was COM3...
  

  

  Device Manager (Windows 10) showing Arduino COM port

• choose External Tools... from the Tools menu
  

  

  Atmel Studio 7 - External Tools dialog (before)

• in the External Tools dialog...
• change the Title to Program
• enter the following in the Command box:
  

         
  c:\Program Files (x86)\Arduino\hardware\tools\avr\bin\avrdude.exe
  
  

• enter the following in the Arguments box, changing -P COM3 to match the COM port that the Arduino is connected to on your machine:
  

         
  -v -C"C:\Program Files (x86)\Arduino\hardware\tools\avr\etc\avrdude.conf" -p atmega2560 -c wiring -P COM3 -b 115200 -D -U flash:w:$(TargetDir)$(TargetName).hex:i
  
  

• check the Use Output window box
• click OK
  

  

  Atmel Studio 7 - External Tools dialog (after)

• there is now a Program option on the Tools menu which will flash the compiled code into the Arduino board - let's add a corresponding shortcut to the UI:
• select Add or Remove Buttons to the right of No Tool, then Customize...
  

  

  Atmel Studio 7 - adding a custom button

• the Customize dialog appears:
  

  

  Atmel Studio 7 - Customize dialog

• click Add Command...
• under Categories select Tools
• under Commands scroll down and select External Command 1
  

  

  Atmel Studio 7 - Add Command dialog

• click OK then Close the Customize dialog
• there is now a Program button left of the device (ATmega2560) button
• to build the code, choose Build Solution from the Build menu (or hit F7):
  

  

  Atmel Studio 7 - after build

• now program the board by clicking the Program button - this will produce some rapid progress messages from the avrdude programming utility in the Output pane:
  

  

  Atmel Studio 7 - after programming

• voila - you should see the "L" LED blinking once then twice, repeatedly