USB Doodad 5: Working USB

Now that I know the processor on the Doodad is working, I want to try out USB.  As we'll be using a software USB library, it's important that we test this out as soon as possible, in case we need to make circuit changes, or it's unreliable, or it just doesn't work at all.

The software USB stack we want to use is called V-USB:

  http://www.obdev.at/products/vusb

I downloaded V-USB and extracted it:

[mjd@onza src]$ wget http://www.obdev.at/downloads/vusb/vusb-20100715.tar.gz
 --2010-11-23 18:17:39--  http://www.obdev.at/downloads/vusb/vusb-20100715.tar.gz
Resolving www.obdev.at... 78.46.114.187
Connecting to www.obdev.at|78.46.114.187|:80... connected.
HTTP request sent, awaiting response... 302 Found
Location: http://www.obdev.at/ftp/pub/Products/vusb/vusb-20100715.tar.gz [following]
--2010-11-23 18:17:40--  http://www.obdev.at/ftp/pub/Products/vusb/vusb-20100715.tar.gz
Reusing existing connection to www.obdev.at:80.
HTTP request sent, awaiting response... 200 OK
Length: 417848 (408K) [application/x-gzip]
Saving to: “vusb-20100715.tar.gz”

100%[======================================>] 417,848     94.8K/s   in 4.3s  

2010-11-23 18:17:45 (94.8 KB/s) - “vusb-20100715.tar.gz” saved [417848/417848]
[mjd@onza src]$ tar -xzf vusb-20100715.tar.gz
[mjd@onza src]$

V-USB comes with several demos.  I chose the mouse demo because it doesn't require any extra hardware such as switches or chips.

I have made some changes to the mouse demo source to reflect that we're using different pins for the USB, and also to make the LEDs blink.  I've also changed the MCU type, the fuses, and the right command to run avrdude for my USB programmer:

--- vusb-20100715-orig/examples/hid-mouse/firmware/main.c
+++ vusb-20100715/examples/hid-mouse/firmware/main.c
@@ -124,9 +124,19 @@

 /* ------------------------------------------------------------------------- */

+static const unsigned char portCMask = _BV(PC0) | _BV(PC1) | _BV(PC2) | _BV(PC3);
+
+static void setLED(const unsigned char v)
+{
+    PORTC &= ~portCMask;
+    PORTC |= (v & portCMask);
+}
+
 int __attribute__((noreturn)) main(void)
 {
-uchar   i;
+    uchar   i;
+    DDRC |= _BV(PC0) | _BV(PC1) | _BV(PC2) | _BV(PC3) | _BV(PC4) | _BV(PC5);
+    PORTC |= _BV(4);

     wdt_enable(WDTO_1S);
     /* Even if you don't use the watchdog, turn it off here. On newer devices,
@@ -148,12 +158,16 @@
     usbDeviceConnect();
     sei();
     DBG1(0x01, 0, 0);       /* debug output: main loop starts */
+    unsigned char a = 0;
     for(;;){                /* main event loop */
         DBG1(0x02, 0, 0);   /* debug output: main loop iterates */
         wdt_reset();
         usbPoll();
         if(usbInterruptIsReady()){
             /* called after every poll of the interrupt endpoint */
+            ++a;
+            a &= 0b111111;
+            setLED(a >> 2);
             advanceCircleByFixedAngle();
             DBG1(0x03, 0, 0);   /* debug output: interrupt report prepared */
             usbSetInterrupt((void *)&reportBuffer, sizeof(reportBuffer));
--- vusb-20100715-orig/examples/hid-mouse/firmware/Makefile+++ vusb-20100715/examples/hid-mouse/firmware/Makefile
@@ -7,11 +7,11 @@
 # License: GNU GPL v2 (see License.txt), GNU GPL v3 or proprietary (CommercialLicense.txt)
 # This Revision: $Id: Makefile 692 2008-11-07 15:07:40Z cs $

-DEVICE  = atmega168
-F_CPU   = 16000000    # in Hz
-FUSE_L  = # see below for fuse values for particular devices
-FUSE_H  =
-AVRDUDE = avrdude -c usbasp -p $(DEVICE) # edit this line for your programmer
+DEVICE  = atmega328p
+F_CPU   = 16000000 # in Hz
+FUSE_L  = 0xff# see below for fuse values for particular devices
+FUSE_H  = 0xd9
+AVRDUDE = avrdude -c avrisp2 -P usb -p $(DEVICE) # edit this line for your programmer

 CFLAGS  = -Iusbdrv -I. -DDEBUG_LEVEL=0
 OBJECTS = usbdrv/usbdrv.o usbdrv/usbdrvasm.o usbdrv/oddebug.o main.o
@@ -125,6 +125,8 @@
 clean:
     rm -f main.hex main.lst main.obj main.cof main.list main.map main.eep.hex main.elf *.o usbdrv/*.o main.s usbdrv/oddebug.s usbdrv/usbdrv.s

+main.o: main.c usbconfig.h
+
 # Generic rule for compiling C files:
 .c.o:
     $(COMPILE) -c $< -o $@
diff -x usbdrv -X lufa-dontdiff.txt -u -r vusb-20100715-orig/examples/hid-mouse/firmware/usbconfig.h vusb-20100715/examples/hid-mouse/firmware/usbconfig.h
--- vusb-20100715-orig/examples/hid-mouse/firmware/usbconfig.h    2010-07-16 02:43:47.000000000 +1000
+++ vusb-20100715/examples/hid-mouse/firmware/usbconfig.h    2010-11-22 09:07:33.000000000 +1100
@@ -27,7 +27,7 @@
 /* This is the port where the USB bus is connected. When you configure it to
  * "B", the registers PORTB, PINB and DDRB will be used.
  */
-#define USB_CFG_DMINUS_BIT      4
+#define USB_CFG_DMINUS_BIT      3
 /* This is the bit number in USB_CFG_IOPORT where the USB D- line is connected.
  * This may be any bit in the port.
  */
@@ -116,7 +116,7 @@
 /* Define this to 1 if the device has its own power supply. Set it to 0 if the
  * device is powered from the USB bus.
  */
-#define USB_CFG_MAX_BUS_POWER           20
+#define USB_CFG_MAX_BUS_POWER           400
 /* Set this variable to the maximum USB bus power consumption of your device.
  * The value is in milliamperes. [It will be divided by two since USB
  * communicates power requirements in units of 2 mA.]

I applied this patch:

[mjd@onza src]$ cd vusb-20100715
[mjd@onza vusb-20100715]$ patch -p1 < ../vusb-20100715-mjd-1.diff
patching file examples/hid-mouse/firmware/main.c
patching file examples/hid-mouse/firmware/Makefile
patching file examples/hid-mouse/firmware/usbconfig.h
[mjd@onza vusb-20100715]$

Then compiled the software:

[mjd@onza vusb-20100715]$ cd examples/hid-mouse/firmware
[mjd@onza firmware]$ make hex
cp -r ../../../usbdrv .
avr-gcc -Wall -Os -DF_CPU=16000000  -Iusbdrv -I. -DDEBUG_LEVEL=0 -mmcu=atmega328p -c usbdrv/usbdrv.c -o usbdrv/usbdrv.o
avr-gcc -Wall -Os -DF_CPU=16000000  -Iusbdrv -I. -DDEBUG_LEVEL=0 -mmcu=atmega328p -x assembler-with-cpp -c usbdrv/usbdrvasm.S -o usbdrv/usbdrvasm.o
avr-gcc -Wall -Os -DF_CPU=16000000  -Iusbdrv -I. -DDEBUG_LEVEL=0 -mmcu=atmega328p -c usbdrv/oddebug.c -o usbdrv/oddebug.o
avr-gcc -Wall -Os -DF_CPU=16000000  -Iusbdrv -I. -DDEBUG_LEVEL=0 -mmcu=atmega328p -c main.c -o main.o
avr-gcc -Wall -Os -DF_CPU=16000000  -Iusbdrv -I. -DDEBUG_LEVEL=0 -mmcu=atmega328p -o main.elf usbdrv/usbdrv.o usbdrv/usbdrvasm.o usbdrv/oddebug.o main.o
rm -f main.hex main.eep.hex
avr-objcopy -j .text -j .data -O ihex main.elf main.hex
avr-size main.hex
   text       data        bss        dec        hex    filename
      0       1872          0       1872        750    main.hex

And now I can program the Doodad:

[mjd@onza firmware]$ su
Password:
[root@onza firmware]# make flash
avrdude -c avrisp2 -P usb -p atmega328p  -U flash:w:main.hex:i

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.01s

avrdude: Device signature = 0x1e950f
avrdude: NOTE: FLASH memory has been specified, an erase cycle will be performed
         To disable this feature, specify the -D option.
avrdude: erasing chip
avrdude: reading input file "main.hex"
avrdude: writing flash (1872 bytes):

Writing | ################################################## | 100% 0.65s

avrdude: 1872 bytes of flash written
avrdude: verifying flash memory against main.hex:
avrdude: load data flash data from input file main.hex:
avrdude: input file main.hex contains 1872 bytes
avrdude: reading on-chip flash data:

Reading | ################################################## | 100% 0.52s

avrdude: verifying ...
avrdude: 1872 bytes of flash verified

avrdude: safemode: Fuses OK

avrdude done.  Thank you.

[root@onza firmware]# make fuse
avrdude -c avrisp2 -P usb -p atmega328p  -U hfuse:w:0xd9:m -U lfuse:w:0xff:m

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.01s

avrdude: Device signature = 0x1e950f
avrdude: reading input file "0xd9"
avrdude: writing hfuse (1 bytes):

Writing | ################################################## | 100% 0.00s

avrdude: 1 bytes of hfuse written
avrdude: verifying hfuse memory against 0xd9:
avrdude: load data hfuse data from input file 0xd9:
avrdude: input file 0xd9 contains 1 bytes
avrdude: reading on-chip hfuse data:

Reading | ################################################## | 100% 0.00s

avrdude: verifying ...
avrdude: 1 bytes of hfuse verified
avrdude: reading input file "0xff"
avrdude: writing lfuse (1 bytes):

Writing | ################################################## | 100% 0.00s

avrdude: 1 bytes of lfuse written
avrdude: verifying lfuse memory against 0xff:
avrdude: load data lfuse data from input file 0xff:
avrdude: input file 0xff contains 1 bytes
avrdude: reading on-chip lfuse data:

Reading | ################################################## | 100% 0.00s

avrdude: verifying ...
avrdude: 1 bytes of lfuse verified

avrdude: safemode: Fuses OK

avrdude done.  Thank you.

[root@onza firmware]#

As a prelude to trying it I also filed away the end of the board so it can fit into a USB port.

Well after that, I power cycled the Doodad, and the second last LED is lit.  And if that wasn't impressive enough, After I plugged it into my computer's USB port, the mouse cursor started moving around in a large circle, and the LEDs are counting!

Here's what lsusb shows:

[root@onza mjd]# lsusb
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 002: ID 1a40:0101 TERMINUS TECHNOLOGY INC.
Bus 001 Device 003: ID 0951:1607 Kingston Technology Data Traveler 2.0
Bus 001 Device 004: ID 0951:1607 Kingston Technology Data Traveler 2.0
Bus 001 Device 005: ID 0951:1607 Kingston Technology Data Traveler 2.0
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 002: ID 16c0:03e8 VOTI free for internal lab use 1000

The mouse is the "VOTI" entry. The USB vendor ID and product ID can be set in the software.

Now to see the details:

[root@onza mjd]# lsusb -v

Bus 003 Device 002: ID 16c0:03e8 VOTI free for internal lab use 1000
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               1.10
  bDeviceClass            0 (Defined at Interface level)
  bDeviceSubClass         0
  bDeviceProtocol         0
  bMaxPacketSize0         8
  idVendor           0x16c0 VOTI
  idProduct          0x03e8 free for internal lab use 1000
  bcdDevice            1.00
  iManufacturer           1 obdev.at
  iProduct                2 Mouse
  iSerial                 0
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength           34
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          0
    bmAttributes         0x80
      (Bus Powered)
    MaxPower              400mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           1
      bInterfaceClass         3 Human Interface Device
      bInterfaceSubClass      0 No Subclass
      bInterfaceProtocol      0 None
      iInterface              0
        HID Device Descriptor:
          bLength                 9
          bDescriptorType        33
          bcdHID               1.01
          bCountryCode            0 Not supported
          bNumDescriptors         1
          bDescriptorType        34 Report
          wDescriptorLength      52
         Report Descriptors:
           ** UNAVAILABLE **
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            3
          Transfer Type            Interrupt
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0008  1x 8 bytes
        bInterval             100
Device Status:     0x0000
  (Bus Powered)
⋮
[root@onza mjd]#

I'm not sure why it says the report descriptors are unavailable.  But at least it works.

Now I'll start working on the bootloader.  If I can get the bootloader working, I can take the three pins we're currently using for ISP, and make them LED outputs.

The success continues!

USB Doodad 4: Blinkenleds

Progress on the USB Doodad has been going well with several successes.
After soldering on the processor, I added the reset pullup resistor, and rigged up a 6-pin programming header on the end of the board.

(Note this is a prototype at a later stage)

There's no permanent position for a programming header on the board.  This is because once the board has a bootloader, loading software on the Doodad will be done over USB.  Our plan is to use pogo pins to make a "bed of nails" that we can temporarily sit the Doodad board on to get the bootloader into it.

I plugged the Doodad into my programmer, and plugged the programmer into my PC.  There was no smoke (!), and the avrdude program could recognise the '328!  That's always a good sign.

[root@onza mjd]# /usr/bin/avrdude -c avrisp2 -P usb -p m328p -q

avrdude: AVR device initialized and ready to accept instructions
avrdude: Device signature = 0x1e950f

avrdude: safemode: Fuses OK

avrdude done.  Thank you.

[root@onza mjd]#

I then soldered a LED and a dropper resistor onto the pin for bit 5 of port C, searched for a simple "blinkenled" program on the 'net, and compiled it for the '328:

[mjd@onza 328hello]$ cat hello.c
#include <avr/io.h>
#include <util/delay.h>

int main()
{
    // Set bit 5 of port C to be an output
    DDRC |= _BV(PC5);

    while (1)
    {
        // Invert the value of pin 5 on port C
        PORTC ^= _BV(PC5);
        _delay_ms(500);
    }
}

[mjd@onza 328hello]$ avr-gcc -Os -DF_CPU=8000000 -mmcu=atmega328p -o hello.o -c hello.c
[mjd@onza 328hello]$ avr-gcc -Os -DF_CPU=8000000 -mmcu=atmega328p -o hello.elf hello.o
[mjd@onza 328hello]$ avr-objcopy -O ihex hello.elf hello.hex
[mjd@onza 328hello]$ ls -l hello.hex
-rw-rw-r--. 1 mjd mjd 582 2010-11-21 14:52 hello.hex
[mjd@onza 328hello]$

I then programmed it into the '328:

[root@onza 328hello]# /usr/bin/avrdude -c avrisp2 -P usb -p m328p -U flash:w:hello.hex
avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.01s

avrdude: Device signature = 0x1e950f
avrdude: NOTE: FLASH memory has been specified, an erase cycle will be performed
         To disable this feature, specify the -D option.
avrdude: erasing chip
avrdude: reading input file "hello.hex"
avrdude: input file hello.hex auto detected as Intel Hex
avrdude: writing flash (200 bytes):

Writing | ################################################## | 100% 0.08s

avrdude: 200 bytes of flash written
avrdude: verifying flash memory against hello.hex:
avrdude: load data flash data from input file hello.hex:
avrdude: input file hello.hex auto detected as Intel Hex
avrdude: input file hello.hex contains 200 bytes
avrdude: reading on-chip flash data:

Reading | ################################################## | 100% 0.06s

avrdude: verifying ...
avrdude: 200 bytes of flash verified

avrdude: safemode: Fuses OK

avrdude done.  Thank you.

[root@onza 328hello]#

Yay, the LED blinks!  But there's a slight problem: The LED should blink once a second, but I found it was actually blinking once every eight seconds.  This is because by default, there's a fuse in the '328 which divides the clock by 8.

AVR chips have a few bytes of persistent memory called "fuses".  The bits in these fuses control things such as what source the chip uses for clock, and what areas of memory are protected.  The name for the fuse that's dividing the clock by 8 is called CKDIV8.

Here's how I retrieved the fuse values:

[root@onza 328hello]# /usr/bin/avrdude -c avrisp2 -P usb -p m328p -t

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.01s

avrdude: Device signature = 0x1e950f
avrdude> dump lfuse
0000  62                                               |.               |

avrdude> dump hfuse
0000  d9                                                |.               |

avrdude> dump efuse
0000  07                                                |.               |

avrdude> quit

avrdude: safemode: Fuses OK

avrdude done.  Thank you.

[root@onza 328hello]#

I then looked up an AVR fuse calculator:

  http://frank.circleofcurrent.com/fusecalc/

Turning off the CKDIV8 fuse means the "low fuse" value changes from 0x62 to 0xE2.  The fuse calculator also helpfully shows the avrdude parameters to make this change:

[root@onza 328hello]# /usr/bin/avrdude -c avrisp2 -P usb -p m328p -U lfuse:w:0xE2:m

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.01s

avrdude: Device signature = 0x1e950f
avrdude: reading input file "0xE2"
avrdude: writing lfuse (1 bytes):

Writing | ################################################## | 100% 0.02s

avrdude: 1 bytes of lfuse written
avrdude: verifying lfuse memory against 0xE2:
avrdude: load data lfuse data from input file 0xE2:
avrdude: input file 0xE2 contains 1 bytes
avrdude: reading on-chip lfuse data:

Reading | ################################################## | 100% 0.00s

avrdude: verifying ...
avrdude: 1 bytes of lfuse verified

avrdude: safemode: Fuses OK

avrdude done.  Thank you.

I can then check that the fuse programming has worked:

[root@onza 328hello]# /usr/bin/avrdude -c avrisp2 -P usb -p m328p -t

avrdude: AVR device initialized and ready to accept instructions

Reading | ################################################## | 100% 0.01s

avrdude: Device signature = 0x1e950f
avrdude> dump lfuse
0000  e2                                                |.               |

avrdude> quit
avrdude: safemode: Fuses OK

avrdude done.  Thank you.

[root@onza 328hello]#

After doing this, the LED flashes once per second.

At this point, the '328's clock is being provided from an internal oscillator.  The internals oscillator is convenient if you want to keep the number of parts on the board to a minimum, but the frequency can drift with temperature, so it's not a great choice for a USB device.  So my next step was to solder on a quartz crystal and the two associated capacitors.

Back in the fuse calculator, I used the pull-down menu in the "low fuse presets" to choose "Ext. Crystal Osc.; Frequency 8.0- MHz; Start-up time PWRDWN/RESET: 16K CK/14 CK + 65 ms; [CKSEL=1111 SUT=11]".  That makes the low fuse value 0xFF.  So I programmed that in using avrdude as above:

[root@onza 328hello]# /usr/bin/avrdude -c avrisp2 -P usb -p m328p -U lfuse:w:0xff:m

⋮
⋮
⋮

avrdude done.  Thank you.

The LED is now flashing, but it's twice as fast as it should be.  This is because when I compiled the code, I said the clock frequency was 8MHz, to match the internal oscillator.  That's not going to work very well with a 16MHz external crystal.  So I recompiled the code:

[mjd@onza 328hello]$ avr-gcc -Os -DF_CPU=16000000 -mmcu=atmega328p -o hello.o -c hello.c

I won't show the linking and .hex file creation steps as they're the same as above, but after reprogramming the '328, the LED now blinks at 1Hz.

So, I have a board on which I can run software, and the means to program it.  Winnage!

USB Doodad 3: Hot-air soldering of TQFP-32 chips

The processor we're using in the USB Doodad is an AVR ATmega328P.  This very capable processor is at the heart of the popular Arduino educational microcontroller platform.

The two most common packages for the ATmega328P are the PDIP-28 and the TQFP-32.

  <Good place to insert picture of PDIP vs TQFP>

Because the whole purpose of the USB Doodad is to use surface mount components, we want to use the TQFP-32 package.

Image courtesy of SparkFun.

Armed with a USB Doodad prototype board that Ross etched, I thought it was time to try soldering this chip.

There are several ways to solder SMT ICs.  A good guide is in this SparkFun SMT tutorial.  The method I used was using solder paste and hot air.

In order to do hot air soldering, I use a "hot air rework station" which I bought for about $80 in China.  I found another one at Ameritronics which looks very nice, nicer than mine.

Ideally, I'd use a syringe of solder paste, but I don't have one.  Instead I used the paste I covered previously.

I applied my solder paste with a jeweller's screwdriver.  Not a very precise way of doing it, but it did the job.  I applied one thin line of solder paste along the line of IC pads, then squished the chip pins into the paste.  Then I ran some liquid flux along the pins on all four sides of the chip.

Next, I used the hot air to melt the solder on each corner of the chip.  That was to fix the chip into place.  Finally, I ran the hot air along each side of the chip in turn.  The combination of pins, solder paste and flux means that when the solder paste melted, it blobbed around each pin.  I didn't get any bridges between pins, but if I had, I'm confident I could have fixed it with some desoldering braid.

My success makes me think that beginners could solder it too, if they had the right equipment (a hot air rework station).  And if we had a board with a solder mask (as we'll surely have for Doodad a little further down the track), I think beginners could do it with a regular iron too, if they had a video to watch showing how it's done.

Well, that's how I soldered a TQFN-32!

USB Doodad 2: Board design and layout

Some time ago, Ross and I discussed and agreed on what we'd like this board to do (and perhaps just as importantly, what we don't need it to do).  How do we get from this stage to something people can assemble?  One way is by making a prototype.

A prototype lets us:

• Try out the circuit and different variations,
• Try out the PCB layout and variations,
• Try out the software and variations, and
• Try out different assembly ideas.

Ross started off by finding a project in Elektor, a famous English-language electronics magazine produced in the Netherlands.  What it has in common with the Doodad is that it uses an ATmega328P microcontroller, and uses V-USB to do software USB.

Using that project as proof of what is possible, Ross designed the circuit to our requirements, and did a PCB layout.  Because we may use the underside of the board for other purposes later, Ross faced the additional constraint of having to make the board single-sided.  He and I discussed and improved the circuit and the layout several times, and Ross reworked the circuit and layout to match.

In particular, if we use the same pins as the Elektor project, we'd only have 14 pins free for driving LEDs.  Ideally however, we'd like 16 LEDs.  We think by reassigning some pins, doing away with some features (for example, a bootloader exit button, and a reset button), and being creative with some circuit ideas (for example, using a resistor network to run a number of switches off some otherwise-unused ADC pins), we believe we can recover enough pins to give us 16 LEDs.  We'd also like to be able to use the SDA and SCK pins on the expansion port so we can easily attach I²C devices such as sensors and storage ICs.

In order to do this pin experimentation, the board currently has a number of solder pads we can link to reroute various signals.  Here's the current layout:

USB Doodad PCB artwork (v15)

On the left is the expansion port (+, - and two data pins).  On the right is the USB finger port.  Along the bottom is where the LEDs and dropper resistors will go.  In the top right are the USB analog components.  In the middle is the microcontroller, and to each side are the power supply and clock components.  The switches go in the top left.

While this board may not do everything we want it to, it will let us answer a lot of the questions we need to answer before we can produce a final board ready for sending to a manufacturing house.

I want to say that Ross has done a top job: The single-sided layout constraint is pretty fierce, and the quality of the prototype board is easily as good as a bought one.  I am learning a lot working with him.

USB Doodad: An SMT exercise project

Surface mount soldering doesn't have to be scary!

Way back in the distant past when I learned electronics, components were "through hole". That means that components such as resistors had a wire out each end, and these wires were bent and passed through holes in a circuit board, then soldered.

An example through-hole board.

These days, through hole technology is on the decline. It's becoming harder to get the newer integrated circuits in through-hole form. In its place, more and more people are using "surface mount" technology to build circuits. With SMT, components are often in the form of a tiny block, which is placed on a circuit board, directly in contact with the circuit board tracks, then soldered into place.

An example SMT board.

Many people I've spoken to think that using SMT is harder than through-hole. Sure, many SMT components are very tiny, but it's quite easy to buy larger sized SMT components.

As a project at our hackerspace, Connected Community Hackerspace, Ross McKenzie and I have been working on a project which shows people that doing SMT assembly is not as hard as people think.  We have deliberately used the largest size of SMT components to make assembly as easy as possible.

The board has a USB connector at one end, a small expansion connector at the other, and a row of 16 software controlled LEDs in the middle.

The primary purpose of the board is to act as a practical exercise in surface mount assembly.  The secondary purpose is to show how to use the V-USB software USB stack to make a USB device.  The third purpose is to give the Doodad some post-tutorial value, in a number of intentionally frivolous ways:

• Persistence-of-vision toy. Wave it around or put it on the wheels of your bike. Enjoy making groovy flowing messages in the dark.
• Secure password keystore.
• Log-data-to-serial-flash data logger, where samples could be retrieved over USB at some later time.
• “Messages waiting” / “processor load” / “build progress” indicator.
• Multimedia keys (if your keyboard doesn’t have them).
• Novelty breathalyser.
• Countdown timer for car parking reminder.

Our estimate for the cost of this board is about AUD30, so it's well within hobbyist reach.

We plan to release the design for this project under an open licence, possibly the TAPR OHL, which means that others can copy this design.

The project has been designed to use a single sided PCB.  Although this makes the job of designing and routing the board traces much more difficult, being single sided means people can make this board at home.

For applications which require the doodad to operate independently of a PC, we plan to add a small Li-Ion battery and management chip to the reverse of the board.

More information about the project can be found in the USB Doodad Google doc.

Ross and I are making rapid progress, and I'll post more project updates here soon.