Category Archives: Trinket

Pi Controlled NeoPixel Ring

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OK, so not 100% accurate; a Pi controlled Adafruit Trinket controlled NeoPixel ring 😀

I got the idea from seeing Dave Whale’s serial based Arduino interface with the Pi. The Trinket doesn’t have serial so I had to use something else to talk to it.

Enter I²C;  on the Trinket side I’m using the TinyWireS library, on the Pi side I’m calling smBus.writeList() with a list of the bytes I wish to send.  You’ll notice in the video there’s an extra breakout in the circuit, this is a bi-directional level converter to allow the 5v Trinket to talk to the 3.3v Pi without releasing any magic smoke.

My aim is to implement the commands in the Neopixel library however at the moment it just calls setPixelColor with the 4 values (LED, Red, Green, Blue) provided by the Pi. I have a special case of LED==0xFF which lights all LEDs the specified colour (a concept I have pinched from David).  I have included the code below in it’s current (probably hacky) state.

One problem I did find is that the Pi randomly “loses” the I²C address of the Trinket and reports it as 0x03 for a second or two.  I’m not sure if the problem is the Pi or Trinket end but it seems to be an issue with Arduinos in general.  The basis of the work around I used in my Python code can be found here. If anyone knows the reason for this I would be most grateful; it’s a horrible hack

Arduino Code

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#include <TinyWireS.h>
#include <Adafruit_NeoPixel.h>
 
#define PIN 1
#define I2C_SLAVE_ADDRESS 0x04
 
// Parameter 1 = number of pixels in strip
// Parameter 2 = pin number (most are valid)
// Parameter 3 = pixel type flags, add together as needed:
//   NEO_KHZ800  800 KHz bitstream (most NeoPixel products w/WS2812 LEDs)
//   NEO_KHZ400  400 KHz (classic 'v1' (not v2) FLORA pixels, WS2811 drivers)
//   NEO_GRB     Pixels are wired for GRB bitstream (most NeoPixel products)
//   NEO_RGB     Pixels are wired for RGB bitstream (v1 FLORA pixels, not v2)
Adafruit_NeoPixel strip = Adafruit_NeoPixel(16, PIN, NEO_GRB + NEO_KHZ800);
 
// The default buffer size, Can't recall the scope of defines right now
#ifndef TWI_RX_BUFFER_SIZE
#define TWI_RX_BUFFER_SIZE ( 16 )
#endif
 
int rxIndex = 0; 
 
volatile uint8_t i2c_regs[] =
{
    0x0, // RegisterID
    0x0, // Pin
    0x1, // R
    0x0, // G 
    0x0, // B
};
const byte reg_size = sizeof(i2c_regs);
 
//Handles receiving i2c data.
void receiveEvent(uint8_t howMany)
{
    if (TinyWireS.available()){  
      if (howMany < 1)
      {   // Sanity-check
          return;
      }
      if (howMany > TWI_RX_BUFFER_SIZE)
      {   // Also insane number
          return;
      }
 
      if (howMany == 1)
      {   // This write was only to set the buffer for next read
          return;
      }
      rxIndex = 0;
      while(howMany--)
      {   //Gets i2c data. 
          i2c_regs[rxIndex] = TinyWireS.receive();
          rxIndex++;
          if (rxIndex >= reg_size){
            if (i2c_regs[1] == 0xFF) {
              for (uint8_t p=0;p<strip.numPixels();p++){
                strip.setPixelColor(p,strip.Color(i2c_regs[2], i2c_regs[3], i2c_regs[4]));
              }
            }
            else {
              strip.setPixelColor(i2c_regs[1], strip.Color(i2c_regs[2], i2c_regs[3], i2c_regs[4]));    
            }
            strip.show();
            rxIndex = 0;
          } 
      }
 
    }
}
 
void setup() {
  TinyWireS.begin(I2C_SLAVE_ADDRESS);
  TinyWireS.onReceive(receiveEvent);
 
  strip.begin();
  strip.show(); // Initialize all pixels to 'off'
}
 
void loop() {
  TinyWireS_stop_check();
}

Pi Code

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import time
import smbus
import os
import subprocess
 
i2cBus = 0
i2cAddr = 0x04
bus = smbus.SMBus(i2Bus)
debug = False
 
def writeList(reg, list):
    sent = False
    attempt = 0
    while sent==False:
        if debug:
            print "I2C: Writing list to register 0x%02X:" % reg
            print list
        try:
            bus.write_i2c_block_data(i2cAddr, reg, list)
            sent = True
        except IOError:
            #a 'hack' to catch the Arduino disappearing at random intervals
            #runs i2cdetect to refresh the i2c interface, output to /dev/null so we don't see the output
            FNULL = open(os.devnull, 'w')
            retcode = subprocess.call(['i2cdetect', '-y', str(i2cBus)], stdout=FNULL, stderr=subprocess.STDOUT)
            time.sleep(0.1)
            attempt += 1
            if attempt == 10:
                raise Exception('Unable to connect via I2C')
 
def setLED(led, red, green, blue):
    bytes= [led, red, green, blue]
    writeList(0x01, bytes)
 
setLED(0xFF, 0, 0, 0)
time.sleep(0.5)
setLED(0xFF, 32, 32, 32)
time.sleep(0.5)
setLED(0xFF, 0, 0, 0)
 
try:
    while True:
        for p in range(0, 16):
            setLED(p, 32, 0, 0)
            time.sleep(0.1)
        for p in range(0, 16):
            setLED(p, 0, 32, 0)
            time.sleep(0.1)
        for p in range(0, 16):
            setLED(p, 0, 0, 32)
            time.sleep(0.1)
        for p in range(0, 16):
            setLED(p, 0, 0, 0)
            time.sleep(0.1)
except KeyboardInterrupt:
        pass
 
setLED(0xFF, 0, 0, 0)

Baby steps into Arduino

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I have always programmed in VB (5, 6, .NET even VBA!), I’m also starting to look at C# again after a brief dalliance 5 years ago, and of course I have a passable knowledge of Python from working with the Raspberry Pi.

Ever since picking up and playing with the Pi I have become aware of these Arduino thingies. What really peaked my interest were the wearables like GEMMA and FLORA and the work of Charlotte Godley. They’re small, uncomplicated (in theory) devices to hack about on fairly easily.

Fast forward to the beginning of the month at the July CamJam and I was enticed by The PiHut and their wares, eventually crumbling and buying an Adafruit Trinket along with a NeoPixel ring (mmmm 16 LEDs!). After getting the IDE and drivers setup (more on that later) I loaded the obligatory blink sketch, which is the Arduino equivalent of ‘hello world’.

Flushed with success (or something like that) I moved on to the NeoPixel ring, wired it up and loaded the Larson Scanner sample. It needed a couple of tweaks; namely the data pin and the number of pixels, but it worked very well.

Ultimately it was time to write my first ever bit of Arduino (or C for that matter) code, cribbing from and building on the examples I had come across thus far.

The embedded video shows alternating green and purple filling up the ring, followed by yellow and red. I was working on this while helping run Pi sessions the tech zone at Gilwell 24 so the patterns arise from the Scout brand colours and the colours of my Scout scarf respectively. My code, for what it’s worth, is below:

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#include &lt;Adafruit_NeoPixel.h&gt;
 
#define PIN 1
 
// Parameter 1 = number of pixels in strip
// Parameter 2 = pin number (most are valid)
// Parameter 3 = pixel type flags, add together as needed:
//   NEO_KHZ800  800 KHz bitstream (most NeoPixel products w/WS2812 LEDs)
//   NEO_KHZ400  400 KHz (classic 'v1' (not v2) FLORA pixels, WS2811 drivers)
//   NEO_GRB     Pixels are wired for GRB bitstream (most NeoPixel products)
//   NEO_RGB     Pixels are wired for RGB bitstream (v1 FLORA pixels, not v2)
Adafruit_NeoPixel strip = Adafruit_NeoPixel(16, PIN, NEO_GRB + NEO_KHZ800);
 
void setup() {
  strip.begin();
  strip.show(); // Initialize all pixels to 'off'
}
 
void loop() {
  // Scout green and purple
  spin(strip.Color(132/4, 164/4, 11/4),strip.Color(77/4, 33/4, 119/4));
  // BWV Explorers red and yellow
  spin(strip.Color(255/4,0, 0), strip.Color(255/4,255/4, 0));
}
 
void spin(uint32_t c1, uint32_t c2){
  uint32_t c;
 
  // fill in reverse, so highest pixel number first
  for(int8_t p=strip.numPixels() - 1; p&gt;=0; p--){
    // set even pixels to colour 'c1', odd to 'c2'
    if (p%2==0){
      c = c1;  
    }
    else{
      c = c2;
    }
    cycle(0,p,c);
  }
  delay(100);
}
 
void cycle(int8_t f, int8_t l, uint32_t c){
 
  for(uint8_t i=f;i&lt;=l;i++)
  {
      // clear last pixel, set current pixel
      strip.setPixelColor(i-1, 0);
      strip.setPixelColor(i, c);
      strip.show();
      delay(100);
  }
}

Things I Learnt

The USB is temperamental – After much faffing and cursing I discovered in the Adafruit FAQ about reducing the chip_erase_delay. This made things much happier on my laptop!

Updating AVRDUDE helped too – After working well on another laptop my home one refused to work again. A few Google searches later and the AdaFruit forum came to the rescue with AVRDUDE v1.15.13

Commands in C must end with ‘;’ – Obvious I know but the number of times I forgot!

For loops work differently – I’m used to a For … Until construct (i.e. For n = 0 to 15 or for n in range(0 to 16)). It took me ages to work out why my for (n= 15; p=0; p--) didn’t work correctly!

Debugging is hard – The Trinket has no serial console so you can’t see what the code is doing. To resolve one issue I ended up downloading Tiny C Complier (TCC) and creating test code to see what I was doing wrong. For reference a uint_8 of 0 minus 1 is 255 not -1 because it’s unsigned – doh!

All in all it’s been a very fun learning curve and I’m looking forward to trying out a few more AdaFruit examples. Oh wait, what’s that in the next article?