# Posts Tagged “AVR”

A project I’ve been hacking on for a while is a self-contained 1-wire to IPv6 bridge based on an Atmel AVR ATmega644 and the ENC28J60 Ethernet controller from Microchip.

1-wire: is a serial bus from Dallas Semiconductor/Maxim that only requires 1 data line, there are a number of cheap sensors and other devices for this bus. The strength of this bus is not its speed but that it supports large ranges (up to 300 meters).
Also, each 1-wire device has a permanent unique 64-bit serial number.

IPv6: Insanely large address space. It’s common to use a 64-bit netmask for site networks so that EUI-64 based addresses can be used for auto configuration. This leaves 64-bit for the node address – do you see where this is going now… :)

Yes..I’ve built a device that assigned each 1-wire device it’s connected to its own IPv6 address. Why? you ask, mostly because I can.

### Hardware

As mentioned above, the device is based on an AVR ATmega644. It has 64KB of flash memory for program code and 4KB of RAM. It’s running on its built-in oscillator at 8MHz. The ENC28J60 Ethernet chip is connected to the AVR using SPI. The rest of the hardware is mostly for power distribution and management.

The PCB was manufactured by BatchPCB, cheap service but a bit slow turn-around time.

Unfortunately I screwed up the SPI connection but I managed to fix that with some green wires (or black wires in this case). You’ll note them in the picture above.
I also intended to run the AVR at 5V and the ethernet chip at 3.3V. This is what the quad AND-gate in the upper right
corner was for, but since I screwed up the SPI routing it’s disconnected and the whole circuit is running at 3.3V.
The ENC28J60 can only run at 3.3V, the AVR has a range from 2.8-5V and 1-wire should be ran at 5V but works at 3.3V. Hence the need for TTL voltage translation.

As for the 1-wire devices I had implemented a bus master in software that generated the require waveforms. It worked great up to about 10-15 meters. Any cable length greater than that refused to work.
This was a bit unexpected and without an oscilloscope it was more or less impossible to figure out where and how the signals got mangled. So I simply got a DS2480 1-wire line driver that generates the required signals in hardware with more precise timing.

Add-on board with a 1-wire master

This required an add-on board and because I didn’t want to wait for a new PCB I used a 2.54mm prototype board. With the DS2480 only available in SOIC8 packages it required some “creative” soldering :).
The DS2480 required 5V, thus It had to get its own power supply and also required level translation on the UART line between this device and the AVR. I choose an approach using MOSFETs and a few resistors for this (the TO92 packages in the picture above). This turned out to work really good and I think I’m going to use this for the SPI level translation in the next revision of the board.
The wire leaving the board on the left side leads to the 1-wire sensor devices.

The add-on board is extremely ugly. But hey, it works.

Future improvements for the next revision

• Use of external crystal at 16MHz instead of internal 8MHz clock.
• Use MOSFETs for 3.3-5 V translation. Need to test it at 16MHz before manufacturing a PCB though.
• Obviously fix all PCB errors :)
• All SMD parts (resistors and voltage regulators) to shrink PCB size even more.
• Better power distribution. I was a bit too conservative with the decoupling capacitors resulting in some weird power problems (fixable with some caps)
• Create a real add-on board

I’ll publish the PCB CAD files when the next revision is complete.

### Software

The only small IPv6 stack I know of is the uIPv6 stack in the Contiki operating system created by Adam Dunkel et al. This is unfortunately only available together with Contiki and not as a stand alone package as the originally uIP (IPv4) stack.

Contiki is a great operating system, but when you only have 4KB of RAM it becomes a bit heavy weight. So I broke out the uIPv6 stack from Contiki and made it run stand alone and ported in to AVR. I also ported the web server application from Contiki and made it run on AVR. As I wanted to use multiple IPv6 addresses I also had to add support for IP aliases to the uIPv6 stack.

Since the uIPv6 was integrated with Contiki it used the Contiki process model which it self is based on “proto-threads” (another thing invented by Adam Dunkel). I felt that this didn’t fit so I turned all processes into a polling mode instead. So one has to call a set of polling functions from the main application loop or from timers.

The other major parts of the code are drivers for ENC28J60, DS2480 and DS1820.

##### Software
1-wire devices
30 second polling interval with auto-discovery of new devices.
Each device is assigned its own IPv6 address, requires a /64 network to be available.
Webserver
Integrated web server makes it possible to visit each address. An XML file with the latest sensor reading is returned. An “age timestamp” is also provided which makes it possible to determine how old the reading is.

Currently, with 5 1-wire devices connected it uses about 3KB of RAM.

### In-action

I only have temperature sensors connected at the moment. If you happen to have an IPv6 capable connection you can access the sensors through a web browser.

(If you don’t have IPv6 you should get it, or you can view graphs based on the sensor values at lindberg.tl instead).

Handy system clock for AVR 8-bit microcontrollers suitable for measuring elapsed time or for use with timers. This provides
a monotonic time since system startup (like the POSIX CLOCK_MONOTONIC).

The system clock is based around a 32kHz clock crystal and one of the 8-bit timers provided by the AVR, it is possible to use the CPU frequency as a timer base as long as it’s a nice, dividable, frequency.
To be able to provide a stable 1 Hz clock but still have sub-second precision we divide 1 second into an arbitrary number of
system ticks. To minimize CPU usage the tick counter should be increased at each interrupt, this means that the number of
ticks per second we we choose determines our interrupt frequency and timer resolution.
How to choose number of ticks? It all depends on your required resolution, if you only want a 1 second resolution a tick and a second becomes equal.

The AVR timer is a 8-bit register that simply counts at the rate of its clock source. The clock source can be either the CPU
clock or an external oscillator, the clock source is also subject to a prescaler to further decrease the frequency.
The timer then generates an interrupt on overflow and/or when it hits a pre-configured value.
Since its a 8-bit timer, we have a maximum of 256 cycles before an interrupt is generated, a smaller interval can be achieved by using the comparator match to generate an interrupt at a specific value.

This example is creating a 1/32 second resolution timer (32 ticks per second) using an external 32768 Hz watch crystal.
comp is the comparator value, to avoid re-arming it with different values it should be limited to 128 or 256, otherwise it has to be changed at each interrupt.

$F_{timer} = 32768$
$ticks = 32$

The following to equations can be used to calculate either comp or the prescale value.
prescale is limited by the target device, but common values are powers of 2 (8,32,64,128,256,1024).

$comp = \frac{\frac{F_{timer}}{prescale}}{ticks}$
$prescale = \frac{F_{timer}}{comp \times ticks}$

Using 128 as the comp value and inserting the other values into equation 2 yields the following prescaler

$\frac{32768}{128 \times 32} = 8$

So, a prescaler of 8 gives us two interrupts per 256 cycles, one at 128 and one at 256 (overflow). Using 256 as comp would yield a perscaler of 4 but the target device I used didn’t have a TS/4 prescaler.

Complete source code for a 1/32 (or 31.25ms) second resolution timer for the ATmegaxx4 using Timer 2 and a 32kHz watch crystal connected to the pins TOSC1 and TOSC2.
Requires AVR libc.

#include <avr/io.h> #include <avr/interrupt.h>   typedef uint32_t clock_time_t; static clock_time_t global_system_ticks = 0;   /* ISR for the timer overflow */ ISR(TIMER2_OVF_vect) { global_system_ticks++; }   /* ISR for the comparator */ ISR(TIMER2_COMPA_vect) { global_system_ticks++; }   /* Return number of elapsed ticks */ clock_time_t clock_time() { return (global_system_ticks); }   /* Return number of elapsed seconds */ unsigned long clock_seconds(void) { uint32_t tmp;   TIMSK2 &= ~(1 << OCIE2A) | (1 << TOIE2); tmp = global_system_ticks / 32; TIMSK2 |= (1 << OCIE2A) | (1 << TOIE2); return (tmp); }   void clock_init() {   /* Enable external oscillator (32 kHz crystal) connected to TOSC{1,2} */ ASSR |= (1 << AS2);   /* Reset timer */ TCNT2 = 0;   /* Set TS/8 prescaler, results in a 4096Hz clock */ TCCR2B |= (1 << CS21);   /* Compare at half counter value */ OCR2A = 128;   /* * Enable overflow and compare interrupt. * Triggers each 1/32 secs */ TIMSK2 |= (1 << OCIE2A) | (1 << TOIE2); }