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Stopwatch AVR applications

Stopwatches with AVRs in assembler
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Stopwatches with AVRs

Stopwatches are time measurement devices. What does this have to do with AVRs? Aren't those swiss made mechanical devices with the three knobs better than a micro? Nevertheless, here is the necessary theory to do it with a micro. And even some practical devices.

1 What it needs to have an own stopwatch design

Now, we need
  1. a clock generator to measure times, and
  2. a display to read the measurements results, and
  3. a few knobs or keys to start, stop and restart the measurements, and
  4. an AVR (or whatever micro you prefer).

2 A closer look on these components

2.1 Clock generators

AVRs all have an internal clock generator (only a few very old ones don't). This usually works roughly at 1.0 MHz. "Roughly" means that this is not really exact, more the "near-by" type of accuracy: guaranteed at +/- 10%, depending from operating voltage and temperature. +/- 10% means: if you measure one minute, the result could be by six seconds smaller or larger than expected. If you do some extra work in holding the operating voltage within a small range and by adjusting the calibration byte) you can reduce those inaccuracies down to 2% (which still means that your minute is by 1.2 seconds inaccurate. Those who do not need more accuracy, e.g. for cooking eggs, is done with the 10%. For a stopwatch measuring times of a 100 m hurdle run on three tracks you are done with the 2% because inaccuracies are all the same for the four runners. But if you want to compare those times with those of thousands of other races, it needs to be more exact.

More exact is a crystal oscillator. Crystals on the market have inaccuracies of between 30 and 50 ppm, so with this we are at 0.005% inaccuracy. Over one minute that means 3 ms inaccuracy, close enough for the amateur's needs.

Those who still want to have it more exact, can For fanatic accuracy fans: time measuring is always as exact as the weakest chain link in the whole chain.

Which crystal to select? Because the more exact stopwatch has to measure 10 ms (normal requirement) or 1 ms (fanatic requirement), the crystal's frequency has to be divisable without remainder by 100 or 1,000. Because timers in an AVR have divider rates of 8 or 64, this also haS to fit into the frequency. When using CTC with a timer (Clear Timer on Compare), divider rates between 1 and 256 (with an 8 bit timer) or between 1 and 65,536 (with a 16 bit timer) has to fit in between. And: if you operate the stopwatch with batteries the clock frequency shall not be too high to reduce the power consumption of the AVR.

With an 8 bit timer crystal frequencies of 2.0, 2.048 or 8.0 MHz are convenient. With 2.048 MHz your timer can divide by a prescaler of 8 and then by 256 and you are at 1,000 Hz, so you do not need CTC and the timer overflow can interrupt. With a 16 bit timer additionally a crystal with 4.0 MHz can be used (prescaler 8, CTC at 499).

With an external crystal (or crystal oscillator) to be attached, one third of the AVR types are not compatible with this requirement, because they do not allow that.

2.2 Display

Unless you do not need a display which 50,000 people in a sporting stadium can see, only LCDs come into question. For a battery operation LCDs are a must. As those are available mostly in 4.5 V and above (only a few are at 3.3 V) your battery has to be 3 times 1.5 V AA or AAA size or 4 times 1.2 V rechargeables.

In case of the stadium size display: use two micros. The first for measuring only and the second for display only. And in between those two add two or three lines for communication.

An LCD needs four or eight data bus connections and two or three control connections between the micro and the LCD (see more theory on that under this page here). That makes a minimum of six and a maximum of 11 portpins.

AVR types, minimum version, internal RC AVR types, maximum version, internal RC To the left the available AVR types for the minimum version, to the right those for the maximum version are listed. Both are based on the internal RC oscillator clock.

Hint: The selections were made with the software described here and is available for free download and use.

AVR types, minimum version, crystal AVR types, maximum version, crystal Here the same list but with an external crystal or crystal oscillator as clock. That reduces the possible types largely to the ones listed here.

2.3 Knobs and keys

Each stopwatch needs some keys. At minimum those are:
  1. the reset key, by which the time can be cleared (alternatively you can remove the battery, and
  2. a Start/Stop key to start and stop measuring.
If the stop watch shall be able to store more than one time info (e.g. for comparison), an additional "Store" key is required. If it should be able to measure times on up to four different tracks simultaneously, you need further N keys. A minimum of two and a maximum of 10 pins for keys so are necessary. This limits the number of possible devices further.

Note for C-freaks: From five tracks on and above your favourite type is not among the available AVRs for this task.

Because all keys toggle (some not immediately but later on when corrosion has done its work), protection against this is a must, either by hard- or by software. Hardware protection against toggling e.g. looks like that:

Hardware toogle protection Voltage on input pin after key release When tied to ground with the key, the input pin immediately shows low. When released, the voltage on the capacitor of 1 µF is slowly charged with the internal pull-up resistor of 50 kΩ. It needs roughly 45 ms to reach a high level (3.0 V at 5.0 V operating voltage). If the operating voltage is 3 V, the load process lasts slightly longer (roughly 55 ms).

During that time period, further toggle pulses unload the capacitor again and restart the charging period, but do not lead to low-high signals. That suppresses all signals before and during the load period, but does not lead to delays in detecting key closures immediately.

This simple solution has some serious disadvantages:
  1. The capacitor unloads immediately over the key on closure. This causes a very high peak current. Increased ageing and corrosion of the key contacts are the consequence, those can even fail completely.
  2. When the supply voltage is turned off, the voltage on the VCC pin is dropping very fast. The voltage on the capacitor is still high and exceeds the VCC voltage. The capacitor then will supply current to the input pin and, by that, can destroy this sensible CMOS pin.


Protected debouncing To avoid those consequences the following can be done:
  1. Between the electrolytical capacitor and the key a resistor limits the unload current. The resistor should not be too large to not cause delays above 1 ms between key closure and voltage drop below the High==>Low voltage level. 100 Ω are sufficient.
  2. Between the capacitor and the port input pin a Schottky diode blocks reverse current flow into the pin. The reverse current is small enough to not harm the CMOS pin.


The necessary additional hardware should make clear that a software solution is the simpler and easier alternative to hardware debouncing with an electrolytic capacitor.

Software toggle protection is easy in a stopwatch because the software needs to look at all the inputs every 10 resp. 1 ms. Times in between those periods do not matter, so no external interrupts (INTn or PCINTn) are necessary. So key events can be included in the time measurement scheme. And: key toggle protection can also be included there.

2.4 Which AVR?

With a 4 bit LCD data bus and up to four keys the ATtiny24 is a convenient selection and has all necessary pins. Such a project can use the ATtiny24-LCD module described here. Control of the LCD can use the include software decribed there, but adds a few modifications. The example in Stopwatch_tn24 is driven with this LCD software.

If the LCD should be driven in 8 bit mode or if more keys than four are required, either
  1. ATmega8 or ATmega8A, or
  2. ATmega48(A, PA) or ATmega88(A, PA)
in a 28 pin casing come into question. Those who need more than six keys or need a RS232 or an I2C interface (e.g. for an external display) change to a 40 pin type such as ATmega16 or ATmega8515.

In Stopwatch_m8 a four-channel stopwatch with ATmega8 is described.

Two projects that should cover 90% of all needs.

Praise, error reports, scolding and spam please via the comment page to me.

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