Accurately measures a pulse on external interrupt (e.g. from a diode) and displays the duration on three 7-segment displays driven by three 74HC595 shift registers. The provided code allows the use of either an ATmega 328P (and related), an ATtiny 45/85 and an ATtiny 44/84(A).
Old analog cameras have a shutter that is often entirely mechanical. After decades of use or storage, even more so in unknown conditions, the reliability of indicated shutter speeds is questionable. This is the code for a device which measures the shutter speed by pointing a constant-on light source from the backplane of the camera through the shutter at a photodiode. When shooting, the photodiode gets hit by the light source and the resulting pulse gets evaluated by this piece of code.
Depending on target, CPU clock speed (F_CPU), prescaler values and register names are naturally different in order for the measurement to be precise.
The 8-bit Timer0 is being used.
With 16 MHz clock speed and a prescaler value of 64, a granularity of 4 µs can be achieved. In this case, the uint16_t counter variable which counts timer rollovers will itself roll over after ~67 seconds. With 8 MHz clock speed and a prescaler value of 8, a granularity of 1 µs can be achieved. The aforementioned counter variable will roll-over after 16 seconds.
Since in photography, the most common highest camera speeds available are at 1/1000s - 1/4000s, this device must and can achieve an accuracy in the order of 10e-5 s (10 µs). Therefore, its three-digit display outputs values, depending on the order of magnitude, in the following format, from lowest to highest:
1.23for 1.23 ms (and, accordingly,0.25for 0.25 ms)12.3for 12.3 ms123for 123 ms1.23.for 1.23 s12.3.for 12.3 s123.for 123 s, provided that no roll-over happens beforehand (see above).
Basically, an appended dot at the end of the value is not meant to be a decimal dot, but signifies that the value being displayed is expressed in seconds rather than milliseconds.
In its standard operating mode, the device takes one-shot measurements. The operator would need to copy the results manually and evaluate them separately. This is where the sequential mode comes into play: It is meant to take multiple measurements at a single shutter speed setting and then display the arithmetic mean, minimum and maximum values in order for the operator to get a sense of the deviation and distribution of the results. In this mode, the number of measurements taken before evaluation is up to the user. It is engaged and disengaged with a simple pushbutton press.
In order to transmit the measured values to a connected device via serial connection, the code has implemented a bit-banged UART transmission at 9600 bauds only with the ATtiny84A. The serial connection uses the 16-bit Timer1 and the PA5 pin for output (Pin 1 on the 6-pin AVR ISP connector). The values transmitted are in microseconds, each followed by a \n (newline character).
This way, any further analysis and processing of the values can be done from a connected computer. No longer manually copying values into a spreadsheet!