Write a C program for the $AT90$ that uses the input captures system of the $16-$bit

timer$/$counter to measure the frequency at pin ICP$1(=$PD$4).$

Pin PD$4$ is connected to a PWM frequency generator of the lab board. The generator has $16$

settings that differ in frequency $($up to $400kHz)$ and duty cycle. Button "FREQ SEL" allows you

to change between settings. The current setting is displayed as a hexadecimal number on the

component labelled $"$FRQ$".$

Example settings:

Setting $0=$ off

Setting $1=1Hz$ wave

o Setting $5=1kHz$ wave

The frequency is measured using the "frequency method" from the lecture: Over a certain

measurement duration $d,$ the number $e$ of rising edges are counted.

The frequency $f$ is displayed in $Hz$ as an $8-$bit value on the LED panel $($Port C$).$

To account for frequencies that exceed the $8-$bit range, the switches on Port A can be used to

change the unit $($e$.$g$.$ to $0\mathrm{.}1Hz,1Hz,100Hz).$

Values higher than $255$ are displayed as $255.$ Hence, a value of $255$ indicates that the

switches should be used to change to another unit.

More specifically, the switches effect the

measurement duration $d,$

timer$/$counter prescaler, and

a multiplier for the displayed value $f$

as follows:

Switch PA$7$ switches between a 'configuration mode' and a 'measurement mode'.

In the 'measurement mode', the white lamp $($PB$6)$ is turned on to indicate ongoing

measurements. Measurements are being taken repeatedly until PA$7$ is turned to

'configuration mode'. Whenever the frequency measurement is updated, the value is

displayed on Port C$.$

In the 'configuration mode', the white light is off and no frequency measurements

are conducted. Instead, the user can configure the measurement process via

switches PA$0,$ PA$1,$ PA$2,$ PA$3.$ The LEDs on PC$0,$ PC$1,$ PC$2,$ PC$3$ light up according to

the switch positions.

If PA$0=$ off, then a prescaler of $8$ is set $($resulting in timer$/$counter increments every $1us).$

If PA$0=$ on$,$ then a prescaler of $1$ is set $($resulting in timer$/$counter increments every $125ns).$

PA$2$ and PA$1$ set the measurement duration as follows:

PA$2=$ off, PA$1=$ off $d=10$ measurement duration

PA$2=$ on$,$ PA$1=$ off $d=1$ s measurement duration

PA$2=$ on$,PA1=$ on $d=10$ ms measurement duration

To handle very high frequencies, $PA3=$ on allows to divide the number of rising edges $e$

detected over the measurement duration to be divided by $100$ before being displayed.