Midterm 2 Design Problem

In this design example, we are going to design a Slot Machine

Assume Port D is wired as shown in the table below.

		Direction	DDRD bit(s)	Type / Initialization	PORTD
PD bits 3-0	switches	Input	000	Passive Input	1
PD bit 4	Win light	Output	1	Initially Off	0
PD bit 5	new Account button	Input	0	Active output of a DFF	0
PD bit 6	add Account button	Input	0	Active output of a DFF	0
PD bit 7	Lose light	Output	1	Initially Off	0

We begin by defining SRAM variable account and initializing Port D as defined in the table.

Now let’s generate a clock to the two D flip-flops so we can read our new and add account button.

Given clock is 20 Mhz. I want to clock the DFF at a Frequency is 20Khz.

Alternative wording:  I want to cycle and test if the button is pressed every 50 microseconds.

If I want to use timer 2, what divide frequency will I need to do that? What would you need to load into Timer 2 to generate this delay?

Calculate Max delay given the following information. You are using Timer 2, a clock frequency of f_clk = 20 MHz, and a clock divider of ÷8. Timer 2 is an 8-bit timer so the maximum number of tics is 2⁸ = 256 tics. To convert to time we need to equate tics to time.

f_1/0  =  f_clk  /8 = 2.5MHz           t_1/0=1/f_1/0 =0.4 µsec/tic

t_max = 0.4µsec/tic × 256 tics = 102.4 µsec

So our timer with given conditions can generate a 25 microseconds delay, now let’s look at what we need to preload our counter with to get a delay of 25 microseconds.

25 µsec ÷ 0.4 µsec/tics = 62.5 tics (Rounding down we leave it at 62). So we would need to preload timer 2 with a value of 256 – 62 = 194₁₀ = 0xC2

We can now use polling or an interrupt service routine to generate our clock.

Now let’s generate the subroutines to be called when the user sets a dollar amount into 4-switches and presses the new account or add account button.

Converting this table into bytes to be saved in Flash.

0x0123  wheel: .DB 0x47, 0x56, …..0x2A