One signal for which all hardware designers have to bow is the almighty CLOCK. In the previous exercise, you’ve already connect up with this signal. The clock on the PYNQ Z2 runs at 125MHz and therefore has a clock period of 8 ns.
As the final aim of this lab is to flash lights like a Christmas tree, having fancy light effects change every 8 ns would be a little quick to the eyes.
The easiest way to do something in the “human-perceptible-order-of-magnitude” is to use a pre-scaler: simply have a counter count in loops. Every time the counter hits a certain threshold, a small tick is given.
Having a counter go to 125’000’000 would result in 1 tick every second. So, to have 2 ticks per second, the counter needs to count to 62’500’000.
The available libraries in the design tools allow for easy counting with integers. Counting with the STD_LOGIC-type is a bit more cumbersome.
...
signal tick : STD_LOGIC;
constant C_PRESCALER_MAX : integer := 125000000/2;
signal prescaler: integer;
begin
P_PRESC: process(clock_i)
begin
if rising_edge(clock_i) then
if reset_i = '1' then
prescaler <= 0;
tick <= '0';
else
if prescaler = C_PRESCALER_MAX then
tick <= '1';
prescaler <= 0;
else
tick <= '0';
prescaler <= prescaler + 1;
end if;
end if;
end if;
end process P_PRESC;
...As a good hardware designer you also test your design using testbenches. When you do things at the speed of around 1 Hz, simulation would take very long. For simulation it would be more convenient that the tick is generated every 250, or so, clock cycles.
It goes without saying that you can just overwrite the value of the constant C_PRESCALER_MAX, from the previous example, for simulation. Before generating a bitstream this could be CTRL+Z-ed. Alas, if I take myself as an example, one forgets to do this. Also, when you want to simulate again, you have to go change the constant again.
A bit of a dirty hack can be a way around this.
...
signal tick : STD_LOGIC;
constant C_PRESCALER_MAX : integer := 125000000/(2
-- synthesis translate_off
* 312500
-- synthesis translate_on
);
signal prescaler: integer;
...The use of pragmas can be compared to C’s #ifdef. The pragma used above, disables the synthesis tool between the translate_off and translate_on statements.
For more (on) pragmas, see here, or ask Google.
For this exercise use a pre-scaler to have all four LEDs flash at 1 Hz. To be more specific: the LEDs are on for half a second and the off for half a second.
For this exercise, each LED flashes at double the frequency of its left neighbour. The left-most LED continues flashing at 1 Hz (as in the previous exercise).