How to calculate the RPM of a hometrainer with VHDL - vhdl

Question

I've got a problem; I need to calculate / measure the RPM of a hometrainer using a hall sensor and a magnet on the wheel, the hardware needs to be described in VHDL, my current method is this:
If the hall sensor detects a pulse, reset a counter
Increment counter every clockcycle
On the next pulse, store the previous value, reset, and repeat.
The code:
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
entity teller is
port(
hallsens : in std_logic;
counter : out std_logic_vector(15 downto 0);
areset : in std_logic;
clk : in std_logic
);
end entity teller;
architecture rtl of teller is
signal counttemp : std_logic_vector(15 downto 0);
signal timerval2 : std_logic_vector(15 downto 0);
signal lastcount : std_logic_vector(15 downto 0);
begin
process(clk, areset)
begin
if areset = '1' then
counter <= "0000000000000000";
counttemp <= "0000000000000000";
timerval2 <= "0000001111101000";
elsif hallsens = '1' then
counter <= lastcount + "1";
timerval2 <= "0000001111101000";
counttemp <= "0000000000000000";
elsif rising_edge(clk) then
timerval2 <= timerval2 - "1";
if timerval2 = "0000000000000000" then
lastcount <= counttemp;
counttemp <= counttemp + "1";
timerval2 <= "0000001111101000";
end if;
end if;
end process;
end rtl;
But to calculate the RPM from this I have to divide the counter by the clockspeed, and multiply by 60. This takes up a lot of hardware on the FPGA (Altera Cyclone 2).
Is there a more efficient way to do this?
TIA

Answer 1

I don't have a computer at hand now, but I'll try to point different things I see:
Don't mix numerical libraries (preferably only use the numeric_std) #tricky suggests.
If handling numerical values, and including libraries for that.. you can should use numerical types for signals (integer, unsigned, signed..) it makes things clear and helps to distinguish numeric signals and no numercial-meant signals.
Hallsens is read as a pseudo-reset, but is not in the sensitivity list of the process, this could cause mismatches between Sims and hw. Anyway this is not a good approach, stick with a simple reset and clock pair.
I would detect hallsens within the clocked region of the process and increment the counter of events there. It should be simpler.
I'm assuming your hallsens asserted time is wide enough to be captured by the clock.
Once timer signal has reached zero (I'm assuming this gives you a known time based on your clk frequency) you can reload again the timer (as you do), output the count value and reset the counter, starting again.
For math operations 1/Freq and *60, you could use some numerical tricks if needed, based on the frequency value.. but you could:
multiply by inverse of frequency instead of dividing.
approximate it to sums of power of 2. (60 = 64-4)
make Freq to be multiple of 60 to simplify calcs.
Ps: to be less error prone, you can initialize your vectors (as theyre multiple of 4) in hex format like: signal<=X"0003" avoiding big binary numbers.