conversion - VHDL - converting types and integer substraction

I am preparing a program in VHDL and I got stuck in type-conversion. I tryed google-search and also here at stack exchange, but I am quite confused since one answer contradicts other and neither one I can get to work. But finally to the question of mine: I have to display ordinary digital clocks. I have got from my professor decoder for 7 segment display which takes as input std_logic_vector(3 downto 0). And my part is to provide data (more specifically set of digits) to display. Most of work is already done but I am struggling to convert from my variables e.g. minutes (it is std_logic_vector(4 downto 0)) to two decimal digits. I have done separation this way:

if (min >= 50) then
  d3 <= 5;
  d4 <= min - 50;
elsif (min >= 40) then
  d3 <= 4;
  d4 <= min - 40;

elsif 
...
end if;

(Maximal value of min is 59 and if it get this high, it resets to 0 and hours counter get +1, just like ordinary clocks). Thus d3 should display tens of minutes and d4 unites of minutes. As you probably had already guessed, since min has length of 5, I can not compile it (d4 accepts length of 4 tops). Thus my idea is to convert min to integer, subtract number (e.g. 50) and convert back to std_logic_vector with right length (actual value would be in range 0 to 9, thus no problem with length of four bits). I tried to preform something like this:

d4 <= std_logic_vector(unsigned(integer(unsigned(min)) - 50));

but without success. I had always ended with errors like unknown function, type mismatch or no matching overload for method, no matter what combination I tried. Apparently I got somewhere some trivial error, but I fail to see it. I use those:

library ieee;
use ieee.numeric_std.all;

use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;

Please can you point me to the right direction? I am running out of ideas.

Thank you very much for your time, Michal

EDIT: Inputs, outputs, variables declarations as requested are:

port (sw2, sw1, sw0: in std_logic;  -- input switches
        clock:       in std_logic;
        reset:       in std_logic;

        d0, d1, d2, d3, d4, d5, d6, d7: out std_logic_vector(3 downto 0);  -- outputs for eight units of 7segment display decoders
        dp0, dp1, dp2, dp3, dp4, dp5, dp6, dp7: out std_logic);  -- outputs for decimal points

  attribute loc : string; 
  attribute loc of sw0   : signal is "P7";
  attribute loc of sw1   : signal is "P9";
  attribute loc of sw2   : signal is "P10";
  attribute loc of clock : signal is "P2"; 
  attribute loc of reset : signal is "P11"; 


end main_1048;

architecture main_1048arch of main_1048 is
begin
process(sw2, sw1, sw0, clock, reset)
variable yrs: std_logic_vector(11 downto 0);
variable min, sec: std_logic_vector(4 downto 0);
variable hrs, day, mth: std_logic_vector(3 downto 0);
begin

EDIT2: After deleting libs std_logic_arith a std_logic_unsigned as sugested by Brian Drummond and redefining variables so they are now naturals. New errors were No matching overload for "-". According to the specifications I found, "-" should work:

function "-" (L: NATURAL; R: UNSIGNED) return UNSIGNED;
  -- Result subtype: UNSIGNED(R'LENGTH-1 downto 0).
  -- Result: Subtracts an UNSIGNED vector, R, from a non-negative INTEGER, L.

Thus final edit was to d4 <= std_logic_vector(min - to_unsigned(50,4)); (with declaration variable min: natural range 0 to 59;) and this (concerning typecast) works.

Thank you all for your help!

Answer

First delete the non-standard std_logic_arith and std_logic_unsigned libraries, leaving std_logic and numeric_std.

As well as being non-standard, they cause problems through a VHDL rule that - if two definitions of a type (or procedure or operator or whatever) are visible - like unsigned or <= - VHDL will try to discover which you intended (e.g. by the parameter types of the operator, because overloaded operators are legal).

But if two definitions are visible and indistinguishable - both are considered to be hidden. This makes you FIX the problem instead of allowing the compiler to pick one - probably wrong - at random.

So with only the standard libraries, things get a lot simpler.

---

Now another basic rule is that if you are fighting with lots of type conversions, something is declared the wrong type, which points to a design error.

I'm going to guess the mistake : min should be declared as unsigned but isn't. (Ditto d3 etc). It represents an unsigned number in bit (std_logic) form. So declare it as such.

Fix that and see what happens. But first, find the source for the numeric_std package (the package spec, don't bother with the package body). You will see a lot of operators that allow unsigned and integer to interact directly.

Then I would expect your code snippet to work directly, without any type conversions.

If you don't actually need the bits of min, go further and declare it as a subtype of natural with range 0 to 59. There is no reason not to do so, even for synthesis, and for ports on a component.

It is usually appropriate to use unsigned (or even std_logic_vector if you must) for ports at the very top level, where you need to connect each bit to an FPGA pin. But internally, convert to the types most appropriate to the design as soon as you can.