problems with inout port

[Cazed]

Hi,

I'm trying to interface my fpga with a 8051-uC using the external memory
feature. My problem is that according to my simulations the bus does not
output anything back to aLow (aLow multiplexes low eight address bits and
bidirectional data) when I try to read...Writing data seems to be working
fine though...

Code:
library IEEE;
use IEEE.STD_LOGIC_1164.all;
use IEEE.STD_LOGIC_UNSIGNED.ALL;



entity interface is
port(
aHigh : in std_logic; --Address MSB only one bit needed
aLow : inout std_logic_vector(7 downto 0); --Address LSB
ALE : in std_logic; --adress latch
WE : in std_logic; --Write enable
RD : in std_logic; --Read data
--Internal registers
RAMctrl : buffer std_logic_vector(1 downto 0);
RAMaddress : buffer std_logic_vector(5 downto 0);
RAMdata : buffer std_logic_vector(12 downto 0));
end interface;


architecture behave of interface is
signal address : std_logic_vector(8 downto 0);
begin
--latch in address
process(aHigh, aLow, ALE)
begin
if falling_edge(ALE) then --Latch in address
address(8) <= aHigh;
address(7 downto 0) <= aLow;
end if;
end process;

process(address, WE)
begin
if rising_edge(WE) then --write to register
case address is
when "100000000" => NULL; --STATUS
when "110000001" => RAMdata(7 downto 0) <= aLow;
--CRDLSB
when "110000010" => RAMdata(12 downto 8) <=aLow(4 downto
0);--CRDMSB
when "110000011" => RAMaddress <= aLow(5 downto 0);
--CRADDRESS
when "110000000" => RAMctrl <= aLow(1 downto 0);
--CRCONTROL
when OTHERS => NULL;
end case;
end if;
end process;

process(address, RD)
begin
if falling_edge(RD) then
case address is
when "100000000" => NULL; --STATUS
when "110000001" => aLow <= RAMdata(7 downto 0);
--CRDLSB
when "110000010" => aLow(4 downto 0) <= RAMdata(12 downto
8);--CRDMSB
when "110000011" => aLow(5 downto 0) <= RAMaddress;
when "110000000" => aLow(1 downto 0) <= RAMctrl;
when OTHERS => NULL;
end case;
end if;
if rising_edge(RD) then
aLow <= "ZZZZZZZZ";
end if;
end process;
end behave;

Does the simulated input be anything special to allow the readprocess to
output data on aLow?

Regards
/Åke "Cazed" Forslund

 

[Mike Treseler]

I'm trying to interface my fpga with a 8051-uC using the external memory
feature. My problem is that according to my simulations the bus does not
output anything back to aLow (aLow multiplexes low eight address bits and
bidirectional data) when I try to read...Writing data seems to be working
fine though.

Consider adding a clock input and
synchronizing your processes, or better
yet, do it all in one process.

-- Mike Treseler

 

[Colin Paul Gloster]

Dear Åke Forslund,

One possible problem with the code is that you had

process(address, WE)
begin
if rising_edge(WE) then --write to register
case address is
when "100000000" => NULL; --STATUS
when "110000001" => RAMdata(7 downto 0) <= aLow;
--CRDLSB
when "110000010" => RAMdata(12 downto 8) <=aLow(4 downto
0);--CRDMSB
when "110000011" => RAMaddress <= aLow(5 downto 0);
--CRADDRESS
when "110000000" => RAMctrl <= aLow(1 downto 0);
--CRCONTROL
when OTHERS => NULL;
end case;
end if;
end process;

in which you mentioned only address, WE in the sensitivity list but
possibly performed assignments with aLow so I suggest putting aLow into
the sensitivity list too.

Similarly for the reading process, I suggest
address, RD, RAMdata, RAMaddress, RAMctrl
for the sensitivity list.

Overall your code looks nice, but I fear that I have not solved the
problem you reported.

I hope you manage to get it working.

Regards,
Colin Paul Gloster

 

[jens]

in which you mentioned only address, WE in the sensitivity list but

possibly performed assignments with aLow so I suggest putting aLow into
the sensitivity list too.

For an edge-triggered process without any asynchronous controls, the
clock (or WE in this case) is the only thing that should be in the
sensitivity list. Any signals inside the clocked part of the process
shouldn't be in the sensitivity list.

The read process looks kind of strange with two edges of RD being used,
it might not synthesize very well. I'd recommend something like this:

aLow <= aLowOut when (RD = '0')
else (others => 'Z');

(where aLowOut replaces aLow in the mux)

Something like that will be compatible with your tri-state buffer- the
RD signal directly controls the output enable.

Another note: the address is being clocked and not latched as the
comments indicate, if it were latched there will be more time for
address decoding (which may not be an issue if you're using a fast FPGA
and slow 8051).

 

[Dave Pollum]

Hi,

I'm trying to interface my fpga with a 8051-uC using the external memory
feature. My problem is that according to my simulations the bus does not
output anything back to aLow (aLow multiplexes low eight address bits and
bidirectional data) when I try to read...Writing data seems to be working
fine though...

Code:
library IEEE;
use IEEE.STD_LOGIC_1164.all;
use IEEE.STD_LOGIC_UNSIGNED.ALL;



entity interface is
port(
aHigh : in std_logic; --Address MSB only one bit needed
aLow : inout std_logic_vector(7 downto 0); --Address LSB
ALE : in std_logic; --adress latch
WE : in std_logic; --Write enable
RD : in std_logic; --Read data
--Internal registers
RAMctrl : buffer std_logic_vector(1 downto 0);
RAMaddress : buffer std_logic_vector(5 downto 0);
RAMdata : buffer std_logic_vector(12 downto 0));
end interface;


architecture behave of interface is
signal address : std_logic_vector(8 downto 0);
begin
--latch in address
process(aHigh, aLow, ALE)
begin
if falling_edge(ALE) then --Latch in address
address(8) <= aHigh;
address(7 downto 0) <= aLow;
end if;
end process;

process(address, WE)
begin
if rising_edge(WE) then --write to register
case address is
when "100000000" => NULL; --STATUS
when "110000001" => RAMdata(7 downto 0) <= aLow;
--CRDLSB
when "110000010" => RAMdata(12 downto 8) <=aLow(4 downto
0);--CRDMSB
when "110000011" => RAMaddress <= aLow(5 downto 0);
--CRADDRESS
when "110000000" => RAMctrl <= aLow(1 downto 0);
--CRCONTROL
when OTHERS => NULL;
end case;
end if;
end process;

process(address, RD)
begin
if falling_edge(RD) then
case address is
when "100000000" => NULL; --STATUS
when "110000001" => aLow <= RAMdata(7 downto 0);
--CRDLSB
when "110000010" => aLow(4 downto 0) <= RAMdata(12 downto
8);--CRDMSB
when "110000011" => aLow(5 downto 0) <= RAMaddress;
when "110000000" => aLow(1 downto 0) <= RAMctrl;
when OTHERS => NULL;
end case;
end if;
if rising_edge(RD) then
aLow <= "ZZZZZZZZ";
end if;
end process;
end behave;

Does the simulated input be anything special to allow the readprocess to
output data on aLow?

Regards
/Åke "Cazed" Forslund

I'm curious what aLow is assigned when the "others" statement is
executed. Does garbage get assigned to aLow?

When creating code to read registers, I've used something like this
("data" is "inout"):

process ( address, rd ) is
begin
data <= (others => 'Z');
if rd = '1' then
case address is
when "00" => data <= reg1;
when "01" => data <= reg2;
when "10" => data <= reg3;
when others => null;
end case;
end if;
end process;

-Dave Pollum

 

[Cazed]

Hi, and thanks for all the help,

I solved the initial problem and it was really located about 40 cm from
the screen . I seemed to have forgotten to unfreeze the signal aLow in
my simulation making any readable data on the port invisible.

However, I still get a bunch of warnings and I will check out all tips I
have gotten here.

Regards
/Åke Forslund

P.S. I apologize for mangling technical terms such as "Latch", I asdumed
the data was "Latched" since the signal ALE is described as a latch. I
will promptly look up the difference between clock and latch. D.S.

 

[ake.forslund]

Hi Jens,

after dealing with some other issues I returned to the 8051-interface
earlier today, looked at your recommendation, checked out the syntax in
my book, and finally saw how to use yor recommendation!

Now the module synthesises without any warnings and simulates
perfectly,

Thanks for the help!

/Åke "Cazed" Forslund