Bidirectional bus in Spartan-3

[rybol]

Hi,

I am using Spartan-3 XC3S200 and trying to build a bidirectional bus
('port_1'). Through this bus I put some data (data_ram_out) from internal
BRAM to the output of the chip (generator mode) in one situation (signal
'generator' =1) and sample data from input to BRAM (analyzer mode) in the
second situation (generator = 0). To make this possible, I used the template
from ISE Webpack:

buf_loop: for i in 0 to 7 generate
begin

IOBUF_inst : IOBUF
generic map (
DRIVE => 12,
IOSTANDARD => "DEFAULT",
SLEW => "SLOW")

port map (
O => data_0_7(i), -- Buffer output
IO => port_1(i), -- Buffer inout port (connect directly to
top-level port)
I => data_ram_out(i), -- Buffer input
T => not generator -- 3-state enable input
);

end generate buf_loop;

(My port and signals:

port_1 : inout std_logic_vector(7 downto 0);

signal data_0_7 : std_logic_vector(7 donwto 0);
signal data_ram_out : std_logic_vector(7 downto 0);
signal generator : std_logic;
)

But it seems like only analyzer mode is working ok. (when I make a little
change: ' T => generator' , only the generator mode works).
What could I do wrong? Did you ever use the IOBUF with success? Or tried any
other way of creating a bidirectional bus?

Regards,
Tomek

 

[Andrew FPGA]

Another way is:

--inside the architecture body
data_in <= data_bus;
data_bus <= data_out when drive_bus = '1' else (others => 'Z');

or

TristateDriver: process(data_out, drive_bus)
begin
if(drive_bus = '1') then
data_bus <= data_out;
else
data_bus <= (others => 'Z');
end if;
end process TristateDriver;

You can also use the .ucf file to assign slew rate, drive strength etc
properties to IO.

Regards
Andrew

 

[ajahn]

Hi Tomek,
you say, both modes work correct when you change the t => (not)
generator assignment?
Without taking a closer look, that sounds like your io-buffers work
okay, but you should check on what the generator signal does: Does it
switch the correct way?
Cheers,
Andreas.

 

[rybol]

Hi Tomek,

you say, both modes work correct when you change the t => (not)
generator assignment?
Without taking a closer look, that sounds like your io-buffers work
okay, but you should check on what the generator signal does: Does it
switch the correct way?

Of course, the generator signal 'didn't switch the correct way', but now
everything works fine.
Thanky you very much!

Tomek