Altera FPGA ring oscillator low level implementation

Thanks for the responses,

The way that I have been trying to implement up to this point is the following.

For each component of the ring oscillator (i.e. 4 buffers 1 inverter) I implement by making use of the LUT_input/LUT_output primitives. This is an example of a buffer, note pre_toggle_1 is simply the reset passed and pre_toggle is the ring_invert signal, the signal that wraps around from the end of the ring oscillator.

--------------------------------------------------------

delay1_lut_in_1: LUT_INPUT

PORT map(

a_in => reset,

a_out=> pre_toggle_1

);

delay1_lut_in_2: LUT_INPUT

PORT map(

a_in => ring_invert,

a_out=> pre_toggle

);

pre_toggle_2 <= (pre_toggle_1) AND (pre_toggle);

delay1_lut_out: LUT_OUTPUT

Port map(

a_in => pre_toggle_2,

a_out => ring_delay1

);

-----------------------------------------------------------------------

I then tried mapping all 3 elements to a particular location on the chip by use of the assignment editor.

-to (these names were found by using the node finder)

|lcd_test|ring_osc:\GEN_RO:0:XILINX_RO|delay1_lut_in_1

|lcd_test|ring_osc:\GEN_RO:0:XILINX_RO|delay1_lut_in_2

|lcd_test|ring_osc:\GEN_RO:0:XILINX_RO|delay1_lut_out

-assignment name

LOCATION

as well as

Implement as output of logic cell

-value

LCCOMB_X14_Y35_N0

--------------------------------------------------------------------------------------

The following components of the ring oscillator are then mapped to sequential cells, so 1 particular ring oscillator could have the locations all within the same LAB:

LCCOMB_X14_Y35_N0 - buffer

LCCOMB_X14_Y35_N2 - buffer

LCCOMB_X14_Y35_N4 - buffer

LCCOMB_X14_Y35_N6 - buffer

LCCOMB_X14_Y35_N8 - inverter

-----------------------------------------------------------------------

Additionally I tried using the attribute keep statements

attribute KEEP : string;

attribute KEEP of pre_toggle : signal is "true";

attribute KEEP of pre_toggle_1 : signal is "true";

attribute KEEP of ring_delay1 : signal is "true";

---------------------------------------------------------------------------

Do I need to use an LCELL to implement the functions and not LUT_input/LUT_output?

Also do you guys know of a way that I can verify that the placements are actually still there after synthesis. At this point it compiles without error, but with that warning. Is there a way that I can see that the placements have been respected?

Thank you for your help.

Paul