[Oberon] RISC5 implementation issues.

[Walter Gallegos]

Have a solid and coherent clock distribution is basic for FPGA design, 
my proposition was keep both 25MHZ and 75MHZ, generated by DCM.

Run all in 75 MHZ is unnecessary; also, clock enable approach add 
unnecessary complexity to the design. Make a design synchronous don't 
necessary means use the same clock for all the design.

Continue using 25MHZ for the core and peripherals, the only concern is 
the CDC (clock domain crossing). As both clock was generated by the same 
DCM is minor problem; correspondent rising edges are aligned by design 
in DCMs; metastability is not an issue. Using DCMs and constraining 
input clock (50MHZ) the constraints propagation rules constraint both 
clocks, 25MHZ and 75MHZ. If no methodology errors all design are 
constrained from first to last register element in the chain. Beware of 
combinational logic in outside this elements.

In my opinion, taking care of CDC keep both clock is the appropriate 
solution.

Best regards,
Walter

El 2016-02-18 a las 19:58, Magnus Karlsson escribió:
> So I have been thinking about this some more and decided to 
> modify/update the design to remove all the concerns raised by Walter 
> and Wojtek.
>
> Just to recap, Walter's concern is that the clocks are generated using 
> flip-flops and use logic fabric interconnect instead of dedicated 
> clocking elements and pathways, and that all clocks should be 
> generated by a DCM module instead (DCM = Digital Clock Manager).  
> Wojtek's concern is that there are unspecified timing relations 
> between the 25MHz and the 75MHz clock domains.
>
> Both concerns are valid and in my opinion the correct way to fix both 
> issues is to make the design completely synchronous.  This means that 
> all clocked elements in the design (like flip-flops, memories etc.) 
> should be clocked with a single clock signal, which in this case is 
> the 75MHz clock.  The CPU and I/O subsystem, which before was clocked 
> by a separate 25MHz clock, are now also clocked by the 75MHz clock but 
> are only enabled to be clocked on every third clock cycle.  This means 
> that all "always @ (posedge clk)" statements have been changed to 
> include "If (enable) ...", where "enable" is a signal that is true on 
> every third clock cycle.  The asynchronous SRAM interface is also 
> changed so that the write signal is asserted on the middle-third clock 
> phase of the three clock CPU cycle.
>
> While the Verilog changes to do this are very straight forward, one 
> complication here is that the Xilinx ISE tool used to create the bit 
> file for the FPGA do not understand that the CPU and I/O subsystem are 
> only clocked on every third clock and will basically try to make the 
> CPU run at 75MHz, and will fail since this is too fast the FPGA.  The 
> solution to this problem is to tell the tool that all clock paths in 
> the CPU and I/O subsystem can actually take three clocks to complete 
> (this is called multi-cycle paths). With the multi-cycle paths added 
> to the .ucf file the design compiles with no timing violations
>
> With those changes the 75MHz clock is now generated by a DCM and the 
> unspecified timing relations that Wojtek brought up are now gone since 
> everything is clocked with a single clock.   The modified design have 
> been tested on Pepino and seems to run fine.
>
> The complete ISE project with those changes are available at the 
> Pepino GitHub repository: 
> https://github.com/Saanlima/Pepino/tree/master/Projects/RISC5Verilog_Pepino
>
> Any comments or critique are welcome.
>
> Cheers,
> Magnus
>
>
>
> On 2/17/2016 2:16 PM, Walter Gallegos wrote:
>> Hi Magnus,
>>
>> You are welcome to continue with FPGA specific topics by private 
>> e-mail if you want.
>>
>> Regards
>> Walter
>>
>> El 2016-02-17 a las 18:30, Magnus Karlsson escribió:
>>> Hi Walter,
>>>
>>> Since this is really Paul's design, I guess it would be more 
>>> appropriate to discuss it with him, I was just trying to explain why 
>>> it looks like it does.
>>>
>>> Cheers,
>>> Magnus
>>>
>>>
>>> On 2/17/2016 1:15 PM, Walter Gallegos wrote:
>>>> Magnus,
>>>>
>>>> Some of messages was delayed; so, I continue from here to not 
>>>> overload the list.
>>>>
>>>> If I understand you correctly, you justify a uncontrolled delay 
>>>> because they simplify the SRAM handling.
>>>> Sorry, is as using the old circuit with an and/inverted to generate 
>>>> a pulse. If you need a delayed signal you should use the DCM 90°, 
>>>> 180° or 270° clock outputs and keep all under control, I think 
>>>> don't need a state machine in this case.
>>>>
>>>> About ISE warnings, be careful, non warning do not means good 
>>>> methodology.
>>>>
>>>> About XILINX docs; really, I don't remember. Doing training, first 
>>>> as Xilinx ATP and now as independent consultant, I touch this 
>>>> problem in my trainings. Have an uncontrolled delay in clock is a 
>>>> big door to random problems. FPGA design must be synchronous all 
>>>> times; no exceptions.
>>>>
>>>> Regards,
>>>> Walter
>>>>
>>>>
>>>> El 2016-02-17 a las 14:41, Magnus Karlsson escribió:
>>>>> Walter,
>>>>>
>>>>> I agree with you that the "pure" way of doing this is as you 
>>>>> stated, with a DCM to directly generate both clk and pclk. So how 
>>>>> come Paul didn't do that?  It's not like he doesn't know how to 
>>>>> use the DCM, after all the current code generates pclk from clk 
>>>>> using a DCM, and there would probably be less code to do it like 
>>>>> you suggest.  No, the reason for this is very subtle and is easy 
>>>>> to miss if you just take a quick look at the code, and it has to 
>>>>> the asynchronous SRAM interface.
>>>>>
>>>>> One of the most critical aspects of using SRAM is to control the 
>>>>> write signal - ideally the write signal should be asserted after 
>>>>> all other control signals (like address, data, byte-enable, read, 
>>>>> oe) are valid, and should be de-asserted well before any of the 
>>>>> other control signals go invalid, to avoid spurious writes. 
>>>>> However, this is not that easy to do in a synchronous system where 
>>>>> all signals change at the clock edge.  The most common way to do 
>>>>> this is to have a state machine that is clocked at say 4x the CPU 
>>>>> clock so that you can divided the SRAM access cycle into several 
>>>>> phases and assert the write signal on some of those phases.
>>>>>
>>>>> However, this is not the way Paul choose to do it, instead he 
>>>>> choose to do a less "pure" clock generation by generating clk from 
>>>>> a flip-flop rather than from a DCM. By doing so, he actually 
>>>>> generates an early version of the clock signal called clk that is 
>>>>> leading the global clock signal clk_BUFG by the delay of the BUFG 
>>>>> buffer.  Since this early version of the clock signal is generated 
>>>>> like any other logic signal, he could use this signal to gate the 
>>>>> write signal to the SRAM such that write signal will be 
>>>>> de-asserted well before the other control signals (clocked by 
>>>>> clk_BUFG) will change, and thus avoiding the need to have a state 
>>>>> machine controlling the write signal.  The price for this is that 
>>>>> the clock signal is now generated in a less "pure" way, but still 
>>>>> a valid way as long as you know what you are doing. The BUFG clock 
>>>>> driver can be driven from a PLL, a DCM or from the logic fabric.  
>>>>> The first two are speed optimized paths going directly from the 
>>>>> PLL or DCM to the BUFG and can be clock at much higher clock rate, 
>>>>> while the logic fabric path is limited by the maximum clock rate 
>>>>> of the logic fabric. However, at the clock rate we use (25 MHz) 
>>>>> this is not an issue.  When you do this there are no warnings 
>>>>> generated by ISE that this is not a good idea, and I have not read 
>>>>> anywhere in the Xilinx clocking resource guide that you should 
>>>>> avoid doing this. Basically, the BUFG clock driver is designed to 
>>>>> do this, the tool will allow you to do it and at the clock rate we 
>>>>> use it has no performance implications.  As I see it, this is 
>>>>> another place where the goal of simplification has driven the 
>>>>> implementation of the system at the expense of a slightly less 
>>>>> "pure" clock generation.
>>>>>
>>>>> Just my 2c
>>>>>
>>>>> Magnus
>
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-- 

Walter Daniel Gallegos
Programmable Logic & Software
Consultoría, Diseño, Entrenamiento.
Montevideo, Uruguay
EMAILwalter at waltergallegos.com   
Tel +598 26 23 44 60 | Cel +598 99 18 58 88


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