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Latching 65816 bank byte in FPGA
http://forum.6502.org/viewtopic.php?f=10&t=8012		 Page 1 of 1
Author:  fachat [ Mon Mar 25, 2024 2:32 pm ]
Post subject:  Latching 65816 bank byte in FPGA
Hi there,

I am trying to speed up my Micro-PET design from 50MHz base / 12.5MHz CPU clock to 70 MHz base / 17.5MHz CPU clock.
I have a stable read timing now, but the system does not correctly read and process the bank byte in time it seems. With bank clamped to zero, I see correct memory accesses (at least at the initial vector pull and subsequent memory fetches - but not with how I try to use the bank byte.

Currently I try to latch the bank byte, and then derive select signals from it. Latching is not really a good FPGA design, registers are preferred, but registering the bank byte directly at the rising edge of phi2 leads to
a) either all select lines are delayed until after phi2 rises or
b) when selecting the data input vs the latch output depending on phi2 itself, the changing data lines already effect the select lines causing glitches.

Note in this example, phi2 rises with the falling edge of qclk (which is 50/70MHz), so registering the value at the rising edge - half a qclk before phi2 rises - avoids the glitches
Code:
        -----------------------------------------------------------------------
        -- CPU address space analysis
        --

        -- note: simply latching D at rising phi2 does not work,
        -- as in the logical part after the latch, the changing D already
        -- bleeds through, before the result is switched back when bankl is in effect.
        -- Therefore we sample D at half-qclk before the transition of phi2.
        -- This may lead to speed limits in faster designs, but works here.
        BankLatch: process(reset, D, phi2, qclk)
        begin
                if (reset ='1') then
                        bankl <= (others => '0');
                elsif (rising_edge(qclk) and phi2='0') then
                        if (forceb0 = '1') then
                                bankl <= (others => '0');
                        else
                                bankl <= D;
                        end if;
                end if;
        end process;

        bank <= bankl;


Now, this does not seem to be fast enough anymore. Any idea / best practice for this?
Author:  Yuri [ Mon Mar 25, 2024 11:01 pm ]
Post subject:  Re: Latching 65816 bank byte in FPGA
Correct me if I'm wrong (as I'm still learning the ins and outs of FPGAs myself), but it seems very odd that an FPGA running at 50MHz or more would be "too slow" for this task.

If the clock rates are 50/12.5MHz and 70/17.5Mhz, I'm presuming you have PHI2 derived from your main clock divided by 4; so it would appear that you are setting D several times before PHI2 goes high again.

At some point in that low phase of PHI2 the 65816 has to start switching those lines over from the multiplexed address bus to the values desired on the data bus; are you sure that 2nd or 3rd main clock cycle isn't picking up on the data pins settling to the new state?
Author:  fachat [ Mon Mar 25, 2024 11:37 pm ]
Post subject:  Re: Latching 65816 bank byte in FPGA
I am not sure what the real reason is.

The memory decoding is quite elaborate. I see a flickering in the RAM select line even when trying to read th reset vector. That indicates that the select is not quick enough.

Here is the mapper https://github.com/fachat/csa_ultracpu/ ... perPET.vhd
(Note this is not the main branch)

The relevant select line is vramsel. Outside the mapper (in Top.vhd) the select line is registered when phi2 goes high, so it is not registered in the mapper code.

I am still analyzing. The bank latch is one of the potential issues I identified, but it may not necessarily be the right or the only one...

André
Author:  BigDumbDinosaur [ Tue Mar 26, 2024 12:30 am ]
Post subject:  Re: Latching 65816 bank byte in FPGA
Yuri wrote:
At some point in that low phase of PHI2 the 65816 has to start switching those lines over from the multiplexed address bus to the values desired on the data bus; are you sure that 2nd or 3rd main clock cycle isn't picking up on the data pins settling to the new state?

Actually, the data bus “turnaround” doesn’t occur until after the rise of Ø2.  The bank bits persist for a few nanoseconds during Ø2 high.
Author:  fachat [ Tue Mar 26, 2024 6:56 am ]
Post subject:  Re: Latching 65816 bank byte in FPGA
The original version was something like:

A) there is a register for the bank, taking in the D lines on rising phi2.
B) depending on phi2, the actual value taken for address decoding would be either D itself when phi2 was low, or the latched value when phi2 was high.

When I was using this I saw glitches in the decoding because switching (B) was faster than the register (A), and the CPU was even faster. So changing D was visible on the bank before the register value came through.

At least that was my interpretation. That was with the XC95288XL CPLD, and I kept it. Now I'm at Spartan 6, so maybe I should revisit...
Author:  Yuri [ Tue Mar 26, 2024 9:25 am ]
Post subject:  Re: Latching 65816 bank byte in FPGA
BigDumbDinosaur wrote:
Yuri wrote:
At some point in that low phase of PHI2 the 65816 has to start switching those lines over from the multiplexed address bus to the values desired on the data bus; are you sure that 2nd or 3rd main clock cycle isn't picking up on the data pins settling to the new state?

Actually, the data bus “turnaround” doesn’t occur until after the rise of Ø2.  The bank bits persist for a few nanoseconds during Ø2 high.


Yea, saw that when I looked at the datasheet. Still though, I can't help but wonder if the extra qclk pulses during PHI2 being low could have an effect.

I think it would be interesting to see what the behavior might be if you sampled on a clock that was 45 degrees out of phase of PHI2 but had the same period.

Edit:

Did some more digging, looks like if I'm reading your schematics right you're running the 65816 @ 3.3V, the data sheet says that the bank address won't be ready until 40ns after PHI2 goes low. If you are sampling on the rising edge of qclk at 50MHz that'd be 20ns. It also means that you're barely giving the 65816 to setup those bank addresses before PHI2 goes high, at which point it will only hold the value for 5ns after that.

Attachment:
timings.jpg
timings.jpg [ 641.02 KiB | Viewed 3056 times ]


That's my reading of it at any rate. I'm sure someone who actually knows what they're talking about will point out how I'm wrong. :mrgreen:
Author:  BigEd [ Tue Mar 26, 2024 11:23 am ]
Post subject:  Re: Latching 65816 bank byte in FPGA
How about you clock both the data bus and the phi2 into flops. Then use this sampled phi2 to drive the mux.
Author:  fachat [ Tue Mar 26, 2024 11:13 pm ]
Post subject:  Re: Latching 65816 bank byte in FPGA
I might just as well be ignoring the FPGA compiler warning about latches and do this...:

Code:
        BankLatch: process(reset, D, phi2, qclk)
        begin
                if (reset ='1') then
                        bankl <= (others => '0');
                elsif (phi2 = '0') then
                        if (forceb0 = '1') then
                                bankl <= (others => '0');
                        else
                                bankl <= D;
                        end if;
                end if;
        end process;

        bank <= bankl;


So far only suggestions about my code.... any experiences you had yourself?
Author:  fachat [ Wed Mar 27, 2024 1:21 pm ]
Post subject:  Re: Latching 65816 bank byte in FPGA
As an update. I went with the actual latch. There might be opportunities for optimization later, but sonfar it works.

After careful examination of the signals I noticed that it can't really be the bank latch anyway - every phi1 phase is the same short intervall no matter if the effective CPU clock is 1 or 17 MHz. So as it runs on 1 MHz reliability, address decoding for 17 MHz should be working just as well as both have a 28ns phi1 phase (phi2 low).

I found the problems more in the data timing and after applying a timing constraint the systemnworks fine under 17MHz. Even when blown with a cold spray or a hot (resoldering) fan
Author:  Rob Finch [ Tue Apr 09, 2024 3:23 pm ]
Post subject:  Re: Latching 65816 bank byte in FPGA
I would not use a latch. Latch timing is very difficult to guarantee with an FPGA. I would use a higher frequency clock (eg. 140 MHz) and delay the phi2 clock by seven clock cycles, which would be the same as a phi2 one clock cycle in advance of the normal phi2. Then use the advanced phi2 edge to register the bank select. However,
You could also have the FPGA provide a phi2 clock to the system that is slightly different than the CPU’s phi2 clock so that address decoding can be made to work. For example, with eight clocks per phi2 the phi2 output by the FPGA could be low for five then high for three, delaying the CPU’s clock by an FPGA clock cycle. The bank could then be registered by the rising edge of the CPU’s phi2 while the peripherals would not see a system phi2 for another FPGA clock cycle.
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