40mhz Quadra 660AV

[zigzagjoe]

This will be quick, time is short...here's some quick steps to run your 660av at 40mhz. Note this is using the unreleased 33mhz timings, so running at 40mhz is overclocking the RAM and VRAM. This changes the gestalt id to 79 instead of 60. I found appletalk to still work at 40mhz. None of this is not strenuously tested, so YMMV.

1. Get a 20mhz clock by doing one of the following
   1. Replace 12.5mhz oscillator with 20mhz oscillator
   2. Short pins 1 and 2 of J36 to disable onboard osc, and piggyback a new 20mhz on top
   3. Short pins 1 and 2 of J36 (GND) and inject a 20mhz clock signal on pin 3 of J36
2. Replace MC88920 with MC88916
   1. MC88916 should be at least 70mhz and preferably 80mhz rating
3. Move R140 to R142
4. Move R143 to R148
5. Add a heatsink to your 68040
6. (For system 7.1) Modify System enabler 088 gbly resource as below to add the gestalt ID for 660av-33mhz to the list of supported machines. Note these gestalts have an offset for 6 for some unknown reason.
7. Note: You'll have to modify the system file for newer OSes, I leave that as an exercise for the reader

(Before)


(After)


There's no way to use the 40mhz timings without ROM modifications, only 840AV has those timings and the lack of Nubus controller prevents that configuration from booting.

 

[Callan]

Great work! Looking forward to trying this out on mine!

 

[zigzagjoe]

Huh.... it's working at 50mhz. Appletalk too. I actually ran out of range on my signal generator. Vroom.

My 256K cache worked but only at 33mhz.

 

[zigzagjoe]

More stuff. I've attached the modified enabler for 7.1 and added a model string so the model name shows up as "Macintosh Quadra 660AV+".

If you're increasing the bus clock past 25mhz you need to make a few changes for SCSI. While it seemed OK without this change, I can't say what effect there might be on data integrity if this wasn't performed. So this changes things to match the 840AV.

1. Install a 25MHZ oscillator at G7 and an inductor at L26
2. Install two decoupling caps at C427, C428 on bottom of logic board
3. Move R108 to R100 on bottom of logic board

You can install a 66.6mhz oscillator at G5/G4 (SMD or TH) to clock the DSP at the same speed as in the 840AV. What does this change? Nothing, as far as I can tell, but hey, you can do it if you wish

The 2MB VRAM modification is an absolute pig and I really don't recommend it unless you love the machine and want the higher bit depths it makes possible. Seriously: It is a *lot* of work; if I had to guess, it probably took me 8 hours or so including some frustrating time to troubleshoot a few soldering issues (I hate SOJ/PLCC!). The only way you're getting these chips on/off safely is with hot air and a board heater/hot plate. Straightening the leads is a huge PITA, I found an extremely sharp pair of tweezers to be the best choice and you'll eventually find you can sort of unroll the leads with a bit of care. You will probably want NOS chips for the new ones, too... solder on some used chips will probably make it impossible to properly straighten leads.

Pins 14 (RAS) and 35 (SE) on each bank must be separated and wired together, then jumped to the appropriate resistor pack.



Lack of nubus and two SIMM slots aside, otherwise this is a pocket 840AV, technically a smidge faster due to the 33mhz timings. Somewhat disappointingly, the framebuffer and memory controller are a hair slower than the wombat if you control for clock speed and timings - not a huge difference, but I was surprised to see the regression. @Arbee, i'd love to peruse the ERS for the YMCA and CIVIC, if you have them

 

[Phipli]

the framebuffer and memory controller are a hair slower than the wombat if you control for clock speed and timings - not a huge difference, but I was surprised to see the regression.

Yeah, the 650 at 40MHz timings has faster video and CPU benchmarks compared to the 840av as well as you'd expect based on what you're seeing.

It's part of the reason that the wombat family of machines are so great

 

[zigzagjoe]

Yeah, the 650 at 40MHz timings has faster video and CPU benchmarks compared to the 840av as well as you'd expect based on what you're seeing.

It's part of the reason that the wombat family of machines are so great

The memory difference is easily explained by the lack of interleaving on the YMCA, but barring specific specs for the Civic it'd be hard to say how it differs from the DAFB variant in the MEMC. The datasheet for the MEMC did mention support for interleaving as well as page mode, but the wombat can't use interleaving, and page mode also seems to be for special cases only. So, it must be something else...

I've also been wondering about the 16MB/bank limitation with the YMCA causing to a max RAM of 68MB for the 660 or 128 for 840. There's an additional bit in the size register that seems to imply that 64MB/bank might be possible, but it'd require a lot of changes to the sizing code as it's currently written to space banks at 16MB intervals in memory during sizing and stitching.