VOGONS

386 Clock Generator Replacement

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First post, by feipoa

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Laying around for nearly 15 years, I had several left-over PLLs from a project I was involved in to run my 486 motherboard at more custom Front-side Buses. These PLLs consisted of:

NB3N502DG
512MILF
502MILF
PI6C4511WE
ICS570ALF
ICS541M
ICS542M (this one is a divider)

I decided it would be convenient to solder one of these PLL multipliers onto a protoboard for use on a 386, similar to what was done here: https://migronelectronics.bigcartel.com/produ … g-underclocking

I do not know which PLL part the above unit is using; there are reports from people who have received one of these that the part number has been rubbed off. Most PLL multipliers which can take an external crystal should work for our purposes. I decided to use the 512MILF PLL, which according to digikey, has been discontinued. I believe NB3N502DG will also work since the pinouts are identical. With today's inflation, these chips now cost about $6 USD.

My goal was to get finer tuning in the 80-100 MHz range, thus I am using crystals from 14.318 MHz to 20.0 MHz (20 pF). With these crystals and multiplier adjustments (4x, 5x, 5.33x, 6x) , we can achieve these results in the 80-100 MHz range (and beyond): 80.0, 81.29, 81.92, 85.33, 85.91, 86.70, 87.38, 88.67, 90.00, 92.16, 94.58, 96.00, 97.54, 98.30, 100.0, 102.4, 104.86, 106.4, 108, etc.

The footprint dimensions of the protoboard is 17.75 mm x 10.15 mm, which looks to be smaller than the above reference unit due to not having an overhang.

Some photos of the working prototype:

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Last edited by feipoa on 2024-11-04, 08:44. Edited 1 time in total.

Plan your life wisely, you'll be dead before you know it.

Reply 1 of 3, by feipoa

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There is another 386 clock generator replacement project I've been working on which, for the most part, renders this prototype obsolete. For those not familiar with the other 386 clock generator project, the link is here: Project: Full Can Clock Oscillator Replacement That particular unit can be programmed to any frequency using an Arduino with the Arduino IDE run in "Arduino ISP" mode. That unit also contans two buttons which can increase or decrease the operating frequency by any desired amount.

Here is a photo of both prototypes next to each other:

The attachment 386_clock_gen_next_to_Wiretap_PCB_buttons.JPG is no longer available

An oscilloscope image of the protoboard prototype running at, I think, 85.33 MHz:

The attachment 386_clock_generator_replacement_scope_85MHz.JPG is no longer available

Here's a table showing which multipliers (pins S0 & S1) yield which operating frequency with various crystals:

The attachment 386_clock_generator_replacement_frequency_table.JPG is no longer available

For those who want to assemble their own, either using a protoboard as I've done, or by designing a PCB, here's my rough connection sheet:

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I forgot to note in that sheet that there is a 10 nF capacitor between Vcc and GND. My crystals are 20 pF. For X1 and X2, I went through a few calculations from various datasheets, and one showed I needed 23 pF caps, the other 16 pF caps. I am using 20 pF caps.

Plan your life wisely, you'll be dead before you know it.

Reply 2 of 3, by stamasd

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It's a nice project. I've done something similar with a SI5351 frequency synthesizer controlled via an arduino.

I/O, I/O,
It's off to disk I go,
With a bit and a byte
And a read and a write,
I/O, I/O

Reply 3 of 3, by feipoa

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stamasd: I didn't realise there was another SI5351 project. Do you have a photo of yours? How are you handling cold-start frequency output? My SI5351 based clock generator is shown above (green PCB). It no longer requires an external Arduino since there's an ATtiny85 on the PCB.

Plan your life wisely, you'll be dead before you know it.

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