I'm confused, too. Why soldering, at all?
I thought the ISA bus clock is usually set in CMOS Setup..
With that big 4 pin quartz oscillator (50 MHz) being relevant for chipset/CPU clocking (actual used frequency is halve of this, 25 MHz).
That's why it's socketed, often, held by a zip tie sometimes. It's being replaced if the CPU is changed.
An ISA bus frequency of 10 or 12 Mhz is not too awkward, I think.
So CPU+Mainboard Clock (25 MHz) divided by 2 (12,5 MHz) or 3 (8,33MHz) makes sense to me.
Some fast 286 PCs from the 80s ran at 12 Mhz directly still, including the 'chipset' and ISA bus
That's because the 286' Front Side Bus (FSB) used to be the bus that connected each component with each other.
The ISA bus or AT bus was nothing different than the 286 signal lines on an edge connector.
Since this is a 386SX motherboard (a 286 derivative),
it maybe can handle that higher clock speed itself just fine ?
Just like it's 286 cousin ? 286 chipsets were still "recycled" for use with 386SX CPUs, after all..
Sometimes directly, sometimes with a little modification.
That's why 386SX boards were so affordable, afaik.
They had kept intelligent features, too, like shadow memory or Expanded Memory support.
Edit: To be precise, the 286 and 386 have an CLK2 input (just called CLK on 286).
This input will divide the incoming operation frequency by two (as coming from, say, a 50 MHz TTL oscillator).
Because it's merely measuring a rising/falling edge signal, I suppose.
It's up to the mainboard if it's doing the same (it does normally).
So it may run at 25 MHz or 50 MHz here, depends on the motherboard.
Edit: I rewrote the text partially. Hope it's better readable now. Though I still have my doubts. 😅
Edit: But what if that SX motherboard and its BIOS were originally made with the 386SX16 CPU in mind ?
The 386SX16 @16MHz was a popular flavor of the 386SX, after all.
Then clk/2 would be perfectly fine as a default setting for the BIOS (= 8,33 MHz). 🙂👍
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