• Please review our updated Terms and Rules here

Crystal Oscillators on 386 Motherboard

D

dvanaria

Experienced Member
Joined
Apr 11, 2013
Messages
110
Location
Denver
I have a 386-era motherboard (VIP M319P1 TAM/40-P1) which came with an Am386DX-40 processor, 64 K of L2 Cache (speed is 20 ns), and has eight ISA expansion slots on it. There is also 4 MB of main memory (speed is 70 ns).

There are 3 crystal oscillators on the motherboard that I can see:

KTS38 (small, barrel shaped)
14.31818 TQE (rectangular, silver)
KOYO KCO-110s 80.000 MHz 91-22 (larger, rectangular, silver)

I'm pretty sure the third one is the main system clock, and the CPU divides it by 2 (internally) to get 40 MHz.

I can't figure out what the other two clocks are for, can anyone help me figure this out?


Also, there is a jumper on the motherboard, "Bus Speed Select" that can be set to either "ATCLK/8" or "ATCLK/6". I'm pretty sure this is to set the ISA Bus speed, which ideally should be at 8.33 MHz I believe. But it's not clear what clock signal this jumper is dividing. Any help with this also would be greatly appreciated!
 
Last edited:
Ditto on the small round one--almost always 32.768KHz.

It doesn't really matter if you have a CGA or not. The 14MHz signal is defined on the ISA interface, so everyone provides it. It also provides a clock signal for the 8254 (or implied one) counter-timer, so it's really a requirement.
 
Chuck(G) said:
Ditto on the small round one--almost always 32.768KHz.

It doesn't really matter if you have a CGA or not. The 14MHz signal is defined on the ISA interface, so everyone provides it. It also provides a clock signal for the 8254 (or implied one) counter-timer, so it's really a requirement.
Click to expand...

Any idea what I should set the "Bus Speed Select" motherboard jumper to? (either "ATCLK/8" or "ATCLK/6")

I'm guessing it also has something to do with the ISA interface...
 
The 14.318 MHz signal is used by video cards. They can obtain it from the "OSC" ISA signal, but since VGA, most cards use their own 14.318 built-in oscillator instead. If you divide it by 3 you get the 4.77 MHz needed for the original PC CPU. And if you divide 4.77 MHz by 4 you obtain the 1.193180 MHz signal used to feed the timer (8253 in the old PCs, now part of the chipset) which is programmed to interrupt the CPU every 65536 cycles, so approximately 18 times per second (that's known as the timer "tick").

As the OSC signal in the ISA bus has to be always 14.318 MHz, you will have this frequency on any ISA PC even if it doesn't have any integrated video card. I don't know about modern PCs without ISA bus, but probably the OSC signal survives in one or another way.

The reason to use a 14.318 MHz (and having to divide it) instead a 4.77 MHz crystal was availability (so low price) because these clocks were used for TV electronics. In fact the 8088 worked at 4.77 MHz just because it was easy to derive that easy-to-find clock frequency, but the chip was rated for 5 MHz.

As you say, the 80 MHz clock is used as the system clock (2 x CPU clock) and the ~32 KHz signal is used for the CMOS chip which also maintains the real time clock (but I'm not sure that the KTS38 is a 32 KHz clock).
 
Last edited:
Rauli said:
The reason to use a 14.318 MHz (and having to divide it) instead a 4.77 MHz crystal was availability (so low price) because these clocks were used for TV electronics. In fact the 8088 worked at 4.77 MHz just because it was easy to derive that easy-to-find clock frequency, but the chip was rated for 5 MHz.
Click to expand...
Refer to the diagram at [here].
In my opinion, the primary reason for the use of 14.31818 (instead of say, 15 MHz) was to facilitate the generation of NTSC's 3.579545 MHz colour burst signal.
 
Rauli said:
As you say, the 80 MHz clock is used as the system clock (2 x CPU clock) and the ~32 KHz signal is used for the CMOS chip which also maintains the real time clock (but I'm not sure that the KTS38 is a 32 KHz clock).
Click to expand...

Why not, because of the "38" part of the part number? As far as I have been able to determine from a quick look around the net thru hole metal package tuning fork crystals from various vendors come in a few standard size ranges where that part of the part number indicates the diameter of the package measure in millimeters.

For example from one vendor an NC15LF is 1.5mm diameter max, an NC26LF is 2.0mm max, and an NC38LF is 3.2mm max. Another vendor has R145 at 1.5mm, R26 at 2.1mm, and R38 at 3.1mm. A third vendor has DS10 at 1.0mm, DS15 at 1.5mm, and DS26 at 1.8mm.

So my guess is that the KTS38 marking on that part just means that the package is somewhere in the 3-4mm range in diameter, and it doesn't mean anything about its frequency.
 
gslick said:
Why not, because of the "38" part of the part number?
Click to expand...

No, I'm not sure just because from the KTS38 marking I can't say that it's a 32 KHz clock.

gslick said:
So my guess is that the KTS38 marking on that part just means that the package is somewhere in the 3-4mm range in diameter, and it doesn't mean anything about its frequency.
Click to expand...

So we agree, don't we?
 
Last edited:
You must log in or register to reply here.
Back
Top