RetroBrew Computers Forum: General Discussion » Micro 8088

Home » RBC Forums » General Discussion » Micro 8088 - Function of C21 Trimmer Cap

Show: Today's Messages :: Show Polls :: Message Navigator

Micro 8088 - Function of C21 Trimmer Cap [message #7271]

Tue, 17 March 2020 10:35

damienc
Messages: 4
Registered: March 2020

Junior Member

I just recently built a Micro 8088 board. Sadly, I forgot to order a PS/2 jack but I decided to fire it up just to make sure it booted. So I can't currently do much besides see the POST screen.

I'm using a Fujitsu 8088-1 and a Faraday FE2010A that I pulled from my uncle's old Samsung S330 XT clone that I was given years ago. The Micro 8088 board boots fine with these, although I noticed that the reported CPU speed was 7MHz. I expected something around 9.5MHz because this is what speed it used to run at in the Samsung with turbo enabled.

I'm pretty sure that I can use a key combination to get the maximum CPU speed, but I was wondering if the trimmer capacitor C21 on the Micro 8088 serves some sort of function in relation to the 28MHz crystal oscillator. It certainly appears that way per the schematic.

I didn't adjust the trimmer at all post-install, so if it does perform some function related to the oscillator it sort of makes sense that if the trimmer is in a "middle" position the CPU would be roughly somewhere between 4.77MHz and 9.5MHz. I have no real clue though, this is just my uneducated guess.

My PS/2 jack came in the mail today so I can fiddle with it some more after work. But in the meantime, can anyone tell me the purpose/function of the C21 trimmer cap?

Thanks

Report message to a moderator

Re: Micro 8088 - Function of C21 Trimmer Cap [message #7273 is a reply to message #7271]

Tue, 17 March 2020 11:04

damienc
Messages: 4
Registered: March 2020

Junior Member

Maybe I answered my own question...

C21 is in series with the oscillator's output which goes to terminal 21 of the FE2010A, which is crystal input. It seems clear to me that the trimmer cap is used for adjusting the output frequency of the crystal.

Would like someone with more experience than myself to confirm that adjusting the cap would affect the oscillator output and therefore the CPU frequency, although I'm pretty sure that's the case.

Report message to a moderator

Re: Micro 8088 - Function of C21 Trimmer Cap [message #7275 is a reply to message #7273]

Tue, 17 March 2020 12:51

Jonas
Messages: 77
Registered: October 2015

Member

The C21 trimmer cap allows adjustment of the frequency at a precision level (very small adjustments).

It is possible to switch the clock frequency in the setup before booting DOS

or after booting DOS https://github.com/skiselev/micro_8088:

The BIOS keyboard handler uses the following key combinations to switch the CPU clock frequency

Ctrl-Alt-Keypad - - Normal 4.77 MHz CPU clock frequency
Ctrl-Alt-Keypad * - Turbo 7.16 MHz CPU clock frequency
Ctrl-Alt-Keypad + - Turbo 9.55 MHz CPU clock frequency (works only on systems with 28.63636 MHz X1 crystal)

[Updated on: Wed, 18 March 2020 05:00]

Report message to a moderator

Re: Micro 8088 - Function of C21 Trimmer Cap [message #7286 is a reply to message #7275]

Wed, 18 March 2020 04:54

Jonas
Messages: 77
Registered: October 2015

Member

See AN826.pdf. A good text from Microchip about crystal oscillator basics, crystal pulling on page 7: Crystal tuning can be helpful when we wish to tune the circuit to the exact operating frequency desired.

Attachment: AN826.pdf
(Size: 242.89KB, Downloaded 341 times)

[Updated on: Wed, 18 March 2020 04:57]

Report message to a moderator

Re: Micro 8088 - Function of C21 Trimmer Cap [message #7289 is a reply to message #7286]

Wed, 18 March 2020 10:08

damienc
Messages: 4
Registered: March 2020

Junior Member

Looks like an interesting read. Thank you!

Report message to a moderator

Previous Topic:	TTL Oscillator Design Questions
Next Topic:	Cutting brackets for Micro 8088/XT-CF-Lite

-=] Back to Top [=-

[ Syndicate this forum (XML) ] [

] [

]

Current Time: Wed Jan 14 20:26:15 PST 2026

Total time taken to generate the page: 0.02548 seconds