555 Timer Circuit Newbie

Thread Starter

bibble235

Joined May 29, 2018
57
Hi

I started building one of these because I need a 1Mhz clock for my 6502 processor and for the fun of learning and this has led me to 2 questions.

1/ Does a clock have to be right or can it be under speed.
2/ How do people get the right speed
I have seen the page on this site to plug in R1, R2 and C1. Do you use a pot to get it right. Is it better to vary the caps or the resisters.

Not using this for anything critical and will end up using a modern 6502 so 1Mhz is not going to be a problem. I am more interested in the way people approach this?

Thanks all. Be kind.
 

Ian Rogers

Joined Dec 12, 2012
1,136
If not time critical I would use a cheap 1Mhz ceramic resonator these are still quite accurate.. The problem with a 555 is, while it's cool to DIY, the components required will be...... Well not stable in my view.

You can get TTL clock sources in a four pin package... If I remember the 6502 was MOS tech.. I used the RCA1802 "kinda the same" and ceramic resonators worked fine..
 
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Thread Starter

bibble235

Joined May 29, 2018
57
Thanks all, do understand I can buy a part to solve this. I was more keen to understand how you would do it without this approach. Perhaps the answer is people don't.
 

Ian0

Joined Aug 7, 2020
9,667
Back in the days of Commodore PETs and Acorn Atoms, the 6502 was driven by a Pierce oscillator made of a single inverter (74LS04), the buffered once or twice with more 74LS04. The crystal used was the 3.57MHz American TV crystal, because they were cheap, and because it paid to synchronise the processor clock to the clock of the video chip, which tended to be a 6847 or 6845.
 

Thread Starter

bibble235

Joined May 29, 2018
57
Back in the days of Commodore PETs and Acorn Atoms, the 6502 was driven by a Pierce oscillator made of a single inverter (74LS04), the buffered once or twice with more 74LS04. The crystal used was the 3.57MHz American TV crystal, because they were cheap, and because it paid to synchronise the processor clock to the clock of the video chip, which tended to be a 6847 or 6845.
Thanks for the background. Started on one of these and led to 40 more years of IT. Been doing Microcontrollers ESP32, picos and now looking back to improve my new hobby of electronics. How things work is more important than using them.
 

Ya’akov

Joined Jan 27, 2019
9,068

sparky 1

Joined Nov 3, 2018
756
The accuracy from the crystal allows software functions to work correctly.
The question: How accurate can a 555 circuit be and how can I make a precision 555 circuit?

Having a really nice 555 precision circuit to insert and compare output reliability of a 6502 processor that normally use a quartz crystal
might show the important role that quartz crystals have in accurate processing and why clock accuracy is necessary.

The Commadore 128 had maximum frequency 2MHz however a television was used for video and consumer demand for graphics had a profound impact on the whole product. Many of us were more interested in writing useful programs and were tired of language Basic.
I believe that C programmers today like to play with hardware. I think they might be disappointed without accuracy. That is why the product could be educational but very difficult to compete with the capability that exists. The simulation for 6502 does exist, even a tube version. so there should be variety of applications having simpler circuitry.
 
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