Minidot 2 - The holoclock

[rgbphil]

Hi,
I've recently updated my Minidot clock with a cross fade function on the charlieplex display which may be of interest to people.
http://www.instructables.com/id/E11GKKELKAEZ7BFZAK/
Phil

[SteveyG]

Looks really pretty! Though I'd have dropped the 32kHz crystal and just derived the time from the 20MHz crystal.

[rgbphil]

Thanks for the compliment.

As for the 32khz crystal....I've looked into that. The problem with deriving timing from 20MHz is that you're divisor will not be an exact multiple or fraction of a second. This means a tiny error will build up over time and the clock will be wrong eventually.

There are ways of correcting and compensating for this, but they're pretty complex. It was easier to use a watch crystal. An alternative would have been to use a 3.2768Mhz crystal (as the main oscillator) or any other crystal which can be a binary divisor...ie 3.2768kHz (or MHz) is 2^15. This means the timer can run free, clicking over the RTC variables pretty accurately.

There are numerous discussions over at the Neons and Nixies yahoo group about this sort of thing.

Phil

[SteveyG]

You can always exactly divide down a crystal provided it is above 32.768kHz, and it isn't difficult in the slightest, take a 20MHz crystal for example:

With a PIC, the period of each instruction will be 200ns. Now preload one of your timers with the value 0x3CB0, and you'll get an interrupt every 0.01s. Then just have a counter at the start of your timer's ISR, to count to 100. That will give you 1 second exactly. No jitter, no error. Uses 7 instructions for your ISR so a total of 700 out of 20000000 instructions per second used.

[justDIY]

Uses 7 instructions for your ISR so a total of 700 out of 20000000 instructions per second used.

a pic at 20mhz should execute 5mips ... since all instructions are at least clk/4

I think the avr flavor of microcontrollers runs each instruction at clk

[SteveyG]

Uses 7 instructions for your ISR so a total of 700 out of 20000000 instructions per second used.

a pic at 20mhz should execute 5mips ... since all instructions are at least clk/4

I think the avr flavor of microcontrollers runs each instruction at clk

Thanks - it was a typo