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External 4MHz Osc vs Internal 8MHz Osc

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Urahara

Member
Hi

Which would use more power : an external 4Mhz crystal or an internal 8MHz oscillator block of a 18F?

Am thinking of using the internal oscillator block, running it in RC_RUN mode in general and using the external 4Mhz crystal for accurate seconds countdown in timing-sensitive subroutines. Would this be more power efficient?

Thks!
 

3v0

Coop Build Coordinator
Forum Supporter
The current used is dependant on the processor speed. The faster you run the more current you use. The current used by the clock circuits should be very small.

Most of the time the internal oscillator is good enough. The two exceptions I know of are generating video signals (very precise) and clocks that keep real time (cumulative error). Your application may work fine without an external crystal.
Hi

Which would use more power : an external 4Mhz crystal or an internal 8MHz oscillator block of a 18F?

Am thinking of using the internal oscillator block, running it in RC_RUN mode in general and using the external 4Mhz crystal for accurate seconds countdown in timing-sensitive subroutines. Would this be more power efficient?

Thks!
 

Urahara

Member
Hi 3vO

Can I clarify what you've posted earlier?

1) Your feel is that an internal 8MHz oscillator block should consume less power than an external 4MHz one?

2) I read that the internal osc can have an error of abt 2%. So potentially, if I run a timer for abt 60mins, potentially, the error is abt 1.2secs. Guess this would be extreme environment. If used in normal room temperature, it should be ok?

Thks!
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi 3vO

Can I clarify what you've posted earlier?

1) Your feel is that an internal 8MHz oscillator block should consume less power than an external 4MHz one?

2) I read that the internal osc can have an error of abt 2%. So potentially, if I run a timer for abt 60mins, potentially, the error is abt 1.2secs. Guess this would be extreme environment. If used in normal room temperature, it should be ok?

Thks!
hi,
As 3v0 is not online, I'll answer.:)

The 8MHz operation will consume more power than the 4MHz.

Can you say exactly what you want to time, whats the project.?
 

Urahara

Member
Hi Eric

Thks!

The 8MHz operation will consume more power than the 4MHz.
So I assume it does not matter even if I pre or post scale the 8MHz down to 4MHz? The crux of the matter is that a slower external crystal will consume less than a higher internal one?

Can you say exactly what you want to time, whats the project.?
The timing part is actually a simple count-down where user enters the no of mins/secs to countdown. Hit enter and the countdown begins in resolution of 0.1sec. When the time is up, it will beep.
 

3v0

Coop Build Coordinator
Forum Supporter
Hi Eric

Thks!


So I assume it does not matter even if I pre or post scale the 8MHz down to 4MHz? The crux of the matter is that a slower external crystal will consume less than a higher internal one?
Regarding power use it does not matter how or where the clock signal is generated. Slower clocks should use less power. When the unit is not in use you could even put it to sleep unless you had a display to refresh.

The timing part is actually a simple count-down where user enters the no of mins/secs to countdown. Hit enter and the countdown begins in resolution of 0.1sec. When the time is up, it will beep.
I would use a crystal unless you do not have the IO pins to spare.

Without knowing more about the system it is difficult to give you a better answer. Is there any display multiplexing going on? Which processor?
 
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