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oscillation mode in lpc2148

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sknoogleplex

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Hi, does anyone know how the oscillation mode is determined in an LPC2148? Page 18 of the manual starts talking about how there is a "slave" mode and an "oscillation mode", but I can't find anywhere that discusses how these are actually selected. Does anyone know or see it?

Secondly, what is the frequency if the internal oscillator? (Where do you find this?)

(I'm specifically trying to find F_OSC.)

Thank you.
 

futz

Active Member
Hi, does anyone know how the oscillation mode is determined in an LPC2148? Page 18 of the manual starts talking about how there is a "slave" mode and an "oscillation mode", but I can't find anywhere that discusses how these are actually selected. Does anyone know or see it?
I could be totally wrong, but by "mode" I think they just mean what type of oscillator source you have connected. You can use either a crystal connected to both X1 and X2 pins (master), or an outboard clock source (another MCU or whatever?) connected to just the X1 pin (slave). I looked at all the docs I have here and found no reference (so far) to "selecting" the mode.

Secondly, what is the frequency if the internal oscillator? (Where do you find this?)

(I'm specifically trying to find F_OSC.)
As far as I know there is no internal oscillator. F_OSC is the clock rate of the oscillator you have connected. You can multiply that up internally with the PLL, but the oscillator base clock is still external.

The oscillator output frequency is called FOSC and the ARM processor clock frequency is referred to as CCLK for purposes of rate equations, etc. elsewhere in this document. FOSC and CCLK are the same value unless the PLL is running and connected.
Now Ubergeek will read this and correct me. :D
 

sknoogleplex

New Member
Ah, that makes lots of sense. I'm not sure what gave me the idea that there was an internal clock, but I've been agonizing over this for a while :p. Thanks!
 

AtomSoft

Well-Known Member
hey all i love jumping into this ARM stuff:
Page 18
"The onboard oscillator in the LPC2141/2/4/6/8 can operate in one of two modes: slave
mode and oscillation mode."
This means it works in 2 modes obviously lol but there is infact no actual register to select it. The actual way to do it is in HARDWARE. They way you have it connected determines if its in SLAVE or OSC. Mode.

Check out page 19 the diagram:

I think this is in-fact how you set the mode of operation.

EDIT
Some more info:
WinARM GCC tutorial - Clocking ARM with Crystal oscillator and PLL
 

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