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'[PIC] 12F675 and ANSEL settings for A2D conversion'
2005\05\02@034928 by Jinx

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> It says set it to x11 = F_RC (clock derived from a dedicated internal
> oscillator = 500kHz max)

> That's a different clock than the other internal 4MHz oscillator, right?

FRC kind of implies the RC oscillator (see Section 9.2.4) but don't
know why it's limited to 500kHz (elsewhere RC is 4MHz). I've never
used this setting. I thought it may have been something to do with
Timer1's OSC but that's only 200kHz

Anyone know for sure ???

Table 7-1 is what you need to look at. Note the shaded cells and

> I assume I don't have to use x11

No, I'd assume that 001 or 101 would do you (with ref to Table 7-1)

> My analog input will take miliseconds to change in value. I think that
> means I can safely select ADCS = 001 = 8T_OSC @ 4MHz = 2.0
> microseconds. If I picked that setting, does that mean that my analog
> input will only have to hold for at least 2*2microseconds = 4 us for
> it to be correctly sampled and converted?

See the data starting page 101 : Tables 12-8/9/10, Figs 12-10/11


A/D & Measurement Tips & Tricks Using the 8-Pin PIC12F675

Tips & Tricks Using the 8-Pin PIC12F675

2005\05\03@095314 by Jinx

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Below is a response from Microchip Engineering Support Team for
your submitted ticket #: 1-20181. A description of the problem and
our proposed resolution is contained below.

Area: PICmicro Products
Product Group: PIC12F67X
Product: PIC12F675

Problem Description:
Hi, can you please explain further this reference in the 12F675
data sheet

Page 44, DS41190C

Register 7-2 ANSEL

bit 6-4 x11 FRC (clock derived from a dedicated internal oscillator = 500kHz

What is the "dedicated internal oscillator" referred to, and why is the
distinction made between it and other sources of Fosc on Page 44 ?


Joe Colquitt
New Zealand

Problem Resolution:

Dear Customer,

Please refer data sheet Page 41 (Section 7.1.4) for the AD Conversion
clock explanation.  The 10-BIT ADC in the PIC12F675 are request
minimum Tad with 1.6uS, and the conversion time need 12 Tad, so the
mininum conversion time are 1.6uS * 12Tad = 19.2uS.

You also will find the TABLE 7-1 show the Tad request for difference
operation frequency. At 5MHz frequency, you should set the ANSEL
<6:4> with Fosc/8 ( 5MHz/8= 1.6uS)  or at 4MHz frequence, the
minimum Tad are 4MHz/8 = 2uS.

When you set the ANSEL<6:4> with FRC mode (dedicated internal
oscillator = 500kHz max), the means the ADC will use own internal RC
oscillator for the ADC clock source, the minimum Tad will be 2uS
(500kHz) to 6uS

Microchip Engineering Support

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