ATA5423-PLSW datasheet(26/100 Pages) ATMEL

26 / 100 page

4841D–WIRE–10/07

ATA5423/ATA5425/ATA5428/ATA5429

0 of the XTAL has to be lower than CLmin/2 = 3.7 pF for a Pierce oscillator type in order to not

enter the steep region of pulling versus load capacitance where there is a risk of an unstable

oscillation.

To ensure proper start-up behavior the small signal gain, and thus the negative resistance, pro-

vided by this XTO at start is very large; for example, oscillation starts up even in worst case with

a crystal series resistance of 1.5 k

Ω at C

0 ≤2.2 pF with this XTO. The negative resistance is

approximately given by

with Z

1, Z2 as complex impedances at pin XTAL1 and XTAL2, hence

1 =–j/(2 ×π × fXTO × CL1) + 5Ω and Z2 = –j/(2 × π × fXTO × CL2) + 5Ω.

3 consists of crystals C0 in parallel with an internal 110 kΩ resistor hence

3 =–j/(2 ×π × fXTO × C0) /110 kΩ, gm is the internal transconductance between XTAL1 and

XTAL2 with typically 19 mS at 25°C.

With f

XTO = 13.5 MHz, gm = 19 mS, CL = 9 pF, and C0 = 2.2 pF, this results in a negative resis-

tance of about 2 k

Ω. The worst case for technological, temperature and supply voltage variations

is then for C

0 ≤2.2 pF always higher than 1.5 kΩ.

Due to the large gain at startup, the XTO is able to meet a very low start-up time. The oscillation

start-up time can be estimated with the time constant

τ.

After 10

τ to 20 τ an amplitude detector detects the oscillation amplitude and sets XTO_OK to

High if the amplitude is large enough. This sets N_RESET to High and activates the CLK output

if CLK_ON in control register 3 is High (see Table 7-7). Note that the necessary conditions of the

VSOUT and DVCC voltage also have to be fulfilled (see Figure 4-2 and Figure 5-1).

To save current in IDLE and Sleep modes, the load capacitors are partially switched off in these

modes with S1 and S2, as seen in Figure 4-2.

It is recommended to use a crystal with C

m = 3.0 fF to 7.0 fF, CLN =9 pF, Rm <120Ω and

0 = 1.0 pF to 2.2 pF.

Lower values of C

m can be used, this increases the start-up time slightly. Lower values of C0 or

higher values of C

m (up to 15 fF) can also be used, this has only little influence on pulling.

Re Z

XTOcore

{} Re

+++

------------------------------------------------------------------------------------------------------

⎩⎭

⎨⎬

⎧⎫

Re Z

XTOcore

() R

()

-----------------------------------------------------------------------------------------------------------

ATA5423-PLSW Datasheet(PDF) 26 Page - ATMEL Corporation

ATA5423-PLSW Datasheet(HTML) 26 Page - ATMEL Corporation

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