MICRF405 datasheet(30/46 Pages) MICREL

30 / 46 page

Micrel

MICRF405

April 2006

M9999-041906

(408) 955-1690

Bit Rate Settings

The bit rate is set separately for ASK and FSK

modulation to support ASK modulation with FSK

spreading. The FSK bit rate is set with the two

parameters FSKClk_K and FSKn. The relationship is

FSKn

XTAL

FSKClk

⋅

(1)

where:

BR: Bit rate.

fXTAL: Crystal oscillator frequency.

FSKClk_K: Integer in the range [1..63] (6 bit).

FSKn: Integer in the range [0..5] (3 bit).

A procedure to find the settings for the desired bit

rate is described below:

1. Set FSKn to 0.

2. Calculate FSKClk_K by using this formula:

FSKn

XTAL

FSKClk

⋅

3. If FSKClk_K is too high, increment FSKn by

one, and then jump to 2.

In some cases, several combinations of FSKClk_K

and FSKn will give the bit rate. If VCO modulation is

selected, Modulation[1:0] < 2, then the lower FSKn

the better shaping of the signal. However in some

cases, low FSKn causes the modulator to saturate

(Please see the section Modulator for details). When

Manchester encoding is enabled, Manchester_en=1,

FSKn needs to bigger than zero for the modulator to

operate properly.

When sending ASK with FSK spreading, FSKClk_K

and FSKn set the speed of the FSK spreading.

The bit rate of the ASK is set similar with the two

parameters ASKClk_K and ASKn. The relationship

is now:

ASKn

XTAL

ASKClk

⋅

(2)

where the new parameters are:

ASKClk_K: Integer in the range [1..63] (6 bits).

ASKn: Integer in the range [0..3] (2 bits).

A procedure for finding a bit rate will be equal to

FSK, but with this formula:

ASKn

XTAL

ASKClk

⋅

Due to the high flexibility of the modulator, a brief

explanation of how it works will ease the use of it.

Figure 15 shows a block diagram of the components

of the modulator.

Modulator

Adr

Data

A6..A0

0001110

Mod_I4=0

Mod_I3=1

Mod_I2=0

Mod_I1=0

Mod_I0=1

Mod_A2=0

Mod_A1=1

Mod_A0=1

0001111

VCO_Fr_Chk=0

VCO_Fr_Auto=0

FSKn2=1

FSKn1=0

FSKn0=0

Mod_F2=1

Mod_F1=0

Mod_F0=0

0010000

MOD_TEST1=0

Prescaler_Sel=0

FSKClk_K5=1

FSKClk_K4=1

FSKClk_K3=0

FSKClk_K2=1

FSKClk_K1=0

FSKClk_K0=0

Figure 15. Modulator Block Diagram.

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