CYRF8935 datasheet(13/40 Pages) CYPRESS

13 / 40 page

CYRF8935

Document Number: 001-61351 Rev. *K

Page 13 of 40

Setting the Radio Frequency

Programming by channel number is the easiest way to set

frequency. In the CYRF8935, RF carrier frequency and RF

channel number are always related by the expression:

Freq. = 2402 + Ch. #

Channel number is loaded into bits [6:0] of Register 7. Bits 7 and

8 initiate the desired Rx or Tx operation, respectively.

Some sample Register 7 examples are as shown in Table 4.

During Regulatory Compliance testing, you can jump directly to

another frequency any time without going through IDLE state. If

you change between Tx and Rx, however, you must pass

through IDLE state. For IDLE state, write Register 7 to clear bits

8 and 7. Tx or Rx operation is initiated when Register 7 bit 8 or

7 is set. Radio frequency is also determined at that time.

Crystal Oscillator

The CYRF8935 contains the on-chip gain block for the quartz

crystal frequency standard.

Quartz Crystal Application

As shown in Figure 10 on page 14, the series resistor Rs limits

power to the crystal and contributes to the phase-shift necessary

for oscillation. The ideal Rs value may need to be determined

empirically, adjusted for certain crystal manufacturer part

numbers and designs. The series equivalent combinations of C1

and C2 largely determine the capacitive load seen by the crystal,

which should match the crystal vendor's specification. These

capacitor values are chosen to center the crystal oscillator

frequency at the correct value, 12 MHz. The feedback resistor Rf

from the buffer output to input serves to self-bias the on-chip

buffer to the center of the linear region for maximum gain.

Verifying correct crystal oscillator frequency may require special

test methods. Because connecting a frequency counter probe to

either XTALi or XTALo adds capacitive loading and alters the

crystal oscillation frequency, other methods must be used. For

bare die applications involving COB packaging, use the

BRCLK[5] test point to verify correct frequency of oscillation. This

requires register 32[3:1] set accordingly (see Register Defini-

tions on page 21). For 24-QFN packaged parts, the correct

crystal frequency is determined by transmitting a continuous

carrier frequency (see Register Settings for Test Purposes on

page 19) and using a RF frequency counter to ensure correct

frequency. Irrespective of which method is used, initial tolerance

should be within budget as recommended in Table 5, such that

the total frequency error stays within budget.

Note

For proper operation, the total frequency error must not

exceed what is shown in Table 5. Individual error contributions

can be adjusted; for example 10+20+5+5=40, or 5+30+2+3=40.

Table 4. Sample Register 7 Settings

Carrier Frequency,

MHz

DUT Channel Number

(decimal)

DUT Channel

Number

(hex)

Tx setting:

Reg. 7 value

for TX_EN= 1

Rx setting:

Reg. 7 value

for RX_EN= 1

2402

0100

0080

2403

0101

0081

2404

0102

0082

2434

0120

00A0

2441

0127

00A7

2480

014E

00CE

Table 5. Crystal Specifications

Crystal Parameter

Specification

Frequency

12.000 MHz

Initial frequency tolerance

±15 ppm

Frequency tolerance over

temperature

±15 ppm

Frequency tolerance after

aging

±5 ppm

Frequency drift due to load

cap. drift

±5 ppm

Total

±40 ppm

Equivalent series resistance 80

 max

Resonance mode

Fundamental, parallel resonant

Load capacitance

In accordance with external load

capacitors (see C1 and C2 in

Figure 10)

Note

5. BRCLK signal is available on bare die only, not packaged parts.

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