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Document No. 001-95089 Rev. *A

1

AN95089

PSoC

Author: Prakhar Agarwal

Associated Part Family: CY8C4XX7-BL, CY8C4xx8-BL, CYBL10X6X, CYBL10x7x

Related Application Notes: None

AN95089 provides insights into the selection and tuning of the external crystal oscillator (ECO) and watch crystal oscillator

(WCO) for PSoC 4/ PRoC BLE devices to achieve a good RF performance. This application note introduces basics of

crystals and clock accuracy measurements. Cypress-recommended crystals and tuning techniques for optimum

performance are also discussed.

Contents

1

Introduction...............................................................1

2

Crystal Oscillator Basics ...........................................2

2.1

Crystal Oscillator Circuitry................................2

2.2

Load Cap Value (CL)........................................2

2.3

Crystal Equivalent RLC Circuit.........................4

2.4

Drive Level.......................................................5

2.5

PPM Error ........................................................5

3

Effects of Inaccurate ECO Crystal Frequency

on RF Performance ..................................................5

4

Crystal Tuning Technique for ECO...........................6

4.1

Steps to Correct Clock Inaccuracy...................7

5

Is Tuning Required for Each Board? ........................8

6

Crystal Analysis for ECO ........................................10

6.1

Frequency Variation with Temperature ..........10

6.2

Frequency Variation with Load Cap Value.....10

7

Recommendation for ECO .....................................11

8

Crystal Analysis for WCO .......................................11

8.1

Frequency Variation with Temperature..........11

8.2

Start-Up Time and ESR .................................12

8.3

Load Capacitance..........................................12

8.4

CL and Clock Accuracy .................................13

8.5

Frequency Variation Across Boards ..............13

9

Recommendations for WCO...................................14

10

Layout Considerations for PCB ..............................14

11

Summary ................................................................15

12

References .............................................................15

13

Appendix: Frequency Error

(Transmit Center Frequency Tolerance) ................17

Worldwide Sales and Design Support.............................19

1

Introduction

This application note helps you select the ECO crystal and the WCO crystal for PSoC 4/PRoC BLE devices and tune

them for optimum performance.

Bluetooth Low Energy (BLE) is a timing-sensitive technology in which an inaccurate ECO clock can degrade the

physical layer RF performance; similarly, an inaccurate WCO clock can lead to increased power consumption in a

peripheral.

The on-chip ECO circuit with an external crystal is used to synthesize a 24-MHz clock to run the BLE subsystem. The

clock sets the protocol timing for link-layer operations and derives the carrier frequency for physical-layer RF circuits.

An external WCO crystal is used to derive the 32.768-kHz clock that maintains link-layer timing synchronization when

the BLE subsystem is in a low-power mode.