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AN32 S i3 21 X , S i 3 2 3 0 , A N D S i3 2 3 3 F R E Q U E N C Y S H I FT K E Y I N G ( F S K ) M O D U L A T I O N

Introduction The ProSLIC tone-generator Oscillator 1 may be used to create frequency shift keying (FSK) frequency modulation for caller ID. The ProSLIC has two independently programmable voice-band oscillators (see ProSLIC specification for details). While both oscillators are necessary for the generation of call progress and signaling tones such as busy tone, dial tone, or DTMF, only Oscillator 1 is necessary for phase continuous FSK. Oscillator 1 will preserve phase continuity during frequency shifts; Oscillator 2 will not. Oscillator 1 has some special built-in features to greatly simplify the FSK switching between two frequencies.

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The screen-capture below shows a paired trace and two singular scope traces during FSK between 2200 and 1200 Hz. The paired trace is the mirror of the Tip and Ring ac voltages. The first singular trace is the difference between these voltages. The second singular trace is the interrupt bit of the ProSLIC chip. This interrupt, generated by the Oscillator 1 timers, signals the software that the bit data has been loaded, and that the registers are free to update new data. The traces below represent a 1, 0, 1, 0, 1 pattern.

Copyright © 2007 by Silicon Laboratories

AN32

AN32 The FSK mode is selected in the Oscillator Control register (direct Register 32). This enables the oscillator timers to be used as the indicator for FSK bit duration. Oscillator 1 is automatically loaded with the last register setting upon the expiration of Oscillator 1’s active timer. Oscillator 1’s inactive timer is only used for the expiration of the first bit. Otherwise, Oscillator 1’s inactive timer is disabled automatically by the FSK mode bit. The interrupts caused by Oscillator 1 timers informs the user of a window of time to update the FSK register (direct Register 52) for the next FSK bit.

Generation of FSK signals consists of four steps: 1. Initialize the registers for transmission. 2. Enable Oscillator 1 in FSK mode, and engage the timers for Oscillator 1 with their interrupts. 3. Write subsequent data bit to direct Register 52, one bit every interrupt. 4. Disable the timers and Oscillator 1. The FSK indirect registers are listed in Table 1 and Table 2.

Table 1. FSK Indirect Registers (Si3210, Si3211, Si3230) Indirect Register Number

Purpose

Example Value

99

Amplitude Parameter for Space(0)

0x01B4

100

Frequency Parameter for Space(0)

0x6B60

101

Amplitude Parameter for Mark(1)

0x00E9

102

Frequency Parameter for Mark(1)

0x79C0

103

Transition Parameter from 0 to 1

0x1110

104

Transition Parameter from 1 to 0

0x3C00

Table 2. FSK Indirect Registers (Si3215, Si3216, Si3233) Indirect Register Number

Purpose

Example Value

99

Amplitude Parameter for Space(0)

0x02A8

100

Frequency Parameter for Space(0)

0x5320

101

Amplitude Parameter for Mark(1)

0x0162

102

Frequency Parameter for Mark(1)

0x7210

103

Transition Parameter from 0 to 1

0x10B0

104

Transition Parameter from 1 to 0

0x3D60

Table 3. FSK Direct Registers

2

Direct Register Number

Purpose

Example Value

32

OSC1 Control Register

0x56

36

Low Byte Active Timer

0x13 for 1200 baud

37

High Byte Active Timer

0x00

52

FSK Data

1/0 Data Value

108.6

Enhancement Register

1

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AN32 1. Initialize the Registers

3. Send the Caller ID Frame

To take advantage of the advanced FSK mechanism (direct Register 108—Enhancement Enable register), bit 6 should be set to 1. Set the Oscillator 1 Active Timer—Low Byte register (direct Register 36), to the duration of a bit in 1/24000 of a second minus one. This would be decimal 19 for 1200 baud. Decimal 19 represents twenty increments. Each increment is 1/24000 of a second, so 20/24000 is 1/1200. The Oscillator 1 Active Timer—High Byte register (direct Register 37) is 0 because the most significant bits of the active timer are 0.

A frame of caller ID most likely consists of a group of bytes (octets) with start and stop bits. If the start bit is a 0 (Space), this must be written to direct Register 52. This causes an active timer interrupt. Next, load the first bit into direct Register 52. Wait for an interrupt, then write the next bit. Wait for the active timer interrupt and so on. Once the end of the byte has been reached, write a stop bit. Continue to write bytes till the end of the frame. A group of Marks between a group of alternating 1s and 0s will probably have to be included depending on the caller ID specification. Starting the frame with “U” 0x55 and not sending characters for as many bits as the spacing specification requires will achieve this.

The FSK Data register (direct Register 52) should be written to a logical 1 twice. (This sets up the double buffer.) The next registers to initialize are the amplitude, coefficient, and phase registers on Oscillator 1 for both their Mark (logical 1) and Space (logical 0) values. For the North American values of 1200 Hz (Mark) and 2200 Hz (Space) at .20 VRMS or –13.5 dBm at 900 Ω, the register values are shown in Table 1 and Table 2.

2. Enable the Oscillator To enable the interrupt for the Oscillator 1 active timer, the LSB is enabled on direct Register 21. The Oscillator 1 Control register (direct Register 32) is programmed to hex 56 (0x56). This bit engages the active timer and relative bit for Oscillator 1. The REL (reload) bit is the FSK mode bit.

4. Disable the Oscillator Disable the oscillator by writing a 0x50 to direct Register 37. This will leave the line quiet.

5. Helpful Hints There are two things to remember when coding the FSK modulation. One, most caller ID boxes will not work without ringing. Two, a valid checksum and byte count will need to be generated for the caller ID frame to be registered as valid. Silicon Labs has example source code that will demonstrate caller ID using a fast PC on the Si3210-EVB.

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AN32 CONTACT INFORMATION Silicon Laboratories Inc. 400 West Cesar Chavez Austin, TX 78701 Tel: 1+(512) 416-8500 Fax: 1+(512) 416-9669 Toll Free: 1+(877) 444-3032 Email: [email protected] Internet: www.silabs.com

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