TUSB3200A datasheet(34/102 Pages) TI1

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TUSB3200A

SLES018B – OCTOBER 2001 – REVISED FEBRUARY 2011

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FUNCTIONAL

ACTUAL BYTE-WIDE REGISTERS

24-bit Frequency

ACGFRQ2

ACGFRQ1

ACGFRQ0

16-bit MCLK capture

ACGCAPH

ACGCAPL

8-bit Divider control

ACGDCTL

8-bit ACG control

ACGCTL

The main functional modules of the ACG are described in the following sections.

2.2.8.1

Programmable Frequency Synthesizer

The 24-bit ACG frequency register value is used to program the frequency synthesizer. This results in high

resolution to accurately select the desired codec master clock frequency. The value of the frequency

adjust the frequency value by ±LSB or more to lock onto the USB start-of-frame (SOF) signal to achieve a

synchronous mode of operation. The 24-bit frequency register value is updated and used by the frequency

synthesizer only when MCU writes to the ACGFRQ0 register.

Depending on the application, a smaller number of bits for controlling the synthesizer frequency can be

chosen. The frequency resolution also depends on the actual frequency being used. In general, the

frequency resolution is less for higher frequencies and more for lower frequencies. This is due to the fact

that the 208 ps frequency resolution becomes more significant compared to the period at higher

frequencies than at lower frequencies. The resolution increases with the number of bits used to represent

the frequency as the quantization error reduces as more bits are used to represent a fractional number.

The clock frequency of the MCLKO output signal is calculated by using the formula:

For N > 24 and N < 50: MCLKO frequency = (25/N) × 192/8 MHz

For N = 50: MCLKO frequency = 96/8 MHz

Where N is the value in the 24-bit frequency register (ACGFRQ). The value of N can range from 24 to 50.

The 6 most significant bits of the 24-bit frequency register are used to represent the integer portion of N

and the remaining 18 bits of the frequency register are used to represent the fractional portion of N. An

example is shown below.

Example Frequency Register Calculation

Suppose the desired MCLKO frequency is 24.576 MHz. Using the above formula, N = 24.4140625

decimal. To determine the binary value to be written to the ACGFRQ register, separately convert the

integer value (24) to 6-bit binary and the fractional value (4140625) to 18-bit binary. As a result, the 24-bit

binary value is 011000.011010100000000000.

The corresponding values to program into the ACGFRQ registers are:

ACGFRQ2 = 01100001b = 61h

ACGFRQ1 = 10101000b = A8h

ACGFRQ0 = 00000000b = 00h

Keep in mind that writing to the ACGFRQ0 register loads the frequency synthesizer with the new 24-bit

value.

Example Frequency Resolution Calculation

To illustrate the frequency resolution capabilities of the ACG, the next possible higher and lower

frequencies for MCLKO can be calculated.

To get the next possible higher frequency of MCLKO equal to 24.57600384 MHz, increase the value of N

by 1 LSB. Thus, N = 011000.011010100000000001 binary.

Description

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