TUSB3200 datasheet(31/93 Pages) TI

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2–15

6 MHz

PLL

Frequency

Synthesizer

Capture Counter

Mux

MCLKO

MCLKI

Divide

by I

Mux

MCLKI2

MCLKO2

CSCLK

Divide

by B

Mux

Oscillator

Divide

by M

Figure 2–1. Adaptive Clock Generator

The ACG is controller by the following registers. Refer to section A.5.3 for details.

FUNCTIONAL REGISTER

ACTUAL BYTE-WIDE REGISTERS

24-bit Frequency register

ACGFRQ2

ACGFRQ1

ACGFRQ0

16-bit MCLK capture register

ACGCAPH

ACGCAPL

8-bit Divider control register

ACGDCTL

8-bit ACG control register

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 register may be updated

by the MCU while the ACG is running. In audio applications, the firmware can 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

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.

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