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Funcitonal Description (Continued)

3.3.3.1

DCLK Programming

The PLL contains an input divider (ID), feedback divider

(FD) and a post divider (PD). The programming of the

dividers is through F4BAR+Memory Offset 24h (see Table

3-9). The maximum output frequency is 300 MHz. The

output frequency is given by equation #1:

Equation #1:

DCLK = [CLK_14MHZ * FD] ÷ [PD *ID]

Condition:

140 MHz < [DCLK * PD] < 300 MHz

Where:

CLK_14MHZ is pin P24

FD is derived from N see equation #2 and #3:

PD is derived from bits [28:24]

ID is derived from bits [2:0]

Equation #2:

If FD is an odd number then: FD = 2*N +1

Equation #3:

If FD is an even number then: FD = 2*N +0

Where:

N is derived from bits [22:12]

+1 is achieved by setting bit 23 to 1.

+0 is achieved by clearing bit 23 to 0.

Example

Define Target Frequency:

Target frequency = 135 MHz

Satisfy the “Condition”:

(140 MHz < [DCLK * PD] < 300 MHz)

140 MHz < [135 MHz * 2] < 300 MHz

Therefore PD = 2

Solve Equation #1:

DCLK = [CLK_14MHZ * FD] ÷ [PD *ID]

135 = [14.31818 * FD] ÷ [2 * ID]

135 = [7.159 * FD] ÷ ID

18.86 = FD ÷ ID

Guess: ID = 7, Solve for FD

FD = 132.02

Solve Equation #2 or #3:

FD = 2*N +1 for odd FD

FD = 2*N +0 for even FD

FD is 132, therefore even

132 = 2*N +0

N= 66

Summarize:

PD = 2: Bits [28:24] = 00111

ID = 7: Bits [2:0] = 101

N = 66: Bits [22:12] = 073h (found in Table 3-10),

clear bit 23

Result:

DCLK = 135

The BIOS has been provided with a complete table of divi-

sor values for supported video clock frequencies. Many

combinations of divider values and VCO frequencies are

possible to achieve a certain output clock frequency.

TheseBIOS valuesmay be adjustedfromtimetotimeto

meet system frequency accuracy and jitter requirements.

For applications that do not use the GXLV processor’s

video, this is an available clock for general purpose use.

The transition from one DCLK frequency to another is not

guaranteed to be smooth or bounded; therefore, new

divider coefficients should only be programmed while the

PLL is off line in a situation where the transition character-

istics of the clock are “don't care”. The steps below

describe (in order) how to change the DCLK frequency.

Program the new clock frequency

Program Reset (bit 31) high and Bypass PLL (bit 8)

high.

Wait at least 500 µs for PLL to settle.

Program Reset (bit 31) low.

Program Bypass PLL (bit 8) low.

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