GS4900B datasheet(39/102 Pages) SEMTECH

39 / 102 page

GS4901B/GS4900B SD Clock and Timing Generator

with GENLOCK

Data Sheet

37703 - 4

December 2009

39 of 102

Figure 3-2: Output Accuracy and Modes of Operation

3.3 Output Timing Format Selection

At device power-up (described in Section 3.12 on page 94), the application layer should

immediately set the external VID_STD[5:0] and ASR_SEL[2:0] pins. The VID_STD[5:0]

pins are used to select a pre-programmed output video format. The ASR_SEL[2:0] pins

are only available on the GS4901B, and are used to select the fundamental audio

frequency or to turn off audio clock generation.

The output timing formats selectable by the user via the VID_STD[5:0] pins are listed in

Section 1.4 on page 19. Table 3-7 in Section 3.7.2 on page 54 lists the audio sample rates

available via the ASR_SEL[2:0] pins.

NOTE: The VID_STD[5:4] pins should be grounded by the application layer since these

pins are not required to select output video standards 1 to 10.

On power-up, the device will first check the status of the GENLOCK pin. If GENLOCK is

set LOW and a valid reference has been applied to the inputs, the device will output the

selected video standard while attempting to genlock. However, if a reference signal has

not been applied and GENLOCK=LOW, the initial clock and timing outputs may be

determined by the internal default settings of the chip. If GENLOCK is set HIGH, the

device will immediately enter Free Run mode and will correctly output the selected

video standard.

When operating in Free Run or Genlock mode, the GS4901B/GS4900B will continuously

monitor the settings of the VID_STD[5:0] and ASR_SEL[2:0] pins. If the user wishes to

change the format of the output clocks and timing signals, these pins may be

reconfigured at any time, although it is recommended that the device be reset when

changing output video standards.

Free Run

Genlock

Freeze

27 MHz

-2ppm

No Input

Reference

Applied

Reference

Lost

Time

Assumption: Reference

XTAL is 27MHz+/-100ppm

-100ppm

+100ppm

+2ppm

NOTES:

t represents the temperature variability of the crystal

2. Diagram not to scale.

- t

+ t

- t

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