STV2050A datasheet(53/83 Pages) STMICROELECTRONICS

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STV2050A - DYNAMIC FOCUS

10 DYNAMIC FOCUS

10.1 PARABOLA CURVE

The focus function delivers a current with 6-bit resolution, which is constant for horizontal

lines. In vertical direction, the current has the shape of a second order parabola.

The parabola is defined by three points, FV1, FV2 and FV3, whose values correspond to the

FV1[5:0], FV2[5:0] and FV3[5:0] values which are stored in the DE register.

The position of these last 3 points is a multiple of (VGD+1). See Figure 29 "Focus Parabola"

on page 53. The parabola is therefore linked to the value of the Vertical Grid Distance.

Figure 29. Focus Parabola

The focus DAC will provide a programmable constant value in the FVR[5:0] bits in the DE reg-

ister at the bottom of the screen and during vertical retrace.

The start of the retrace value is programmable using the FSB[7:0] bits in the DA register as

shown above.

The position is indicated by the VFP[8:0] bits in the D9 register.

Notes:

If VRES becomes active before the number of lines for the FSB have been counted, the parabola will

be terminated and replaced by the FVR retrace value.

The hardware is not equipped with a safeguard device against DAC range overflow. This may occur

if the FV2 bit is near the max. positive or negative value and the parabola is not symmetrical to the

FV2 centre value. Also, large values for the FSB have to be taken into account.

x/2

FV1

FV2

FV3

FVR

VFP+3

field n

field n+1

TV lines

FSB

x = 12 * (VGD + 1)

y = 11 * (VGD + 1)

Focus DAC

vertical retrace

(VRES)

FV1

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