VD5377 datasheet(31/88 Pages) STMICROELECTRONICS

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VD5377

Key features

This chapter gives an overview of some of the most important registers and functions.

6.1

Feature count

Feature count is a measure of the useful detail in an image which is used to match

successive frames. Generally, the higher the feature count the better the tracking. The

FEATURES register (0x31) in Figure 17 is an 8-bit value representing the 8 MSBs of a 12-bit

internal register. A maximum value of 255 represents a feature count of 16 x 255 = 4080. A

reasonable average target feature count is around 2000. Feature counts averaging less

than 1000 are likely to result in missing counts and sluggish navigation. This is usually as a

result of low contrast in the image or significant vignetting due to the lens.

Note:

On some textured surfaces the feature count may exceed 4080. When this occurs the

FEATURES register clips at 255. This is normal and does not affect tracking.

6.2

Minimum features threshold

Without any object on the sensor the feature count will be non-zero, typically around 200.

This residual value is usually due to the characteristics of the lens and/or pixel noise but

may also be caused by internal or external light reflection which can sometimes result in

unintentional cursor movement (or jitter). To prevent this unwanted movement, the motion

engine is inhibited until the feature count register exceeds the value in the MIN_FEATURES

Default value is 16d x 16 = 256.

6.3

X/Y scaling

The VD5377 outputs a single count for each one pixel displacement of the object. The

physical dimension of one pixel is 30 µm. The actual displacement depends on the

magnification of the lens used. For a lens of magnification M = 0.5 one pixel displacement

equates to 60 µm physical displacement of the object.

Cursor movement is typically expressed in Counts or Dots per Inch (CPI or DPI). In this

case (M = 0.5):

Counts per Inch = 25.4mm/60µm = 423 CPI

The X/Y scaling registers (Table 17: Features and scaling on page 32) can be used to

increase or decrease the native CPI according to the following equation:

Counts per Inch = register_value x M x 100

Scale factors can be applied to X and Y independently to compensate for any lens

distortion.

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