CHAPTER 16 FIP CONTROLLER/DRIVER
289
16.8 Light Leakage of FIP
Light may leak when a FIP is driven using the
μ
PD78044F Subseries. Two possible causes are as follows.
(1) Light leakage due to a short blanking
Figure 16-10 shows the signal waveforms when only the first digit of two digits to be displayed is lit.
As shown in this figure, when the blanking time is short, the T1 signal rises before the segment signal is
eliminated, resulting in light leaking.
Generally, as approximately 20 ms is required for the blanking time, consider values for setting the display
mode register 1 (DSPM1) carefully.
Figure 16-10. Light Leakage due to Short Blanking Time
T0
Blanking time
T1
S0
Light leakage
(2) Light leakage due to capacity between segment and grid of FIP
As shown in Figure 16-12, light may leak even if the blanking time is sufficient. As shown by C
SG
in Figure
16-11, as there is capacity between the grid and segment of the FIP, the timing signal pin voltage will be
increased via C
SG
when the segment signal turns on.
As shown in Figure 16-12, when this voltage exceeds the cut-off voltage (E
K
), light will leak.
This spike noise voltage depends on the size of C
SG
and the on-chip pull-down resistor (R
L
). The greater
the value of C
SG
or the RL value, the greater is the voltage, making it easy for light to leak.
This C
SG
value differs according to the area of the data displayed on the FIP. The greater the area, the
greater the C
SG
is. Consequently, pull-down resistor values with which light will not leak also depend on
the size of the FIP.
As the value of the pull-down resistor incorporated by the mask option is comparatively great, in some cases,
light leakage cannot be controlled with the resistance only.
If the quality of the display is insufficient, increase the back bias (increase the E
K
), place a filter over the
FIP, or attach a 10-k
pull-down resistor externally to the timing signal pin.
Depending on the duty cycle of the spike noise voltage for the whole display period, the easiness with which
light leaks due to the C
SG
varies. The lesser the number of digits displayed, the easier it is for light to leak.
Lowering the luminance of the display is also effective.