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Technical Supplement
9.6.4
Audio Output
9.6.5
Display Control Circuitry
9-20
Crystal Y1 provides an accurate 32.768 KHz clock input whenever the
timekeeping circuitry of U29 is activated. The CPU only enables the
timekeeping function when an optional printer is installed. If no printer is
installed, the CPU switches off timekeeping, thereby extending battery life.
Also, with no printer installed, the RTC clock is only used during diagnostic
testing to verify the CPU clock timing.
The lithium battery BT1 and diodes CR22 and CR23 provide the power switch
over and constant power needed to keep the time and RAM data while the unit is
not in use. Whenever the unit is powered on, Vcc is at 5 V and U29 is powered
via CR22. CR 23 is reverse biased because BT1 at 3 V is at a lower potential
than Vcc. Whenever the unit is powered off, the potential between Vcc and
switched ground is 0 V, CR23 is forward biased, and U29 is powered by BT1.
CR22 is reverse biased, isolating BT1 from Vcc. This circuit design allows BT1
life of up to 5 years typical, without the unit being powered on.
U29 holds 24 bytes of RAM, which is used for nonvolatile storage of CPU data.
The audio output circuit is illustrated in Figure 9-22.
BEEP_1
BEEP_2
Figure 9-22: Audio Output Circuit
BZ1, a piezo ceramic sounder, is the audio output device. Due to its low drive
current of 2 mA maximum, no drive circuitry is needed and is driven directly
from the external output port. The audio output device is differentially driven
with 2 square waves 180 degrees out of phase. The drive frequency is
approximately 1480 Hz or 740 Hz and is generated by the CPU. BZ1 is
differentially driven to obtain maximum audible volume.
The display control circuit is illustrated in Figure 9-23, at the end of Section 9.
Figure 9-5 shows the display control block diagram.
The Taliq display is controlled by the display control circuitry. A photosensor
measures ambient light and automatically switches on the electroluminscent
display backlight during low-light conditions. The display control circuitry is
divided into the following subsections:
•
Control Conditioning Circuit (main PCB) — The control conditioning
circuit processes signals generated by the CPU to produce timing signals for
the display drivers.
•
Display Driver ICs (auxiliary PCB) — Each of the two display driver ICs
have 32 high-voltage outputs that enable individual segments of the display
to be turned on or off.
BZ1
TP74
TEST
TP75
TEST
AT17
BEEPER
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