Overload, Power Limitation, Burst Mode; Standby Mode; Operating Unit Dc (Dc Part); Evaluation Of The Keyboard Matrix - Hitachi VTMX900ECT Wartungshandbuch

2-10
The polarity of the voltages on the transformer is reversed, which
means that the diodes [6300, 6301, 6306, 6308 and 6309] become
conductive and current flows into the capacitors [2301, 2305, 2309,
2311 and 2312] and the load. This current is also ramp-shaped (di/
dt negative, therefore decreasing).
The control adjustment for the switched-mode power supply is
made by changing the conductive phase of the switching transistor
(see Fig.2), so that either more or less energy is transferred from
the rectified mains voltage to [2318] in the transformer. The control
information is provided by the control element [7301]. This element
compares the 5V output voltage via the voltage dividers [3300,
3306, 3336] with an internal 2.5V reference voltage. The output
voltage from [7301] passes via an optocoupler [7300] (for the me-
tallic isolation of the primary and secondary parts) as the current
value to pin 3 on the IC [7303]. The switch-on time for the switching
transistor [7302] is inversely proportional to the value of this
current.

1.6 Overload, power limitation, burst mode:

With an increasing load on one or more power supply outputs, the
switch-on time for the power transistor [7302] also increases, and
thus also the peak value of the delta-shaped current through this
power transistor. The equivalent voltage circuit for this current
profile is passed from resistors [3314] and [3331] via [3312] and
[3347] to pin 2 on the IC [7305]. If the voltage on pin 2 reaches 1V
in one switching cycle, the conductive phase of the switching
transistor is ended immediately. This check is made in each
individual switching cycle. This process ensures that no more than
approx. 48W can be taken out of the mains ( = power limitation ).
If the power supply reaches the power limit, the output voltages
and the supply voltage V on pin 6 of the IC [7303] will be reduced
cc
following further loading. If V
cc
during this process, the output on the IC [7303, pin 5] is blocked. All
output voltages and V are reduced. Once V
cc
below approx. 6.5V, a new start-up cycle begins. If the overload
status or short-circuit remains, the power limitation will be activated
immediately and the voltages will continue to be reduced, followed
by another start-up attempt ( Burst Mode ). The amount of power
taken up from the mains in burst mode is low.

1.7 Standby mode:

In the 'Standby' operating mode on the device, the 'STBY' control
line is used to shut off the output voltages 14AL, 5VA and 5VD on
the power supply to minimise the amount of power taken up from
the mains. The supply to the display heating can also be switched
off using the 'I1WSTBY' control line. The power supply itself will
continue to function continuously in the 'Standby' operating mode
with a switching frequency of 40kHz.
U
3
= U GS
I
D
UDS
Fig. 2
GB
is less than approx. 10V at any point
has dropped to
cc
t
I
Dmax
point of reversal
t
t

2. Operating unit DC (DC part)

The microcontroller TMP93CT76F [7899-A] is a 16 bit
microcontroller fitted with 128Kb ROM and 2.5Kb RAM.
It is the core element of the operating unit, fulfilling the following
tasks with the respective functional groups:
•
Integrated VFD driver
•
Timer
•

Evaluation of the keyboard matrix

•
Decoding the remote control commands from the infra-red
receiver pos. 6170
•
Activation of the display
•
Back-up mode
In normal operation, the µP is operated in dual-clock mode, i.e.
both quartzes [1170, 1171] oscillate. The time is derived from the
slow quartz [1170] (32.768 kHz), and the fast quartz [1171]
(16MHz) is used to generate the system clock frequency.
In case of a mains failure (back-up mode) the µP is not reset, but
instead the mains failure is registered by the IPOR interrupt 3
[7899-B] (pin 67) and the µP is moved into "Sleep mode" (low
power consumption). The 16MHz quartz is turned off and the
32kHz quartz is then used as the clock and system clock frequency.
The operating voltage for the AIO is buffered by a back-up cell [pos.
2174, 2172]. A diode [6171] prevents this gold capacity from
discharging.
2.1 Evaluation of the keyboard matrix
There are 12 different keys. Each key function is assigned a fixed
voltage value. This value is decoded using an analogue/digital (A/
D) port (7899-B, pin 56). Each mechanical key position on the
printed board can adopt any key function via a coding resistor.
Pressing keys simultaneously may lead to undesired functions!
Schematic:
DC-KEY
[7899-B, pin56]

2.2 IR receiver and signal evaluation

The IR receiver [6170] includes a selective, controlled amplifier in
addition to a photo-diode. The photo-diode changes the received
transmission (approx. 940nm) in electrical pulses, which are then
amplified and demodulated. On the output of the IR receiver [7220]
a level lift 0V/5V pulse sequence, which corresponds to the
envelope curve of the received IR remote control command, can be
measured. This pulse sequence is input into the controller for furt-
her signal evaluation via input IRR [7899-B, pin 46].

2.3 Activation and function of the VFD display

In principle, the VFD display [7170] is a tube triode in which the
heating filaments in the tube serve as cathodes (F+,F-). The 7 grids
(G1 - G7) are activated via PC2 - PC7, PD0 on the controller, and
the 16 anodes (P1 - P16) are controlled via ports PE0 - PE7, PF0 -
PF7, PC0, PC1 on the controller, each with a positive potential
compared to the cathode.
The grids and anodes (digits and symbols to be displayed) are
activated in the time-multiplex procedure, voltage lift 5V/-18V. A
dimmer function is generated using pulse-width modulation of the
grid control signals. At maximum display brightness, the pulse
width for each grid is 2.16 ms. It can be reduced, controlled using
software, which reduces the visual brightness of the VFD display
accordingly.
A digit or symbol is only illuminated if the corresponding anode and
the surrounding grid are switched simultaneously to 5V for a certain
time within a scanning period. The electrons emitted from the
cathode are accelerated by the positively charged grid and hit the
luminous layer of the anode which is also positively charged.