U3750BM
TELEFUNKEN Semiconductors
Power-on Reset and Pin RESET
Pin RESET
It is the power on reset output.
To avoid undefined states of the device when it is powered
on, an internal reset clears the control logic. When the
power supply rises above the internal reference level, the
In test mode, it permits to force the IC U 3750 BM in
permanent DTMF dialing by applying a negative voltage
(test schematic figure 3).
pin RESET goes to high during t . After a line break
rt
longer than t a reset is generated. A short line break
lb,
(< t does not affect the reset. Power on reset timings
lb)
Data Acquisition
(t > t ).
lb
Input data is derived from any standard matrix keyboard
(15 keys) or from a remote microcontroller.
1)
V
RAM
> V
at t = 0 . . . . . . . . . a)
SON
Trt = Tr + T on . . . . . . . . . . . . . . . . . . . . . . b)
Keyboard
The keyboard is connected to the IC by six pins, (see
figure 8). Its matrix is triangular.
a)
b)
Figure 6
Figure 8 Keyboard inputs to the U 3750 BM
2)
V
< V
at t = 0 . . . . . . . . . a)
) + Tr + T on . . . . b)
RAM
SON
Internal pull down resistors (typical value: 120 k ) are
connected to the inputs Ci.
Trt = T (V
to V
S off
RAM
A push button is made by a short-circuit of two pins
among the six.
Entries are scanned every 12 ms or 13.2 ms and go to the
logical state 1 during this scanning.
a)
The scanning is inhibited as soon as a calibrated line-
break is produced at the OL output.
The scanning-cycle has eight phases: six of them are
reserved for the scanning of the six pins, the two others are
kept for the reset of the logic keyboard (T0) and the
acquisition (T7).
A push button is valid, if it is unique and if it’s pressed
long enough (see table of pressed and released push
buttons).
b)
Every acquisition time, T7, the input code is decoded into
5 bits (with or without a pressed push button). The micro-
processor can read it as soon as the logic keyboard has set
a flag, tested about every 800 ms and which indicates that
a pushbutton has been correctly detected.
Figure 7
12 (20)
Rev. A1: 16.07.1996
TEMIC [ TEMIC SEMICONDUCTORS ]
