U3750BM
Power-on Reset and Pin RESET
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
pin RESET goes to high during t
rt
. After a line break
longer than t
lb,
a reset is generated. A short line break
(< t
lb)
does not affect the reset. Power on reset timings
(t > t
lb
).
1)
TELEFUNKEN Semiconductors
Pin RESET
It is the power on reset output.
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).
Data Acquisition
Input data is derived from any standard matrix keyboard
(15 keys) or from a remote microcontroller.
V
RAM
> V
SON
at t = 0 . . . . . . . . . a)
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
2)
V
RAM
< V
SON
at t = 0 . . . . . . . . . a)
Figure 8
Keyboard inputs to the U 3750 BM
Trt = T (V
RAM
to V
S off
) + Tr + T on . . . . b)
Internal pull down resistors (typical value: 120 k
W
) are
connected to the inputs Ci.
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.
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).
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.
Rev. A1: 16.07.1996
a)
b)
Figure 7
12 (20)
TEMIC [ TEMIC SEMICONDUCTORS ]
