Datasheet
January 1995 4
Philips Semiconductors Product specification
Programmable timer
HEF4541B
MSI
PINNING
FREQUENCY SELECTION TABLE
A
0
,A
1
address inputs
MODE mode select input
AR auto reset input
MR master reset input
PH phase input
R
TC
external resistor connection (R
t
)
C
TC
external capacitor connection (C
t
)
RS external resistor connection (R
S
)or
external clock input
A
0
A
1
NUMBER OF
COUNTER STAGES n
L L 13 8 192
L H 10 1 024
H L 8 256
H H 16 65 536
f
osc
f
out
--------- 2
n
=
FUNCTION TABLE
Notes
1. For correct power-on reset, the supply voltage should
be above 8.5 V. For V
DD
< 8.5 V, disable the autoreset
and connect AR to V
DD
.
2. The timer is initialized on a reset pulse and the output
changes state after 2
n-1
counts and remains in that
state (latched). Reset of this latch is obtained by
master reset or by a LOW to HIGH transition on the
MODE input.
H = HIGH state (the more positive voltage)
L = LOW state (the less positive voltage)
X = state is immaterial
INPUTS
MODE
AR MR PH MODE
H L X X auto reset disabled
L L X X auto reset enabled
(1)
X H X X master reset active
X L X H normal operation selected
division to output
X L X L single-cycle mode
(2)
X L L X output initially LOW,
after reset
X L H X output initially HIGH,
after reset
RC oscillator
Fig.4 External component connection for RC
oscillator; R
S
≈ 2 R
t
.
Typical formula for oscillator
frequency:
f
osc
1
23 R
t
C
t
××,
---------------------------------=
Timing component limitations
The oscillator frequency is mainly determined by
R
t
C
t
, provided R
t
<< R
S
and R
S
C2 << R
t
C
t
. The function of
R
S
is to minimize the influence of the forward voltage
across the input protection diodes on the frequency. The
stray capacitance C2 should be kept as small as possible.
In consideration of accuracy, C
t
must be larger than the
inherent stray capacitance. R
t
must be larger than the
LOCMOS ‘ON’ resistance in series with it, which typically
is 500 Ω at V
DD
= 5 V, 300 Ω at V
DD
= 10 V and 200 Ω at
V
DD
= 15 V.
The recommended values for these components to
maintain agreement with the typical oscillation formula are:
C
t
≥ 100 pF, up to any typical value,
10 kΩ≤R
t
≤1MΩ.