Philips Semiconductors
Product specification
Programmable timer
PINNING
A
0
, A
1
MODE
AR
MR
PH
R
TC
C
TC
RS
address inputs
mode select input
auto reset input
master reset input
phase input
external resistor connection (R
t
)
external capacitor connection (C
t
)
external resistor connection (R
S
) or
external clock input
AR
H
L
X
X
X
X
FREQUENCY SELECTION TABLE
X
A
0
L
L
H
H
A
1
L
H
L
H
NUMBER OF
COUNTER STAGES n
13
10
8
16
f
osc
n
-------- =
2
-
f
out
8 192
1 024
256
65 536
L
H
X
FUNCTION TABLE
INPUTS
MODE
MR
L
L
H
L
L
L
PH
X
X
X
X
X
L
MODE
X
X
X
H
L
X
HEF4541B
MSI
auto reset disabled
auto reset enabled
(1)
master reset active
normal operation selected
division to output
single-cycle mode
(2)
output initially LOW,
after reset
output initially HIGH,
after reset
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
RC oscillator
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
≤
1 MΩ.
Typical formula for oscillator
frequency:
1
f
osc
= ---------------------------------
2
,
3
×
R
t
×
C
t
Fig.4
External component connection for RC
oscillator; R
S
≈
2 R
t
.
January 1995
4
PHILIPS [ NXP SEMICONDUCTORS ]
