74HC_HCT4060_3 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 03 — 14 July 2008 15 of 25
NXP Semiconductors
74HC4060; 74HCT4060
14-stage binary ripple counter with oscillator
13. RC oscillator
13.1 Timing component limitations
The oscillator frequency is mainly determined by R
t
C
t
, provided R2 ≈ 2R
t
and
R2C2 << R
t
C
t
. The function of R2 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 ON resistance in series with it, which
typically is 280 Ω at V
CC
= 2.0 V, 130 Ω at V
CC
= 4.5 V and 100 Ω at V
CC
= 6.0 V.
The recommended values for these components to maintain agreement with the typical
oscillation formula are:
C
t
> 50 pF, up to any practical value and 10 kΩ <R
t
<1 MΩ.
In order to avoid start-up problems, R
t
≥ 1 kΩ.
13.2 Typical crystal oscillator circuit
In Figure 13, R2 is the power limiting resistor. For starting and maintaining oscillation a
minimum transconductance is necessary, so R2 should not be too large. A practical value
for R2 is 2.2 kΩ.
Typical formula for oscillator frequency:
Fig 12. Example of a RC oscillator
001aai121
MR (from logic)
C2 R2 R
t
RTC
11 RS
CTC
10 9
C
t
74HC4060
74HCT4060
f
osc
1
2.5 R
t
× C
t
×
-------------------------------
=
