HT37B90/HT37B70/HT37B50/HT37B30
Oscillator
Various oscillator options offer the user a wide range of
functions according to their various application require-
ments. Two types of system clocks can be selected
while various clock source options for the Watchdog
Timer are provided for maximum flexibility. All oscillator
options are selected through the configuration options.
with the crystal or resonator manufacturer¢s specifica-
tion. The external parallel feedback resistor, Rp, is nor-
mally not required but in some cases may be needed to
assist with oscillation start up.
Internal Ca, Cb, Rf Typical Values @ 5V, 25°C
Ca
Cb
Rf
The two methods of generating the system clock are:
·
7pF~9pF
9pF~11pF
External crystal/resonator oscillator
300kW
·
External RC oscillator
Oscillator Internal Component Values
External RC Oscillator
One of these two methods must be selected using the
configuration options.
Using the external system RC oscillator requires that a
resistor, with a value between 82kW and 180kW, is con-
nected between OSC1 and VSS. The generated system
clock divided by 8 will be provided on OSC2 as an output
which can be used for external synchronization pur-
poses. Note that as the OSC2 output is an NMOS
open-drain type, a pull high resistor should be connected
if it to be used to monitor the internal frequency. Although
this is a cost effective oscillator configuration, the oscilla-
tion frequency can vary with VDD, temperature and pro-
cess variations and is therefore not suitable for
applications where timing is critical or where accurate os-
cillator frequencies are required. For the value of the ex-
ternal resistor. Note that it is the only microcontroller
internal circuitry together with the external resistor, that
determine the frequency of the oscillator. The external
capacitor shown on the diagram does not influence the
frequency of oscillation.
More information regarding the oscillator is located in
Application Note HA0075E on the Holtek website.
External Crystal/Resonator Oscillator
The simple connection of a crystal across OSC1 and
OSC2 will create the necessary phase shift and feed-
back for oscillation, and will normally not require exter-
nal capacitors. However, for some crystals and most
resonator types, to ensure oscillation and accurate fre-
quency generation, it may be necessary to add two
small value external capacitors, C1 and C2. The exact
values of C1 and C2 should be selected in consultation
Crystal/Resonator Oscillator
External RC Oscillator
Rev. 1.00
43
June 22, 2017
HOLTEK [ HOLTEK SEMICONDUCTOR INC ]
