EM78P312N
|
|
| 4.7 | 10 |
|
|
|
| K | K | Vdd |
OSCI |
|
| 740 |
| |
|
|
|
| ||
| 4 | 10 | 4 |
|
|
EM78P809N |
| K |
|
|
|
EM78P312N |
|
|
|
|
|
|
|
| XTAL |
|
|
|
|
| C1 | C2 |
|
Fig. |
|
| |||
5.16.3 External RC Oscillator Mode
For applications that do not need very precise timing calculation, the RC oscillator offers a lot of cost savings. Nevertheless, it should be noted that the frequency of the RC oscillator is influenced by the supply voltage, the values of the resistor (Rext), the capacitor (Cext), and even by the operation temperature. Moreover, the frequency also varies slightly from one chip to another due to the manufacturing process variation.
In order to maintain a stable system frequency, the values of the Cext should not be less than 20pF, and the value of Rext should not be greater than 1 MΩ, otherwise, the frequency is easily affected by noise, humidity, and leakage.
The smaller the Rext in the RC oscillator, the faster its frequency will be. On the contrary, for very low Rext values, for instance, 1 KΩ, the oscillator becomes unstable because the NMOS cannot correctly discharge the current of the capacitance.
Hence, it must be noted that the supply voltage, the operation temperature, the RC oscillator components, the package types, and the PCB layout, will affect the system frequency.
| Vdd | |
| Rext | |
1 |
| |
OSCI | Cext | |
EM78P809NEM78P312N | ||
|
Fig. 5-33 External RC Oscillator Mode Circuit
52 • | Product Specification (V1.0) 10.03.2006 |
(This specification is subject to change without further notice)
