Z8/Z8Plus Microcontrollers -> Z8 FAQ -> Predicting the Z8 RC Oscillator Frequency
Randal Thornley
21 Sep 1998
The RC oscillator in the Z8 uses a Schmidt trigger in conjunction with an external capacitor and resistor. Normal and low EMI oscillator modes are selected by enabling one of two internal Schmidt triggers.
The oscillator frequency is dependent on the switch points of the Schmidt trigger and the external capacitor and resistor. The following equations model the oscillator.
R and C are the external resistor and capacitor and Vcc is the supply voltage.
The low EMI Schmidt trigger is highly sensitive to the value of the external components due to the low drive strength of this device.
The low EMI oscillator was simulated using multiple values for the external capacitor and resistor. Results from these simulations (shown below) demonstrate that for resistor values below 100Kohms and capacitor values below 100pF the trigger points change dramatically and the frequency approximation is no longer valid.
When using EMI mode with resistor and capacitor values greater than 100k ohms and 100pF respectively, with a nominal value for V1 and V2 and Vcc = 5 V the equations can be simplified to the following.
Operation of the normal mode RC oscillator can be simplified in similar fashion.
The normal mode Schmidt trigger is not greatly affected by external component sizing. Therefore this equation is valid over a wide range of component values.
*V1 and V2 values come from simulation
|
LOW EMI MODE |
Simulated |
|
Calculated |
|
|
Simplified |
||||||
|
Vcc |
Cap Value |
Resistor value |
V1 |
V2 |
T |
F |
T1 |
T2 |
T |
F |
F |
|
|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
5 |
2.00E-10 |
1.00E+04 |
1.12 |
2.82 |
3.01E-06 |
3.32E+05 |
1.15E-06 |
1.84E-06 |
2.99E-06 |
3.35E+05 |
||
|
5 |
2.00E-10 |
2.50E+04 |
1.28 |
2.67 |
5.88E-06 |
1.70E+05 |
2.33E-06 |
3.67E-06 |
6.00E-06 |
1.67E+05 |
|
|
|
5 |
2.00E-10 |
5.00E+04 |
1.36 |
2.58 |
1.04E-05 |
9.62E+04 |
4.09E-06 |
6.43E-06 |
1.05E-05 |
9.50E+04 |
|
|
|
5 |
2.00E-10 |
1.00E+05 |
1.41 |
2.52 |
1.90E-05 |
5.25E+04 |
7.39E-06 |
1.16E-05 |
1.90E-05 |
5.26E+04 |
5.56E+04 |
|
|
5 |
2.00E-10 |
1.47E+05 |
1.43 |
2.49 |
2.69E-05 |
3.71E+04 |
1.04E-05 |
1.64E-05 |
2.68E-05 |
3.74E+04 |
3.78E+04 |
|
|
5 |
2.00E-10 |
2.00E+05 |
1.44 |
2.48 |
3.54E-05 |
2.83E+04 |
1.37E-05 |
2.16E-05 |
3.54E-05 |
2.83E+04 |
2.78E+04 |
|
|
5 |
2.00E-10 |
3.00E+05 |
1.46 |
2.46 |
5.18E-05 |
1.93E+04 |
2.00E-05 |
3.15E-05 |
5.16E-05 |
1.94E+04 |
1.85E+04 |
|
|
Simulated |
Calculated |
Simplified |
||||||||||
|
Vcc |
Cap Value |
Resistor value |
V1 |
V2 |
T |
F |
T1 |
T2 |
T |
F |
F |
|
|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
5 |
1.00E-11 |
1.47E+05 |
1.08 |
2.85 |
2.60E-06 |
3.84E+05 |
8.80E-07 |
1.42E-06 |
2.30E-06 |
4.35E+05 |
|
|
|
5 |
5.00E-11 |
1.47E+05 |
1.33 |
2.61 |
8.20E-06 |
1.22E+05 |
3.15E-06 |
4.95E-06 |
8.10E-06 |
1.23E+05 |
|
|
|
5 |
1.00E-10 |
1.47E+05 |
1.39 |
2.54 |
1.46E-05 |
6.85E+04 |
5.63E-06 |
8.85E-06 |
1.45E-05 |
6.90E+04 |
7.56E+04 |
|
|
5 |
2.00E-10 |
1.47E+05 |
1.43 |
2.49 |
2.69E-05 |
3.71E+04 |
1.04E-05 |
1.64E-05 |
2.68E-05 |
3.74E+04 |
3.78E+04 |
|
|
5 |
3.00E-10 |
1.47E+05 |
1.45 |
2.47 |
3.87E-05 |
2.59E+04 |
1.51E-05 |
2.37E-05 |
3.88E-05 |
2.58E+04 |
2.52E+04 |
|
|
5 |
5.00E-10 |
1.47E+05 |
1.46 |
2.45 |
|
|
2.42E-05 |
3.81E-05 |
6.22E-05 |
1.61E+04 |
1.51E+04 |
