C8051F850-C-GM Datasheet, PDF (171/328 Pages) Silicon Laboratories – Low-Cost 8-bit MCU Family with up to 8 kB of Flash

20.3.5. Frequency Output Mode
Frequency Output Mode produces a programmable-frequency square wave on the module’s associated
CEXn pin. The capture/compare module high byte holds the number of PCA clocks to count before the
output is toggled. The frequency of the square wave is then defined by Equation 20.1.
F_CEXn
=
---------------^F----^P---^C---^A---------------
2 × PCA0CPHn
Note: A value of 0x00 in the PCA0CPHn register is equal to 256 for this equation.
Equation 20.1. Square Wave Frequency Output
Where F_PCA is the frequency of the clock selected by the CPS2–0 bits in the PCA mode register,
PCA0MD. The lower byte of the capture/compare module is compared to the PCA counter low byte; on a
match, n is toggled and the offset held in the high byte is added to the matched value in PCA0CPLn.
Frequency Output Mode is enabled by setting the ECOMn, TOGn, and PWMn bits in the PCA0CPMn
register. Note that the MATn bit should normally be set to 0 in this mode. If the MATn bit is set to 1, the
CCFn flag for the channel will be set when the 16-bit PCA0 counter and the 16-bit capture/compare
register for the channel are equal.
ECOMn
(Compare Enable)
PCA0CPLn
8-bit Adder
Adder
Enable
8-bit
match
Comparator
Toggle
PCA0CPHn
CEXn
TOGn (Toggle Enable)
PCA Clock
PCA0L
Figure 20.5. PCA Frequency Output Mode
Rev. 1.0
151