C8051F970-A-GM Datasheet, PDF (427/454 Pages) Silicon Laboratories – Low Power Capacitive Sensing MCU with up to 32 kB of Flash

C8051F97x
33.3.4. 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 33.1.
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 33.1. 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, CEXn 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.
Write to
PCA0CPLn
0
ENB
Reset
Write to
PCA0CPHn ^ENB
1
PCA0CPMn
PECCMT PE
WC A A A O WC
MO P P T GMC
1 MPN n n n F
6nnn
n
n
x 000
x
PCA0CPLn
Enable
8-bit
Comparator
8-bit Adder
PCA0CPHn
Adder
Enable
Toggle
match
TOGn
0 _{CEXn Crossbar}
1
Port I/O
PCA Timebase
PCA0L
Figure 33.7. PCA Frequency Output Mode
33.3.5. 8-bit, 9-bit, 10-bit and 11-bit Pulse Width Modulator Modes
Each module can be used independently to generate a pulse width modulated (PWM) output on its associated
CEXn pin. The frequency of the output is dependent on the timebase for the PCA counter/timer, and the setting of
the PWM cycle length (8, 9, 10 or 11-bits). For backwards-compatibility with the 8-bit PWM mode available on other
devices, the 8-bit PWM mode operates slightly different than 9, 10 and 11-bit PWM modes. It is important to note
that all channels configured for 8/9/10/11-bit PWM mode will use the same cycle length. It is not possible to
configure one channel for 8-bit PWM mode and another for 11-bit mode (for example). However, other PCA
channels can be configured to Pin Capture, High-Speed Output, Software Timer, Frequency Output, or 16-bit PWM
mode independently.
428
Rev 1.0