Maxim-integrated MAXQ622 Manual do Utilizador descarregar pdf (Página 121)

MAXQ612/MAXQ622 User’s Guide

Maxim Integrated 7-7

The set and reset functions for the autoreload up-counting mode essentially provide the same functionality. They pro-

vide a 16-bit PWM with the ability to change the frequency using the TBR reload value. The toggle mode allows a 50%

duty cycle waveform to be created (when the TBC register is configured to a value inside the counting range, i.e., 0 <

TBC < TBR, with Timer B running). With the TBC register outside of the count range, the set and reset functions allow a

timed clear or set of a given pin without need of polling or interrupting the CPU such that it can manually be performed.

Up-count set, reset PWM duty cycle can be calculated as follows (where period = TBR + 1):

Set mode = (TBR - TBC)/(TBR + 1)

Reset mode = TBC/(TBR + 1)

7.1.5.2 Timer B Mode: Up/Down-Count PWM Output Mode

The 16-bit up/down-count timer is utilized for the up/down-count PWM output mode to produce center-aligned PWM

output. When the Timer B is configured in the up/down-count mode, the external pin (TBB) is used to control the direc-

tion of the timer count. When the Timer B PWM output functionality is enabled at the same time as the up/down-count

autoreload mode, the TBB pin no longer controls the direction of counting. Instead, the up/count count is controlled

internally. When the timer is up counting upward and reaches TBR, in the next cycle, it reverses its direction of count-

ing. When the timer is down counting and reaches 0000h, it reverses direction so as to begin up counting (as illustrated

in Figure 7-6). When the up/down autoreload and PWM output modes are both enabled, the TBB input function can

still be enabled by the EXENB = 1 configuration. Enabling the TBB input function during this mode allows detection

of PWM output negative edges to set the EXFB interrupt flag. Note that the EXFB flag can be set independent of the

state of the TRB bit (e.g., EXFB can still be set on detection of a negative edge when TRB = 0). While it is most likely

that TRB = 1 when EXFB is set, since TRB = 1 is required to enable the PWM output, a negative edge on the TBB pin

while TRB = 0 can still result in setting of EXFB. Using the standard GPI/O port controls to generate a negative edge

when the PWM is not running, for instance, can set EXFB.

Figure 7-6. Timer B PWM Output Up/Down-Count Examples

1 2 3 4 5 6 7 8 7 6 5 4 3 2 1 0 1 2 3

TBR = 0008h

TBV = 0000

TBB PIN

SET, TBC = 7

SET, TBC = 0

SET, TBC = 5

SET, TBC = 6

SET, TBC = 8

SET, TBC = 4

UPDOWN

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Maxim-integrated MAXQ622 Manual do Utilizador Página 121