MAXQ612/MAXQ622 User’s Guide
7-4 Maxim Integrated
7.1.3 Timer B Mode: Up/Down Autoreload Mode
The up/down-count autoreload option is enabled by the DCEN (TBCN.4) bit. When DCEN is set to 1, Timer B counts up
or down as controlled by the state of TBB pin. TBB causes upward counting when a high is applied and down counting
when a low is applied. When DCEN = 0, Timer B only counts up.
When an upward counting overflow occurs (TBV overflow occurs after reaching TBR), a 0000h value loads into TBV. In
the down-count direction, an underflow occurs when TBV reaches 0000h. When an underflow occurs, the TBR value
is loaded into TBV counting continues.
Note that in this mode, the overflow/underflow output of the timer is provided to an edge-detection circuit as well as
to the TFB bit (TBCN.7). This edge-detection circuit toggles the EXFB bit (TBCN.6) on every overflow or underflow.
Therefore, the EXFB bit behaves as a 17th bit of the counter, and can be used as such.
7.1.4 Timer B Mode: Clock Output Mode
Timer B can also be configured to drive a clock output on the TBA port pin as shown in Figure 7-4. To configure Timer
B for this mode, first it must be set to 16-bit autoreload timer mode (CP/RLB = 0, C/TB = 0). Next, the TBOE (TBCN.5)
bit must be set to 1. The output state for this mode is always set to 1 each time the TBOE bit is changed from 0 to 1.
TRB (TBCN.2) must also be set to 1 to enable the timer and the corresponding output. If the timer is stopped (TRB =
0) and subsequently restarted (TRB = 1) while leaving TBOE = 1, the previous timer clock output state is restored on
the TBA pin. The DCEN bit has no effect in this mode. This mode produces a 50% duty-cycle square-wave output. The
frequency of the square wave is given by the formula in the figure. Each timer overflow causes an edge transition on
the pin, i.e., the state of the pin toggles. The timer overflow flag (TFB) is still set on an overflow in clock output mode,
however, the TBOE = 1 condition prevents this flag from causing an interrupt. The Timer B external interrupt is still
available for use when enabled (EXENB = 1). 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).
Figure 7-3. Timer B Up/Down Autoreload Mode
TFB
EXFB
TIMER B INTERRUPT
TBB PIN
1
TBA PIN
TRB
0
SYSTEM
CLOCK
CLOCK
PRESCALER
TBPS[2:0]
COUNT DIRECTION
(1 = UP, 0 = DOWN)
C/TB
150
TBR
(DOWN-COUNTING RELOAD)
(UP-COUNTING RELOAD VALUE)
TBV
150
0000h
(93 paginas)
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