7.10 Pulse Accumulator
The pulse accumulator counter (PACNT) is an 8-bit read/write up-counter. PACNT can
operate in external event counting or gated time accumulation modes. Figure 7-5 is a
block diagram of the pulse accumulator.
In event counting mode, the counter increments each time a selected transition of the
pulse accumulator input (PAI) pin is detected. The maximum clocking rate is the sys-
tem clock divided by four.
In gated time accumulation mode a clock increments PACNT while the PAI pin is in
the active state. There are four possible clock sources.
Two bits in the TFLG2 register show pulse accumulator status. The pulse accumulator
flag (PAIF) indicates that a selected edge has been detected at the PAI pin. The pulse
accumulator overflow flag (PAOVF) indicates that the pulse accumulator count has
rolled over from $FF to $00. This can be used to extend the range of the counter be-
yond eight bits.
An interrupt request can be made when each of the status flags is set. However, op-
eration of the PAI interrupt depends on operating mode. In event counting mode, an
interrupt is requested when the edge being counted is detected. In gated mode, the
request is made when the PAI input changes from active to inactive state. Interrupt re-
quests are enabled by the PAOVI and PAII bits in the TMSK2 register.
7
Bits in the pulse accumulator control register (PACTL) control the operation of PACNT.
The PAMOD bit selects event counting or gated operation. In event counting mode,
the PEDGE control bit determines whether a rising or falling edge is detected; in gated
mode, PEDGE specifies the active state of the gate signal. Bits PACLK[1:0] select the
clock source used in gated mode.
PACTL and PACNT are implemented as one 16-bit register, but can be accessed with
byte or word access cycles. Both registers are cleared at reset, but the PAIS and
PCLKS bits show the state of the PAI and PCLK pins.
The PAI pin can also be used for general-purpose input. The logic state of the PAIS
bit in PACTL shows the state of the pin.
MOTOROLA
7-14
GENERAL-PURPOSE TIMER
MC68331
USER’S MANUAL
MOTOROLA [ MOTOROLA ]
