MC68HC05C8A Datasheet, PDF (61/158 Pages) Motorola, Inc – Microcontrollers

8.3 Counter
Timer
Counter
The key element in the programmable timer is a 16-bit, free-running
counter or counter register, preceded by a prescaler that divides the
internal processor clock by four. The prescaler gives the timer a
resolution of 2.0 microseconds if the internal bus clock is 2.0 MHz. The
counter is incremented during the low portion of the internal bus clock.
Software can read the counter at any time without affecting its value.
The double-byte, free-running counter can be read from either of two
locations, $18, $19 (counter register) or $1A, $1B (counter alternate
register). A read from only the least significant byte (LSB) of the free-
running counter ($19, $1B) receives the count value at the time of the
read. If a read of the free-running counter or counter alternate register
first addresses the most significant byte (MSB) ($18, $1A), the LSB ($19,
$1B) is transferred to a buffer. This buffer value remains fixed after the
first MSB read, even if the user reads the MSB several times. This buffer
is accessed when reading the free-running counter or counter alternate
register LSB ($19 or $1B) and, thus, completes a read sequence of the
total counter value. In reading either the free-running counter or counter
alternate register, if the MSB is read, the LSB must also be read to
complete the sequence.
The counter alternate register differs from the counter register in one
respect: A read of the counter register MSB can clear the timer overflow
flag (TOF). Therefore, the counter alternate register can be read at any
time without the possibility of missing timer overflow interrupts due to
clearing of the TOF.
The free-running counter is configured to $FFFC during reset and is
always a read-only register. During a power-on reset, the counter is also
preset to $FFFC and begins running after the oscillator start-up delay.
Because the free-running counter is 16 bits preceded by a fixed divide-
by-four prescaler, the value in the free-running counter repeats every
262,144 internal bus clock cycles. When the counter rolls over from
$FFFF to $0000, the TOF bit is set. An interrupt can also be enabled
whenever counter rollover occurs by setting its interrupt enable bit
(TOIE).
MC68HC05C8A • MC68HCL05C8A • MC68HSC05C8A — Rev. 5.0
MOTOROLA
Timer
Technical Data
61