Freescale Semiconductor, Inc.
16-Bit Tim e r
8.3 Counte r Re g iste r
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). If a read of the free-running counter or counter alternate
register is from the least significant byte (LSB) ($19, $1B), the LSB
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 also must 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). The counter alternate register can be read at any time
without the possibility of missing timer overflow interrupts due to clearing
of the TOF.
General Release Specification
MC68HC05CT4 — Rev. 2.0
16-Bit Timer
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