MC68HC05C8A Datasheet, PDF (41/116 Pages) Motorola, Inc – Microcontrollers

Chapter 8
Timer
8.1 Introduction
The timer consists of a 16-bit, software-programmable counter driven by a fixed divide-by-four prescaler.
This timer can be used for many purposes, including input waveform measurements while simultaneously
generating an output waveform. Pulse widths can vary from several microseconds to many seconds.
Refer to Figure 8-1 for a timer block diagram.
Because the timer has a 16-bit architecture, each specific functional segment (capability) is represented
by two registers. These registers contain the high and low byte of that functional segment. Generally,
accessing the low byte of a specific timer function allows full control of that function; however, an access
of the high byte inhibits that specific timer function until the low byte is also accessed.
NOTE
The I bit in the condition code register should be set while manipulating both
the high and low byte register of a specific timer function to ensure that an
interrupt does not occur.
8.2 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.
MC68HC05C8A • MC68HCL05C8A • MC68HSC05C8A Data Sheet, Rev. 5.1
Freescale Semiconductor
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