MC68340 datasheet(346/441 Pages) FREESCALE

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8- 2

MC68340 USER’S MANUAL

MOTOROLA

— Pulse-Width Measurement

— Period Measurement

— Event Counting

• Seven Maskable Interrupt Conditions Based on Programmable Events

8.1.1 Timer and Counter Functions

The term 'timer' is used to reference either timer 1 or timer 2, since the two are functionally

equivalent.

The timer can perform virtually any application traditionally assigned to timers and

counters. The timer can be used to generate timed events that are independent of the

timing errors to which real-time programmed microprocessors are susceptible—for

example, those of dynamic memory refreshing, DMA cycle steals, and interrupt servicing.

The timer has several functional areas: an 8-bit countdown prescaler, a 16-bit

downcounter, timeout logic, compare logic, and clock selection logic. Figure 8-2 shows a

functional diagram of the timer module.

8.1.1.1 PRESCALER AND COUNTER. The counter can be driven directly by the selected

clock or the prescaler output. Both the counter and prescaler are updated on the falling

edge of the clock. During reset, the prescaler is set to $FF, and the counter is set to

$0000. The counter is loaded with a programmed value on the first falling edge of the

counter clock after the timer is enabled and again when a timeout occurs (counter reaches

$0000). The prescaler and counter can be used as one 24-bit counter by enabling the

prescaler and selecting the divide-by-256 prescaler output. Refer to 8.4 Register

Description for additional information on how to program the timer.

8.1.1.2 TIMEOUT DETECTION. Timeout is achieved when all 16 stages of the counter

transition to zero, a counter value of $0000. Timeout is a defined counter event which

triggers specific actions depending upon the programmed mode of operation. Refer to 8.3

Operating Modes for descriptions of the individual modes.

8.1.1.3 COMPARATOR. The comparator block compares the value in the 16-bit compare

bits in the status register (SR) are set to indicate this condition. When in the input

capture/output compare mode, a match is a defined counter event that can affect the

output of the timer (TOUTx). Refer to 8.3.1 Input Capture/Output Compare for additional

information on this mode.

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MC68340 Datasheet(PDF) 346 Page - Freescale Semiconductor, Inc