MC68340 datasheet(359/441 Pages) FREESCALE

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MOTOROLA

MC68340 USER’S MANUAL

8- 15

COUNTER

CLOCK

TGATE

COUNTER

TG BIT SET

TIMEOUT

TO BIT SET

ENABLE

MODEx Bits in Control Register = 110

TGE Bit of the Control Register = 1

Figure 8-10. Event Count Mode

The timer is enabled by setting the SWR and CPE bits in the CR and, if

TGATE

≈ is

enabled (TGE bit of the CR is set), then asserting

TGATE

≈. When the timer is enabled,

the SR ON bit is set. On the next falling edge of the counter clock, the counter is loaded

with the value of $FFFF. With each successive falling edge of the counter clock, the

counter decrements. The PREL1 and PREL2 registers are not used in this mode.

TGATE

≈ is not enabled (CR TGE bit is cleared), then TGATE≈ does not start or stop

the timer or affect the TG bit of the SR. In this case, the counter would begin counting on

the falling edge of the counter clock immediately after the SWR and CPE bits in the CR

are set.

TGATE

≈ is enabled (CR TGE bit is set), then the assertion of TGATE≈ starts the

counter. The negation of

TGATE

≈ disables the counter, sets the SR TG bit, and clears the

ON bit in the SR. If

TGATE

≈ is reasserted, the timer resumes counting from where it was

stopped, and the ON bit is set again. Further assertions and negations of

TGATE

≈ have

the same effect. The TGL bit in the SR reflects the level of

TGATE

≈ at all times.

If the counter counts down to the value stored in the COM register, the COM and TC bits

in the SR are set. If the counter counts down to $0000, a timeout is detected. This event

sets the TO in the SR and clears the COM bit. At timeout, the next falling edge of the

counter clock reloads the counter with $FFFF. TOUTx transitions at timeout or is disabled

as programmed by the CR OC bits. The SR OUT bit reflects the level on TOUTx.

To determine the number of cycles counted, the value in the CNTR must be read,

inverted, and incremented by 1 (the first count is $FFFF which, in effect, includes a count

of zero). The counter counts in a true 216 fashion. For measuring pulses of even greater

duration, the value in the POx bits in the SR are readable and can be thought of as an

extension of the least significant bits in the CNTR.

Freescale Semiconductor, Inc.

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