C281x Timer

Configure general-purpose timer in Event Manager module

Description

The C281x contains two event-manager (EV) modules. Each module contains two general-purpose (GP) timers. You can use these timers as independent time bases for various applications.

Use the C281x Timer block to set the periodicity of one GP timer and the conditions under which it posts interrupts. Each model can contain up to four C281x Timer blocks.

The C281x Timer module configures GP Timers that other C281 blocks share. For more information and guidance on sharing timers, see Sharing General Purpose Timers Between C281x Peripherals.

• 

Parameters

Module, Timer no

Select which of four possible timers to configure. Setting Module to A lets you select Timer 1 or Timer 2 in Timer no. Setting Module to B lets you select Timer 3 or Timer 4 in Timer no.

Clock source

When Timer no has a value of Timer 2 or Timer 4, use this parameter to select the clock source for the event timer. You can choose either Internal or QEP circuit. When you select Internal, you can configure other options such as Timer period source, Counting mode, and Timer prescaler.

Timer period source

Select the source of the event timer period. Use Specify via dialog to set the period using Timer period. Select Input port to create an input, T, that accepts the value of the timer period in clock cycles, from 0 to 65535. Timer period source becomes unavailable when Clock source is set to QEP circuit.

Timer period

Set the length of the timer period in clock cycles. Enter a value from 0 to 65535. The value defaults to 10000.

If you know the length of a clock cycle, you can easily calculate how many clock cycles to set for the timer period. The following calculation determines the length of one clock cycle:

$$Sysclk(150MHz)\to HISPCLK(1/2)\to InputClock\mathrm{Pr}escaler(1/128)$$

In this calculation, you divide the System clock frequency of 150 MHz by the high-speed clock prescaler of 2. Then, you divide the resulting value by the timer control input clock prescaler, 128. The resulting frequency is 0.586 MHz. Thus, one clock cycle is 1/.586 MHz, which is 1.706 µs.

Compare value source

Select the source of the compare value. Use Specify via dialog to set the period using the Compare value parameter. Select Input port to create a block input, W, that accepts the value of the compare value, from 0 to 65535.

Compare value

Enter a constant value for comparison to the running timer value for generating interrupts. Enter a value from 0 to 65535. The value defaults to 5000. The timer only generates interrupts if you enable Post interrupt on compare match.

Counting mode

Select Up to generate an asymmetrical waveform output, or Up-down to generate a symmetrical waveform output, as shown in the following illustration.

When you specify the Counting mode as Up (asymmetric) the waveform:

• Starts low

• Goes high when the rising period counter value matches the Compare value

• Goes low at the end of the period

When you specify the Counting mode as Up-down (symmetric) the waveform:

• Starts low

• Goes high when the increasing period counter value matches the Compare value

• Goes low when the decreasing period counter value matches the Compare value

Counting mode becomes unavailable when Clock source is set to QEP circuit.

Timer prescaler

Divide the clock input to produce the desired timer counting rate.

Timer prescaler becomes unavailable when Clock source is set to QEP circuit.

Post interrupt on period match

Generate an interrupt when the value of the timer reaches its maximum value as specified in Timer period.

Post interrupt on underflow

Generate an interrupt when the value of the timer cycles back to 0.

Post interrupt on overflow

Generate an interrupt when the value of the timer reaches its maximum, 65535. Also set Timer period to 65535 for this parameter to work.

Post interrupt on compare match

Generate an interrupt when the value of the timer equals Compare value.

References

TMS320x281x DSP Event Manager (EV) Reference Guide, Literature Number: SPRU065, available from the Texas Instruments Web site.