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C281x QEP

Quadrature encoder pulse circuit


Each F2812 Event Manager has three capture units, which can log transitions on its capture unit pins. Event Manager A (EVA) uses capture units 1, 2, and 3. Event Manager B (EVB) uses capture units 4, 5, and 6.

The quadrature encoder pulse (QEP) circuit decodes and counts quadrature encoded input pulses on these capture unit pins. QEP pulses are two sequences of pulses with varying frequency and a fixed phase shift of 90 degrees (or one-quarter of a period). The circuit counts both edges of the QEP pulses, so the frequency of the QEP clock is four times the input sequence frequency.

The QEP, in combination with an optical encoder, is useful for obtaining speed and position information from a rotating machine. Logic in the QEP circuit determines the direction of rotation by which sequence is leading. For module A, if the QEP1 sequence leads, the general-purpose (GP) Timer counts up and if the QEP2 sequence leads, the timer counts down. The pulse count and frequency determine the angular position and speed.

The C281x QEP module shares GP Timers with other C281 blocks. For more information and guidance on sharing timers, see Sharing General Purpose Timers Between C281x Peripherals.

  • C281x QEP block



Specify which QEP pins to use:

  • A — Uses QEP1 and QEP2 pins.

  • B — Uses QEP3 and QEP4 pins.

Counting mode

Specify how to count the QEP pulses:

  • Counter — Count the pulses based on GP Timer 2 (or GP Timer 4 for EVB).

  • RPM — Count the rotations per minute.

Positive rotation

Defines whether to use Clockwise or Counterclockwise as the direction to use as positive rotation. This field appears only if you select RPM.

Initial count

Initial value for the counter. The value defaults to 0.

Encoder resolution (pulse/revolution)

Number of QEP pulses per revolution. This field appears only if you select RPM.

Enable QEP index

Reset the QEP counter to zero when the QEP index input on CAP3_QEPI1 transitions from low to high.

Enable index qualification mode

Qualify the QEP index input on CAP3_QEPI1. Check that the levels on CAP1_QEP1 and CAP2_QEP2 are high before asserting the index signal as valid.

Timer period

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

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:


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.

Sample time

Time interval, in seconds, between consecutive reads from the QEP pins.

Data type

Data type of the QEP pin data. The circuit reads the data as 16-bit data and then casts it to the selected data type. Valid data types are auto, double, single, int8, uint8, int16, uint16, int32, uint32 or boolean.


For more information on the QEP module, consult the following documents, available at the Texas Instruments® Web site:

  • TMS320x280x, 2801x, 2804x Enhanced Quadrature Encoder Pulse (eQEP) Module Reference Guide, Literature Number SPRU790

  • Using the Enhanced Quadrature Encoder Pulse (eQEP) Module in TMS320x280x, 28xxx as a Dedicated Capture Application Report, Literature Number SPRAAH1