DRA790 Datasheet, PDF (357/436 Pages) Texas Instruments – Infotainment Applications Processor

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DRA790, DRA791
DRA793, DRA797
SPRS968A – AUGUST 2016 – REVISED FEBRUARY 2017
• Period and duty cycle measurements of pulse train signals
• Decoding current or voltage amplitude derived from duty cycle encoded current/voltage sensors
The eCAP module includes the following features:
• 32-bit time base counter
• 4-event time-stamp registers (each 32 bits)
• Edge polarity selection for up to four sequenced time-stamp capture events
• Interrupt on either of the four events
• Single shot capture of up to four event time-stamps
• Continuous mode capture of time-stamps in a four-deep circular buffer
• Absolute time-stamp capture
• Difference (Delta) mode time-stamp capture
• All above resources dedicated to a single input pin
• When not used in capture mode, the ECAP module can be configured as a single channel PWM output
For more information, see section Enhanced Capture (eCAP) Module in chapter Pulse-Width Modulation
Subsystem of the device TRM.
6.11.17 eQEP
A single track of slots patterns the periphery of an incremental encoder disk, as shown in Figure 6-2.
These slots create an alternating pattern of dark and light lines. The disk count is defined as the number
of dark/light line pairs that occur per revolution (lines per revolution). As a rule, a second track is added to
generate a signal that occurs once per revolution (index signal: QEPI), which can be used to indicate an
absolute position. Encoder manufacturers identify the index pulse using different terms such as index,
marker, home position, and zero reference.
QEPA
QEPB
QEPI
Figure 6-2. Optical Encoder Disk
eqep-001
To derive direction information, the lines on the disk are read out by two different photo-elements that
"look" at the disk pattern with a mechanical shift of 1/4 the pitch of a line pair between them. This shift is
realized with a reticle or mask that restricts the view of the photo-element to the desired part of the disk
lines. As the disk rotates, the two photo-elements generate signals that are shifted 90 degrees out of
phase from each other. These are commonly called the quadrature QEPA and QEPB signals. The
clockwise direction for most encoders is defined as the QEPA channel going positive before the QEPB
channel.
The encoder wheel typically makes one revolution for every revolution of the motor or the wheel may be at
a geared rotation ratio with respect to the motor. Therefore, the frequency of the digital signal coming from
the QEPA and QEPB outputs varies proportionally with the velocity of the motor. For example, a 2000-line
encoder directly coupled to a motor running at 5000 revolutions per minute (rpm) results in a frequency of
166.6 KHz, so by measuring the frequency of either the QEPA or QEPB output, the processor can
determine the velocity of the motor.
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