LM3S818 Luminary Micro, Inc, LM3S818 Datasheet - Page 345

• Manufacturer Part Number: LM3S818
• Description: Lm3s818 Arm Microcontroller
• Manufacturer: Luminary Micro, Inc

LM3S818 Data Sheet
the meaning of forward and backward, and to correct for miswiring of the system. Alternatively, the
phase signals can be interpreted as a clock and direction signal as output by some encoders.
The QEI module supports two modes of signal operation: quadrature phase mode and clock/
direction mode. In quadrature phase mode, the encoder produces two clocks that are 90 degrees
out of phase; the edge relationship is used to determine the direction of rotation. In clock/direction
mode, the encoder produces a clock signal to indicate steps and a direction signal to indicate the
direction of rotation. This mode is determined by the SigMode bit of the QEI Control (QEICTL)
register (see page 348).
When the QEI module is set to use the quadrature phase mode (SigMode bit equals zero), the
capture mode for the position integrator can be set to update the position counter on every edge of
the PHA signal or to update on every edge of both PHA and PHB. Updating the position counter on
every PHA and PHB provides more positional resolution at the cost of less range in the positional
counter.
When edges on PHA lead edges on PHB, the position counter is incremented. When edges on PHB
lead edges on PHA, the position counter is decremented. When a rising and falling edge pair is
seen on one of the phases without any edges on the other, the direction of rotation has changed.
The positional counter is automatically reset on one of two conditions: sensing the index pulse or
reaching the maximum position value. Which mode is determined by the ResMode bit of the QEI
Control (QEICTL) register.
When ResMode is 0, the positional counter is reset when the index pulse is sensed. This limits the
positional counter to the values [0:N-1], where N is the number of phase edges in a full revolution
of the encoder wheel. The QEIMAXPOS register must be programmed with N-1 so that the
reverse direction from position 0 can move the position counter to N-1. In this mode, the position
register contains the absolute position of the encoder relative to the index (or home) position once
an index pulse has been seen.
When ResMode is 1, the positional counter is constrained to the range [0:M], where M is the
programmable maximum value. The index pulse is ignored by the positional counter in this mode.
The velocity capture has a configurable timer and a count register. It counts the number of phase
edges (using the same configuration as for the position integrator) in a given time period. The
edge count from the previous time period is available to the controller via the QEISPEED register,
while the edge count for the current time period is being accumulated in the QEICOUNT register.
As soon as the current time period is complete, the total number of edges counted in that time
period is made available in the QEISPEED register (losing the previous value), the QEICOUNT is
reset to 0, and counting commences on a new time period. The number of edges counted in a
given time period is directly proportional to the velocity of the encoder.
Figure 16-2 shows how the Stellaris quadrature encoder converts the phase input signals into
clock pulses, the direction signal, and how the velocity predivider operates (in Divide by 4 mode).
Figure 16-2. Quadrature Encoder and Velocity Predivider Operation
February 6, 2007
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Preliminary