LM3S8G62 Datasheet, PDF (953/1096 Pages) Texas Instruments – Stellaris® LM3S8G62 Microcontroller

OBSOLETE: TI has discontinued production of this device.
Stellaris^® LM3S8G62 Microcontroller
signal or to update on every edge of both PhA and PhB. Updating the position counter on every PhA
and PhB edge 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. The reset mode is determined by the RESMODE bit of the
QEICTL register.
When RESMODE is set, the positional counter is reset when the index pulse is sensed. This mode
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 QEI Maximum Position (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 clear, 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.
Velocity capture uses a configurable timer and a count register. The timer 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 QEI Velocity
(QEISPEED) register, while the edge count for the current time period is being accumulated in the
QEI Velocity Counter (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
(overwriting the previous value), the QEICOUNT register is cleared, 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 20-2 on page 953 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 20-2. Quadrature Encoder and Velocity Predivider Operation
PhA
PhB
clk
clkdiv
dir
pos -1 -1 -1 -1 -1 -1 -1 -1 -1
rel +1
+1
+1
+1 +1 +1 +1 +1 +1 +1 +1
+1
+1
-1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
+1
+1
+1
The period of the timer is configurable by specifying the load value for the timer in the QEI Timer
Load (QEILOAD) register. When the timer reaches zero, an interrupt can be triggered, and the
hardware reloads the timer with the QEILOAD value and continues to count down. At lower encoder
speeds, a longer timer period is required to be able to capture enough edges to have a meaningful
result. At higher encoder speeds, both a shorter timer period and/or the velocity predivider can be
used.
The following equation converts the velocity counter value into an rpm value:
rpm = (clock * (2 ^ VELDIV) * SPEED * 60) ÷ (LOAD * ppr * edges)
July 24, 2012
953
Texas Instruments-Production Data