Datasheet

TMC5130A DATASHEET (Rev. 1.11 / 2015-OCT-08) 88
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17.2 Changing Resolution
Sometimes operation of a motor in reduced microstep resolution is desired, in order to stay
compatible to an older, less performing driver, or, when using motion controllers with limited
frequency capabilities for the STEP/DIR interface. The internal microstep table uses 1024 sine wave
entries to generate the wave. The step width taken within the table depends on the microstep
resolution setting. Depending on the DIR input, the microstep counter is increased (DIR=0) or
decreased (DIR=1) with each STEP pulse by the step width. In principle, the microstep resolution can
be changed at any time. The microstep resolution determines the increment respectively the
decrement, the sequencer uses for advancing in the microstep table. At maximum resolution, it
advances one step for each step pulse. At half resolution, it advances two steps and so on. This way,
a change of resolution is possible transparently at each time.
The sequencer has special provision to allow seamless switching between different microstep rates.
When the microstep resolution becomes switched to a lower resolution, it calculates the nearest step
within the target resolution and reads the current vector at that position. This behavior is especially
important for low resolutions like fullstep and halfstep, because any failure in the step sequence
would lead to asymmetrical run when comparing a motor running clockwise and counterclockwise.
Generally, different microstep resolutions are realized by stepping through the internal 256 entry
microstep table in more coarse steps. In 256 microstep resolution, 1024 steps are done for a full
electrical revolution using an increment of one. The increment is higher for lower resolutions, up to
256 for fullstep. When a lower resolution, each calculated table pointer becomes modified as follows,
in order to point to the nearest valid microstep table address in the target resolution:
Fullstep: The first valid table position is 128 (45° electrical position, i.e. both coils on identical
current). This value is the RMS-Value of 0.7 * sine wave amplitude. Step size is 256 (90°
electrical)
Half step: The first valid table position is 64 (22.5° electrical), Step size is 128 (45° steps)
Quarter step: The first valid table position is 32 (i.e. 90°/8=11.25° electrical), Step size is 64 (22.5°
steps)
etc.
This way equidistant steps result and they are identical in both rotation directions. Some older drivers
also use zero current (table entry 0, 0°) as well as full current (90°) within the step tables. This kind of
stepping is avoided because it provides less torque and has a worse power dissipation in driver and
motor. Especially for full stepping the condition one coil at maximum current and one coil off should
be avoided, because in this condition only one coil contributes to the motion at each point of time.
Step position
MSCNT value
current coil A
current coil B
Half step 0
0
0%
100%
Full step 0
128
70.7%
70.7%
Half step 1
256
100%
0%
Full step 1
384
70.7%
-70.7%
Half step 2
512
0%
-100%
Full step 2
640
-70.7%
-70.7%
Half step 3
768
-100%
0%
Full step 3
896
-70.7%
70.7%