AMIS-30421 Datasheet, PDF (19/41 Pages) ON Semiconductor – Micro-Stepping Stepper Motor Bridge Controller

AMIS−30421
Direction
The direction of rotation can be changed by means of the
DIR−pin and the SPI bit <DIRCTRL>. See also Figure 12
up to Figure 15. Setup and hold times need to be respected
when changing direction (see Figure 6).
NXT Input
Every rising or falling edge on the NXT−pin (selectable
through SPI bit <NXTP>) will move the coil current one
step up or down (dependant on the DIR−pin and
<DIRCTRL> bit) in the translator table (see Table 7). The
motor current will be updated at the next PWM cycle.
Enable
The enable SPI bit <MOTEN> is used to enable the PWM
regulator and drive coil current through the stepper motor
coils. When ‘1’ the motor driver is enabled and coil current
will be conducted. If ‘0’ (zero), the H−bridge drivers are
disabled.
When the motor driver is enabled, the NXT− and DIR−pin
as also the <DIRCTRL> SPI bit can be used to control the
movement of the stepper motor. It’s not allowed to apply
pulses on the NXT−pin when the motor driver is disabled.
Certain errors (see Error Output p24) will automatically
disable the motor driver (<MOTEN> = 0). The errors first
need to be cleared before one is able to enable the motor
driver again.
Setup and hold times need to be respected (see Figure 6).
Microstep Position
To be able to track the position in the current translator
table (Table 7), the microstep position SPI byte can be used
(<MSP[7:0]>). This byte gives the position within the
current translator table in units of 1/64 microsteps. This
means that when working in 1/4^th microstepping the read out
microstep positions will be 0, 16, 32, ...
The microstep position can be used to track/verify the real
position of the stepper motor and as a reference point for
changing the stepping mode (to avoid phase shift (see
further)). See also Application Note AND8399 for more
information on this (this application note is based on
AMIS−305xx but is similar for AMIS−30421).
Keep in mind that <MSP[7:0]> will only be update 1 ms
after the NXT pulse was applied.
VDIR
t
V_NXT
Step up in
Step up in
Step down in
Step down in
t
translator table
translator table
translator table
translator table
Figure 16. Translator table update
Microstep
<SM[2:0]> is used to set the microstep stepping mode.
Changing to another microstep stepping mode can be done
but the setup and hold timings need to be respected (see
Figure 6). Additionally, one needs to be careful to not
introduce an offset (or phase shift) in the translator table.
Increasing to a higher stepping mode (e.g. from 1/2 to 1/4)
can be done at any moment without introducing an offset or
phase shift. Decreasing to a lower stepping mode (e.g. from
1/4 to 1/2) can introduce an offset or phase shift if the change
to the lower stepping mode is not done at the right moment.
One needs to make sure that the translator table position is
shared both by the old and new stepping mode setting.
Figure 17 gives a good and bad example of reducing the
stepping mode.
To avoid the creation of an offset it’s advised to only
change the stepping mode at a full−step position
(<MSP[7:0]> equal to 0, 64, 128 or 192).
Changing the stepping mode to (or from) full step
stepping mode also needs to be done with care. Changing to
full step mode at the moment the coil current is 100% in one
of the coils will result in a movement of the rotor. Reversed,
changing from full step to any other stepping mode will also
result in a movement of the rotor (see Figure 18, top left).
If the stepping mode is changed to full step when the coil
current in both coils is 71%, the coil current in both coils will
only be 71% in full step stepping mode instead of 100% (see
Figure 18, top right). Changing to full step stepping mode
when the coil current in one of the coils is not 100% nor 71%
will result in an offset (see Figure 18, bottom). Notice that
stepping is now done on a rectangle instead of a square.
There will always be coil current present in both coils
when working in full step stepping mode (see Table 7).
When zero current is requested in one of the coils, half step
stepping mode can be used to mimic full step (see section
Full Step Stepping Mode in application note AND8399/D
for more info).
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