AN2044 APPLICATION NOTE

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Figure 3. Motor Phase Currents vs. Step Number

Table 2 also includes the required states for the six logic inputs of the L6207 Stepper Motor

Driver. In order to understand the details of this typical driver chip some background on the

characteristics of the L6207 are required.

4.3 L6207 DUAL FULL BRIGDE DRIVER

A typical stepper motor driver is the L6207 and we will use this part as an example to illustrate

the operation of the Practispin system. The L6207 includes two independent full or H bridges

with separate control inputs and current limit functions. The two bridges are designated A and

B and their output pins designated as OUT1A, OUT2A, OUT1B, and OUT2B. These outputs

are controlled independently by logic inputs IN1A, IN2A, IN1B, and IN2B respectively. A logic

high or low on any of these inputs will drive its corresponding output to the positive supply rail

or to ground.

Both of the A outputs will be forced to an off (high impedance) state if the ENA pin is taken

logic low, as will the B outputs if ENB is taken low.

The L6207 is thus controlled by six logic inputs: IN1A, IN2A, and ENA controlling bridge A and

IN1B, IN2B, and ENB controlling bridge B. Each bridge also has an analog control signal,

VREFA and VREFB, which control the current limit setting.

4.4 STEP STATE

If we know the state of the stepper motor (a number from 0 to 7) then we can use figure 4 to

tell us what logic levels to apply to the control pins of the L6207. The actual state is determined

by a byte variable of the Practispin program called stepstate. Stepstate is initialized to zero on

power up or whenever the power bridge is disabled. Stepstate is incremented whenever the

driving algorithm requires us to "advance the motor". Since eight commutating steps divides

evenly into the natural 256 count of a byte variable, stepstate is just continually incremented

and allowed to roll over. We only consider the lower three bits to be significant. Now, armed

with the stepstate variable, it is a simple matter to construct a lookup table. We will use step-

state (logically anded with a mask to clear all but the low three bits) as the index into a table

which will provide the data to be written out to the L6207. Table 3 shows the hardware map-