Datasheet

TMC429 DATASHEET (v. 1.07 / 2012-AUG-01) 23

If the proportionality factor p is set too small, this results in a slow approach to the target position. If set

too large, it causes overshooting and even oscillations around the target position. The calculation of

the proportionality factor is simple:

The representation of the proportionality factor p by the two parameters p_mul and p_div is some kind

of a fixed point representation. It is

p = p_mul / p_div

with

p_mul = {128, 128+1, 128+2, 128+3, ..., 128+127}

and

p_div = {2

, 2

, ..., 2

, 2

Instead of direct storage of the parameters p_mul and p_div, the TMC429 stores two parameters

called pmul and pdiv, with

p_mul = 128 + pmul and p_div = 2

3+pdiv

= 2^(3+pdiv)

where

pmul = {0, 1, 2, 3, ..., 127} and pdiv = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13}.

The reason why p_mul ranges from 128 to 255 is, that p is divided by p_div which is a power of two

ranging from 8 to 65536. So, values of p less than 128 can be achieved by increasing p_div.

Note: The parameters pmul and pdiv share a single address (IDX=%1001, see Table 8-2, page 18).

The MSB of p_mul pmul internally sets p_mul = 128 + pmul. In other

words, %10000000 = 128 is ORed as bit vector with the content of the register pmul.

v_max

RAMP GENERATOR PULSE GENERATOR (micro) step pulses

v_target

x_target

x_actual

pdivpmul a_maxv_min

pulse_divramp_div

clk32

clock_div32clk

Figure 9-2: Ramp generator and pulse generator

The parameter p has to be calculated for a given acceleration. This calculation is not done by the

TMC429 itself, because this task has to be done only once for a given acceleration limit. The

acceleration limit is a stepper motor parameter, which is usually fixed in most applications. If the

acceleration limit has to be changed nevertheless, the microcontroller could calculate on demand a

pair of p_mul and p_div for each acceleration limit a_max and given ramp_div and pulse_div. Also,

pre-calculated pairs of p_mul and p_div read from a table maybe sufficient.