Datasheet

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TMC429 DATASHEET (v. 1.07 / 2012-AUG-01) 25

p = a_max / ( 128 * 2^( ramp_div-pulse_div ) )

and

p’ = p_mul / p_div = (128+pmul) / 2^(3+pdiv)

and

q = p’ / p

for each pair (pmul , pdiv) and select one of the pairs satisfying the condition 0.95 < q < 1.0. So, the

value q interpreted as a function q(a_max, ramp_div, pulse_div, pmul, pdiv) gives the quality

criterion required. Although q = 1.0 indicates that (pmul , pdiv) perfectly represents the desired p for a

given a_max, this could cause overshooting because of finite numerical precision. In case of high

resolution microstepping, overshooting of one micro step is negligible in most applications. To avoid

overshooting, use pmul-1 instead of the selected pmul or select a pair (pmul, pdiv) with q = 0.95. The



force approach. Some conversions of the base present equations help to reduce the calculation effort

drastically.

9.11.1 Optimized Calculation of p_mul and p_div

With the equations above, one can simplify the calculation of the parameters pmul and pdiv using the

expression

pmul = p * 2^3 * 2^pdiv – 128

with

p = a_max / ( 128 * 2^( ramp_div – pulse_div) ).

To avoid overshooting, use

p_reduced = p * ( 1 – p_reduction[%])

with p_reduction approximately 5% instead of unreduced p. With this, one gets

pmul = p_reduced * 2^3 * 2^pdiv – 128 = 0.95 * p * 2^3 * 2^pdiv – 128.

With this, pmul becomes a function of the parameter pdiv. To find a valid pair {pmul, pdiv} one just

has to choose that pair {pmul, pdiv} out of 14 pairs for pdiv = {0, 1, 2, 3, ..., 13} with pmul within the

valid range 0  pmul       g in C language can be

found on page 60. This source code can directly be copied from the PDF datasheet file.

9.12 lp & ref_conf & ramp_mode (rm) (IDX=%1010)

The bit called lp (latched position) is a read only status bit. The configuration words ref_conf and

ramp_mode are accessed via a common address, because these parameters normally are initialized

only once. The configuration bits ref_conf select the behavior of the reference switches, while the two

bits ramp_mode (rm) select one of the four possible stepping modes.

ramp_mode

mode

Function

%00

RAMP_MODE

default mode for positioning applications with trapezoidal ramp

%01

SOFT_MODE

similar to RAMP_MODE, but with soft target position approaching

%10

VELOCITY_MODE

mode for velocity control applications, change of velocities with linear ramps

%11

HOLD_MODE

velocity is controlled by the microcontroller, motion parameter limits are ignored

Table 9-3 – Outline of TMC429 motion modes