Stepper Motor - Experimenting with Raspberry Pi - page 123

Table 6-1. GPIOs for Motor Control at Reset/Boot Time

GPIO

OK Bad Reason Bad Reason

09 23 00 Pull-up high 08 Pull-up high

10 24 01 Pull-up high 14 TXD0

11 25 02 Pull-up high 15 RXD0

12 28† 03 Pull-up high 16 Output

17 29† 04 Pull-up high 27 Output

18 30 05 Pull-up high

21 31 06 Output

22 07 Pull-up high

(†) No pull-up resistor

The pull-up resistance provided by the Broadcom SoC is weak (50 k W). Because of
this, the worst-case input drive, due to the pull-up resistance, is calculated as follows:

V
R

I =

CC

I

pullup

33
50
66

. V

K
A

=

W

= m

The ULN2003A datasheet states that it takes an input drive of 250 mA to produce an
output current flow of 100 mA. Some GPIO pins are pulled up by external resistors (GPIO
2 and 3 for SDA and SCL). These GPIO pins should not be used as motor drivers, for this
reason.
The Darlington pair includes resistances that naturally pull down the input signal
(review Figure 6-5 ). Resistances R B , R 1 , and R 2 are connected in series between the input
and ground. This effectively provides a pull-down resistance of approximately 13.6 k W.
If the ULN2003A is attached to a floating input like GPIO 28 or 29, the input voltage will
automatically be pulled down as a result (this is desirable here).

Modes of Operation
A unipolar stepper motor may be operated in four modes. The first three of these modes
use digital on/off signals to drive each winding. The fourth micro-stepping mode requires
driving each winding with varying analog signals. Since we are driving with digital GPIO
pins, we'll examine only the first three modes.