2003 Dec 17

5

Philips Semiconductors

Product specification

32 kHz watch circuit with programmable

adaptive motor pulse

PCA2000; PCA2001

handbook, full pagewidth

43.75%

37.5%

DUTY CYCLE

50%

56.25%

62.5%

68.75%

75%

81.25%

100%

MGW351

0.244 ms

0.122 ms

0.98 ms

0.98 ms

0.98 ms

0.98 ms

Fig.4 Possible modulations for a 3.9 ms motor pulse.

Step detection

Figure 5 shows a simplified diagram of the motor driving

and step detection circuit, and Fig.6 shows the step

detection sequence and corresponding sampling current.

Between the motor driving pulses, the switches P1 and P2

are closed, which means the motor is short-circuited. For

a pulse in one direction, P1 and N2 are open, and

P2 and N1 are closed with the appropriate duty cycle; for

a pulse in the opposite direction, P2 and N1 are open, and

P1 and N2 closed.

The step detection phase is initiated after the motor driving

pulse (see Fig.3). P1 and P2 are first closed for 0.98 ms

and then all four drive switches (P1, N1, P2 and N2) are

opened for 0.98 ms.

As a result, the energy stored in the motor inductance is

reduced as fast as possible.

The induced current caused by the residual motor

movement is then sampled in phase 3 (closing P3 and P2)

and in phase 4 (closing P1 and P4). For step detection in

the opposite direction P1 and P4 are closed during

phase 3 and P2 and P3 during phase 4 (see Fig.6).

The condition for a successful motor step is a positive step

detection pulse (current in the same direction as in the

driving phase) followed by a negative detection pulse

within a given time limit. This time limit can be programmed

between 3.9 and 10.7 ms (in steps of 0.98 ms) in order to

ensure a safe and correct step detection under all

conditions (for instance magnetic fields). The step

detection phase stops after the last 31.25 ms, after the

start of the motor driving pulse.