PC33991DHSLASHR2 datasheet(27/36 Pages) MOTOROLA

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MOTOROLA ANALOG INTEGRATED CIRCUIT DEVICE DATA

33991

Return to Zero Calibration

Many stepper motor applications require the integrated

circuit (IC) detect when the motor is stalled after commanded to

return to the zero position for calibration purposes. Stalling

occurs when the pointer hits the end stop on the gauge bezel,

usually at the zero position. It is important when the pointer

reaches the end stop it immediately stops without bouncing

away from the stop.

The 33991device provides the ability to automatically and

independently return each of the two pointers to the zero

position via the RTZR and RTZCR SPI commands. During an

RTZ event, all commands related to the gauge being returned

are ignored, except when the RTZR bit D1 is used to disable the

event, or when the RTZR bits D3 and D2 are changed in order

to look at different RTZ accumulator bits. Once an RTZ event is

initiated, the device will report back via the SO pin, indicating an

RTZ is underway.

The RTZCR command is used to set the RTZ pointer speed,

choose an appropriate blanking time and preload the

integration accumulator with an appropriate offset. Reaching

the end stop, the device reports the RTZ success to the

microcontroller via the SO pin. The RTZ automatically disables

allowing other commands to be valid. In the event the master

determines an RTZ sequence is not working properly, for

example the RTZ taking too long, it can disable the command

via the RTZR bit D1.

RTZCR bits D12:D5 are written to preload the accumulator

with a predetermined value assuring an accurate pointer stall

detection. This preloaded value is determined during

application development by disabling the automatic shutdown

feature of the device with the RTZR bit D4. This operating mode

allows the master to monitor the RTZ event using the

accumulator information available in the SO status bits D15:D8.

Once the optimal value is determined, the RTZ event can be

turned off using the RTZR bit D1.

During an RTZ event, the pointer is returned counter-clock-

wise (CCW) using full steps at a constant speed determined by

the RTZCR D3:D0 bits during RTZ configuration. See Figure 9.

Full steps are used because only one coil of the motor is being

driven at any time. The coil not being driven is used to

determine whether the pointer is moving. If the pointer is

moving, a back EMF signal can be processed and detected in

the non-driven coil. This is achieved by integrating the signal

present on an opened end of the non-driven coil while

grounding the opposite end.

The IC automatically prepares the non-driven coil at each

step, waits for a predetermined blanking time, then processes

the signal for the duration of the full step. When the pointer

reaches the stop and no longer moves, the dissipating back

EMF is detected. The processed results are placed in the RTZ

accumulator, then compared to a decision threshold. If the

signal exceeds the decision threshold, the pointer is assumed

to be moving. When the threshold value is not exceeded, the

drive sequence is stopped when RTZR bit D4 is logic [0]. If bit

D4 is logic [1], the RTZ movement will continue indefinitely until

the RTZR bit D1 is used to stop the RTZ event.

A pointer not on a full step location, or in magnetic alignment

prior to the RTZ event, may result in a false RTZ detection.

More specifically, an RTZ event beginning from a non-full step

position may result in an abbreviated integration, interpreted as

a stalled pointer. Similarly, if the magnetic fields of the

energized coils and the rotor are not aligned prior to initiating

the RTZ, the integration results may mistakenly indicate the

pointer has stopped moving.

Advancing the pointer by at least 24 microsteps clock-wise

(CW) to the nearest full step position, e.g., 24,30, 36, 42, 48

...prior to initiating an RTZ, ensures the magnetic fields are

aligned. Doing that increases the chances of a successful

pointer stall detection. It is important the pointer be in a static,

or commanded position before starting the RTZ event. Since

the time duration and the number of steps the pointer moves

prior to reaching the commanded position can vary depending

on its status at the time a position change is communicated, the

master should assure sufficient time has elapsed prior to

starting an RTZ. If an RTZ is desired after first enabling the

outputs, or after forcing a reset of the device, the pointer should

first be commanded to move 24 microstep steps clock-wise

(CW) to the nearest full step location. Because the pointer was

in a static position at default, the master could determine the

number of microsteps the device took by monitoring and

counting the ST4 (ST5) status bit transitions, confirming the

pointer is again in a static position.

Only one gauge at a time can be returned to the zero

position. An RTZ should not begin until the gauge to be

calibrated is at a static position and its pointer is at a full step

position. An attempt to calibrate a gauge while the other is in the

process of an RTZ event, will be ignored by the device. In most

applications of the RTZR command, it is possible to avoid a

visually obvious sequential calibration by first bringing the

pointer back to the previous zero position and then re-

calibrating the pointers.

After completion of a RTZ, the 33991 automatically assigns

the zero step position to the full step position at the end stop

location. Because the actual zero position could lie anywhere

within the full step where the zero was detected, the assigned

zero position could be within a window of ± 0.5

°.An RTZ canbe

used to detect stall, even if the pointer already rests on the end

stop when an RTZ sequence is initiated; however, it is

recommended the pointer should be advanced by at least 24

microsteps to the nearest full step prior to initiating the RTZ.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

PC33991DH/R2 Datasheet(PDF) 27 Page - Motorola, Inc

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