33991 datasheet(17/36 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33991

TIMING DESCRIPTIONS AND DIAGRAMS

COMMUNICATION MEMORY MAPS

combination is illustrated in Table 10. The default full step

time is 21.25 ms (0101). If there are two full steps per degree

of pointer movement, the pointer speed is: 1/(FS

×2)°.

Bit D4 determines the provided blanking time immediately

following a full step change, and before enabling the

integration of the non-driven coil signal. The blanking time is

either 512 µs, when D4 is logic [0], or 768 µs when D4 is logic

[1].

Detecting pointer movement is accomplished by

integrating the back EMF present in the non-driven coil during

the RTZ event. The integration circuitry is implemented using

a Sigma-Delta converter resulting in a representative value in

the 15-bit RTZ accumulator at the end of each full step. The

value in the RTZ accumulator represents the change in flux

and is compared to a threshold. Values above the threshold

indicate a pointer is moving. Values below the threshold

indicate a stalled pointer, thereby resulting in the cessation of

the RTZ event.

The RTZ accumulator bits are signed and represented in

two’s complement. If the RTZR D3:D2 bits were written as 10

or 11, the ST14 bit corresponds to bit D14 of the RTZ

accumulator, the sign bit. After a full step of integration, a sign

bit of 0 is the indicator of an accumulator exceeding the

decision threshold of 0, and the pointer is assumed to still be

moving. Similarly, if the sign bit is logic [1] after a full step of

integration, the accumulator value is negative and the pointer

is assumed to be stopped. The integrator and accumulator

are initialized after each full step.

Accurate pointer stall detection depends on a correctly

preloaded accumulator for specific gauge, pointer, and full

step combinations. Bits D12:D5 are used to offset the initial

RTZ accumulator value, properly detecting a stalled motor.

The initial accumulator value at the start of a full step of

integration is negative. If the accumulator was correctly

preloaded, a free moving pointer will result in a positive value

at the end of the integration time. A stalled pointer results in

a negative value. The preloaded values associated with each

combination of bits D12:D5 are illustrated in Table 11. The

accumulator should be loaded with a negative value resulting

in a transition of the accumulator MSB to a logic [1] when the

motor is stalled. After a power-up, or any reset in the Default

mode, the 33991 device sets the accumulator value to -1,

resulting in an unconditional RTZ pointer movement.

These bits are write-only.

RC12:RC5— These bits determine the preloaded value

into the RTZ integration accumulator to adjust the detection

threshold.

Values range from -1 (00000000) to -4081 (11111111)

provided in Table 11.

RC4—This bit determines the RTZ blanking time.

• 0 = 512 µs

• 1 = 768 µs

RC3:RC0— These bits determine the full step time during

an RTZ event, determining the pointer moving rate. Step

times range from 4.86 ms (0000) to 62.21ms (1111). Those

are illustrated in Table 10. The default time is 21.25 ms

(0101).

Table 12. RTZCR SI Register Assignment

Address: 101

D12

D11

D10

Write

RC12

RC11

RC10

RC9

RC8

RC7

RC6

RC5

RC4

RC3

RC2

RC1

RC0

Table 13. RTZCR Full Step Time

RC3

RC2

RC1

RC0

Full Step Time (ms)

4.86

8.96

13.06

17.15

21.25

25.34

29.44

33.54

37.63

41.73

45.82

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