33970 datasheet(17/36 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33970

FUNCTIONAL DEVICE OPERATION

LOGIC COMMANDS AND REGISTERS

RZ1 (D1)—Return to Zero Direction. This bit commands

the selected gauge to return the pointer to zero position.

• 0 = Return to Zero Disabled

• 1 = Return to Zero Enabled

RZ0 (D0)—Gauge Select: Gauge 0/Gauge 1. This bit

selects the gauge to be commanded.

• 0 = Selects Gauge 0

• 1 = Selects Gauge 1

Address 101—Gauge Return to Zero Configuration

Gauge Return to Zero Configuration Register (RTZCR) is

used to configure the Return to Zero Event (refer to

Table 12). It is written to modify the step time, or rate; at

which the pointer moves during an RTZ event. Also, the

integration blanking time, which is the time immediately

following the transition of a coil from a driven state to an open

state in the RTZ mode, is adjustable with this command.

Finally, this command is used to adjust the threshold of the

RTZ integration register.

The values used for this register should be selected during

development to optimize the RTZ for each application.

Selecting an RTZ step rate resulting in consistently

successful zero detections depends on a clear understanding

of the motor characteristics. Specifically, resonant

frequencies exist due to the interaction between the motor

and the pointer. This command allows movement of the RTZ

pointer speed away from these frequencies. Also, some

motors require a significant amount of time for the pointer to

settle to a steady state position when moving from one full

step position to the next. Consistent and accurate integration

values require the pointer be stationary at the end of the full

step time.

Bits RC3:RC0, RC12:RC11, and RC4 determine the time

spent at each full step during an RTZ event. Bits RC3:RC0

are used to select a

∆t ranging from 0 ms (0000) to 61.44 ms

(1111) in increments of 4.096 ms (refer to Table 13). The

∆t

is multiplied by the factor M, which is defined by bits

RC12:RC11. The product is then added to the blanking time,

selected using bit RC4, to generate the full step time. The

multiplier selected with RC12:RC11 will be 1 (00), 2 (01), or

4 (10) as illustrated in the equations below. Note that the

RC12:RC11 value of 8 (11) is not recommended for use in a

product design application, because of the potential for an

RTZ accumulator internal overflow, due to the long time step.

The blanking time is either 512

µs when RC4 is logic [0], or

768

µs when it is logic [1].The full step time is generated

using the following equations:

When D3:D0 (RC3:RC0)

≠ 0000

Full Step (t) =

∆t x M + blanking (t)(1)

When D3:D0 (RC3:RC0) = 0000

Full Step (t) = blanking (t) + 2.048 ms(2)

Note In equation (2), a 2.048 ms offset is added to the full

step time when the RC:3:RC0 = 0000. The full step time

default value after a logic reset is 12.80 ms

(RC12:RC11 = 00, RC4 = 0, and RC3:RC0 = 0011).

If there are two full steps per degree of pointer movement,

the pointer speed is 1/(FullStep x 2) deg/s.

Detecting pointer movement is accomplished by

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

RTZ event. The integration circuitry is implemented using a

Sigma-Delta converter resulting in the placement of a 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. 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. If the PECCR command is

written to clock out the RTZ accumulator values via the SO,

the OD14 bit corresponds to the sign bit of the RTZ

accumulator.

Accurate pointer stall detection depends on a correctly

preloaded accumulator for specific gauge, pointer, and full

step combinations. Bits RC10:RC5 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, and a stalled pointer will

result in a negative value. The preloaded values associated

with each combination of bits RC10:RC5 are illustrated in

Table 14. The accumulator should be loaded with a value

resulting in an accumulator MSB to a logic [1] when the motor

is stalled. For the default mode, after a power-up or any reset,

the 33970 device sets the accumulator value to -1.

Table 12. RTZCR SI Register Assignment

Address 101

Bits

D12

D11

D10

Read

–

Write

RC12

RC11

RC10

RC9

RC8

RC7

RC6

RC5

RC4

RC3

RC2

RC1

RC0

33970 Datasheet(PDF) 17 Page - Freescale Semiconductor, Inc

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