RM0312 datasheet(141/275 Pages) STMICROELECTRONICS

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RM0312

16-bit general purpose timer (TIM2/TIM3)

170

Encoder interface mode acts simply as an external clock with direction selection. This

means that the counter just counts continuously between 0 and the auto-reload value in the

TIMx_ARR register (0 to ARR or ARR down to 0 depending on the direction). So you must

configure TIMx_ARR before starting. In the same way, the capture, compare, prescaler,

trigger output features continue to work as normal. Encoder mode and External clock mode

2 are not compatible and must not be selected together.

In this mode, the counter is modified automatically following the speed and the direction of

the incremental encoder and its content, therefore, always represents the encoder’s

position. The count direction correspond to the rotation direction of the connected sensor.

The table summarizes the possible combinations, assuming TI1 and TI2 don’t switch at the

same time.

An external incremental encoder can be connected directly to the MCU without external

interface logic. However, comparators will normally be used to convert the encoder’s

differential outputs to digital signals. This greatly increases noise immunity. The third

encoder output which indicate the mechanical zero position, may be connected to an

external interrupt input and trigger a counter reset.

The Figure 62 gives an example of counter operation, showing count signal generation and

direction control. It also shows how input jitter is compensated where both edges are

selected. This might occur if the sensor is positioned near to one of the switching points. For

this example we assume that the configuration is the following:

•

CC1S = 0b01 (TIMx_CCMR1 register, IC1 mapped on TI1).

•

CC2S = 0b01 (TIMx_CCMR2 register, IC2 mapped on TI2).

•

CC1P = 0 (TIMx_CCER1 register, IC1 non-inverted, IC1=TI1).

•

CC2P = 0 (TIMx_CCER2 register, IC2 non-inverted, IC2=TI2).

•

SMS = 0b011 (TIMx_SMCR register, both inputs are active on both rising and falling

edges).

•

CEN = 1 (TIMx_CR1 register, Counter is enabled).

Table 34. Counting direction versus encoder signals

Active edge

Level on

opposite

signal (TI1FP1

for TI2,

TI2FP2 for

TI1)

TI1FP1 signal

TI2FP2 signal

Rising

Falling

Rising

Falling

Counting on

TI1 only

High

Down

No count

Low

Down

No count

Counting on

TI2 only

High

No count

Down

Low

No count

Down

Counting on

TI1 and TI2

High

DownUp

UpDown

Low

Down

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