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ST9 USER GUIDE

4.4.3 Incremental Encoder Counter

An incremental encoder is a device that generates two square signals in quadrature when its

shaft rotates. Its main specification is the number of cycles per revolution, i.e. the number of

square signal cycles for one shaft revolution. The greater the number of cycles, the more ac-

curate it is, and the more it costs.

In a stepper motor application, it is used to monitor the rotation of the motor. Here however, we

have taken an encoder that has a great resolution, and this section will show you how to use

an MFT to count encoder pulses, and rescale it to another resolution to meet your project

needs.

4.4.3.1 Description

The MFT includes an input block, a counter block, an event block and an output block. In this

application, no output signal is required, since the counter is expected to produce a value that

is read in registers.

The MFT must provide the following functions:

– Use the inputs to drive the counter according to the amount and direction of rotation

– Keep the count undisturbed by events other than the transitions at the inputs

– Allow reading the current counter value

– Allow setting the counter to an arbitrary initial value

We configure the input block to use the two square signals delivered by the encoder to drive

the up and down counting of the counter. We do this using a special mode of the input block

called “autodiscrimination”. In this mode, the input pulses clock the counter, and the phase re-

lationship between the two signals selects up or down counting. Since the process is incre-

mental, once the MFT configured this way, it automatically follows the rotation of the encoder

shaft and the counter value reflects its position relative to the position it was in when the

counter was configured. In our example, the encoder produces 512 cycles per revolution. Be-

cause the counter is 16-bit, it can monitor shaft positions 65536/512 = 128 (or +/-64) full rev-

olutions from the original position.

To operate with an encoder as the input pulse source, we need to set the MFT to Continuous

Mode. When the counter crosses the FFFF to 0000 boundary in either direction, this is called

an End-Of-Count event. In the normal Continuous mode, the End-Of-Count event reloads the

counter with the contents of REG0R (or REG1R in biload mode). In this application, no auto-

matic reload may occur at any time so we can use the full range of the counter and be able to

cross the boundary. To prevent the reload from occurring, we must set the REG0R register to

Capture Mode. The MFT is then said to be in Free-Running Mode.

We have thus selected Free-Running Mode, yet we need to be able to set the counter to any

arbitrary value that is taken as the initial position. For this we need to use the REG0R register

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