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HCTL2016 Datasheet(PDF) 9 Page - Agilent(Hewlett-Packard) |
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HCTL2016 Datasheet(HTML) 9 Page - Agilent(Hewlett-Packard) |
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9 / 18 page  2-186 channels. Therefore, tES (encoder state period) > tCLK. The designer must account for deviations from the nominal 90 degree phasing of input signals to guarantee that tES > tCLK. Position Counter This section consists of a 12-bit (HCTL-2000) or 16-bit (HCTL- 2016/2020) binary up/down counter which counts on rising clock edges as explained in the Quadrature Decoder Section. All 12 or 16 bits of data are passed to the position data latch. The system can use this count data in several ways: A. System total range is ≤ 12 or 16 bits, so the count repre- sents “absolute” position. B. The system is cyclic with ≤ 12 or 16 bits of count per cycle. RST is used to reset the counter every cycle and the system uses the data to interpolate within the cycle. C. System count is > 8, 12, or 16 bits, so the count data is used as a relative or incre- mental position input for a system software computation of absolute position. In this case counter rollover occurs. In order to prevent loss of position information, the processor must read the outputs of the IC before the count increments one-half of the maximum count capabil- ity (i.e. 127. 2047, or 32,767 quadrature counts). Two’s- complement arithmetic is normally used to compute position from these periodic position updates. Three modes can be used: 1. The IC can be put in 8-bit mode by tying the SEL line high, thus simplify- ing IC interface. The outputs must then be read at least once every 127 quadrature counts. 2. The HCTL-2000 can be used in 12-bit mode and sampled at least once every 2047 quadrature counts. 3. The HCTL-2016 or 2020 can be used in 16-bit mode and sampled at least once every 32,767 quadrature counts. D. The system count is > 16 bits so the HCTL-2020 can be cascaded with other stand- ard counter ICs to give absolute position. Position Data Latch The position data latch is a 12/ 16-bit latch which captures the position counter output data on each rising clock edge, except when its inputs are disabled by the inhibit logic section during two-byte read operations. The output data is passed to the bus interface section. When active, a signal from the inhibit logic section prevents new data from being captured by the latch, keeping the data stable while successive reads are made through the bus section. The latch is automatically reenabled at the end of these reads. The latch is cleared to 0 asynchron- ously by the RST signal. into account finite rise times of the waveforms, asymmetry of the waveforms, and noise. In the presence of large amounts of noise, tE should be much greater than 3tCLK to allow for the interruption of the consecutive level sampling by the three-bit delay filter. It should be noted that a change on the inputs that is qualified by the filter will internally propagate in a maxi- mum of seven clock periods. The quadrature decoder circuitry imposes a second timing con- straint between the external clock and the input signals. There must be at least one clock period between consecutive quadrature states. As shown in Figure 9, a quadrature state is defined by consecutive edges on both Figure 9. 4x Quadrature Decoding. |
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