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PMAC 2 Software Reference

138 PMAC I-Variable Specifiation

Remarks

I907 controls the frequency of four hardware clock frequencies for the second group of

four machine interface channels on PMAC2 (channels 5-8). It is a 12-bit variable

consisting of four independent 3-bit controls, one for each of the clocks. Each of these

clock frequencies can be divided down from a starting 39.3216 MHz frequency by powers

of 2, from 1 to 128 times.

This means that the possible frequency settings for each of these clocks are:

Frequency Divide by Divider N in

1/2^N

39.3216 MHz 1 0

19.6608 MHz 2 1

9.8304 MHz 4 2

4.9152 MHz 8 3

2.4576 MHz 16 4

1.2288 MHz 32 5

614.4 kHz 64 6

307.2 kHz 128 7

Very few PMAC2 users will be required to change the setting of I907 from the default

value.

The encoder sample clock signal SCLK controls how often PMAC2’s digital hardware

looks at the encoder and flag inputs. PMAC2 can take at most one count per SCLK cycle,

so the SCLK frequency is the absolute maximum encoder count frequency. SCLK also

controls the signal propagation through the digital delay filters for the encoders and flags;

the lower the SCLK frequency, the greater the noise pulse that can be filtered out. The

SCLK frequency should optimally be set to the lowest value that can accept encoder

counts at the maximum possible rate.

The pulse-frequency-modulation clock PFM_CLK controls the PFM circuitry that is

commonly used for stepper drives. The maximum pulse frequency possible is 1/4 of the

PFM_CLK frequency. The PFM_CLK frequency should optimally be set to the lowest

value that can generate pulses at the maximum frequency required.

The DAC_CLK controls the serial data frequency into D/A converters. If these converters

are on Delta Tau-provided accessories, the DAC_CLK setting should be left at the default

value.

The ADC_CLK controls the serial data frequency from A/D converters. If these

converters are on Delta Tau-provided accessories, the ADC_CLK setting should be left at

the default value.

Example

See I903 Example