Reference Manual

PMAC 2 Software Reference
136 PMAC I-Variable Specifiation
I904 PWM 1-4 Deadtime / PFM 1-4 Pulse Width Control {PMAC2 only}
Range
0 .. 255
Units
PWM Deadtime = [16 / PWM_CLK (MHz)] * I904 = 0.135 usec * I904
PFM Pulse Width = [1 / PFM_CLK (MHz)] * I904
= PFM_CLK_period (usec) * I904
Default
15
PWM Deadtime = 0.135 usec * 15 = 2.03 usec
PFM Pulse Width = [1 / 9.8304 MHz] * 15 = 1.526 usec (with default I903)
Remarks
I904 controls the deadtime period between top and bottom on-times in PMAC2’s
automatic PWM generation for machine interface channels 1-4. In conjunction with I903,
it also controls the pulse width for PMAC2’s automatic pulse-frequency modulation
generation for machine interface channels 1-4.
The PWM deadtime, which is the delay between the top signal turning off and the bottom
signal turning on, and vice versa, is specified in units of 16 PWM_CLK cycles. This
means that the deadtime can be specified in increments of 0.135 usec. The equation for
I904 as a function of PWM deadtime is:
sec135.0
sec)(DeadTime
904I
µ
µ
=
The PFM pulse width is specified in PFM_CLK cycles, as defined by I903. The equation
for I904 as a function of PFM pulse width and PFM_CLK frequency is:
sec)(Width_Pulse_PFM*)MHz(Freq_CLK_PFM904I
µ
=
In PFM pulse generation, the minimum off time between pulses is equal to the pulse width.
This means that the maximum PFM output frequency is
904I*2
)MHz(Freq_CLK_PFM
)MHz(Freq_Max_PFM =
Example
A PWM deadtime of approximately 1 microsecond is desired:
I904 ≅ 1 usec / 0.135 usec ≅ 7
With a 2.4576 MHz PFM_CLK frequency, a pulse width of 0.4 usec is desired:
I904 ≅ 2.4576 MHz * 0.4 usec ≅ 1
See Also
I908, I994
I905 DAC 1-4 Strobe Word {PMAC2 only}
Range
$000000 .. $FFFFFF
Units
Serial Data Stream (MSB first, starting on rising edge of phase clock)
Default
$7FFFC0
Remarks
I905 controls the DAC strobe signal for machine interface channels 1-4. The 24-bit word
set by I905 is shifted out serially on lines DAC_STROB1-4, MSB first, one bit per
DAC_CLK cycle starting on the rising edge of the phase clock. The value in the LSB is
held until the next phase clock cycle.
The default I905 value of $7FFFC0 is suitable for the 18-bit DACs on the ACC-8E Analog
Interface Board. I905 should not be changed from the default unless different DACs are
used.
For a 16-bit DAC, I905 should be set to $7FFF00. For a 12-bit DAC, I905 should be set to
$7FF000.
See Also
I909