IRCAM Sequences

Sequence Generation

Generating the clock waveforms for the ircamera is central to the entire operation. The key problem is generating the sequence of bytes that is to be downloaded into the AWFG (Arbitrary Waveform Generator). The six chip clocks as well as the analog to digital converter trigger clock have been assigned to the two 4 bit outputs of the AWFG as follows.

X0 Pixel transition increments column counter
X1 Lsync active low to initialize column counter
X2 Fsync active low to initialize row counter
X3 Line transition to increment row counter
Y0 Reset active high resets selected row
Y1 Read active high gating signal (usually on)
Y2 Trigger rising edge causes ADC conversion
Y3 Spare a 5 clock wide pulse with rising edge at start of sequence

The AWFG is capable of storing a waveform of up to 32K steps long. When executing a waveform you must specify:

Length of waveform
Number of times to execute the waveform
clock divider - specifies number of microseconds per step

The operation of the AWFG is implemented in the C++ class CAWFG. Which has functions for loading the waveform and executing them. The awfg class communicates with the awfg card through ioctl calls to a linux device /dev/awfg. The driver for this devices was written in-house.

Unfortunately the maximum waveforme length of 32K is not large enough to readout the Picnic array. Instead we must specify a series of waveforme and execute then sequentially. The AWFG does allow a new sequence to be loaded as the last sequence is finishing, if the length of the sequence is the same and the clock divider does not change.

Elementary Wavefoms

We have settled for a waveform of length of 4128 clocks. The Wave Class defines an object that is used to put together more complex sequences. It is invoked (Generate method) with a integer mode that is the logical OR of the following masks.

WAVE_RESET if the Reset line pulse should be fired
WAVE_FSYNC if the Fsync line pulse should be pulsed low
WAVE_READ if the pixel clock and adc clocks should be activated
WAVE_EXPOSE if the sequence is just to mark time for an exposure
WAVE_QUIET if the clocks turned off for testing

The generate invoked with WAVE_READ creates a basic waveform with

512 transitions of the pixel clock
2048 triggers of the adc
4 Lsync pulses
4 transition of the Line clock

Here is A view of the first couple of transitions. Note that there are four rising ADC triggers per pixel readout. Here is a graph of the full read waveform

The basic step time is created from a 5 MHz clock divided by 25, yielding a 5 microsecond base step time.

Generating the Sequences

The full readout sequence is created by stringing together multiple waveforms. This is done in the Create method of the CIRcam class. The Read sequence just reads thru the entire chip and digitizes the result is the combination of two waveforms:

1 waveform with WAVE_READ | WAVE_FSYNC to start frame
31 waveforms with WAVE_READ to scan thru entire chip

The Frame sequence is more complex is composed of three parts: i) the chip is reset 2) the a variable length wait time (the exposure) 3) The full frame Read sequence above. This sequence is made up of 5 individual waveforms.

1 waveform with WAVE_RESET | WAVE_FSYNC to reset the chip start frame
31 waveforms with WAVE_RESET to scan thru entire chip
adjustable WAVE_EXPOSE to create exposure delay
1 waveform with WAVE_READ | WAVE_FSYNC to read the chip start frame
31 waveforms with WAVE_READ to read thru entire chip

The CDS sequence is similarly complex: This sequence is made up of 5 individual waveforms.

1 waveform with WAVE_RESET | WAVE_FSYNC | WAVE_READ to reset the chip, start the frame and reads
31 waveforms with WAVE_RESET|RESET to scan thru entire chip and reads
adjustable WAVE_EXPOSE to create exposure delay
1 waveform with WAVE_READ | WAVE_FSYNC to read the chip start frame
31 waveforms with WAVE_READ to read thru entire chip

The software then subtracts the first values that are read just after the reset from the second ones (after the exposure) and reports the difference.

Last Revised April 6, 1999