2.1. Named Patterns

Patterns have names, much like variables in computer languages. This allows building complex patterns from simpler base patterns. The syntax of a named pattern definition follows:

META_PATTERN:
  PATTERN_1(n1)   -- PATTERN_1 repeated n1 times
  PATTERN_2(n2)   -- PATTERN_2 repeated n2 times
  PATTERN_3(n3)   -- PATTERN_3 repeated n3 times
  PATTERN_4(n4)   -- PATTERN_4 repeated n4 times
  ...

Here META_PATTERN is the pattern name being defined, PATTERN_1 … PATTERN_4 are previously defined pattern names and n1 … n4 are repeat counts (integers[1] ), specifying how many times to repeat the pattern.

Two pre-defined pattern names exist, with the following definitions:

• 1 – Assert for 1 clock cycle

• 0 – De-assert for 1 clock cycle

All named patterns are built from these two fundamental patterns (obviously the "0" and "1" pattern names would be reserved language keywords).

2.1.1. Example: A Clock-like Pattern

For example to create a clock-like pattern named CLOCK_4 that has a frequency of sys_frequency / 4 with a duty cycle of 50% and lasts for 100 sys_clock cycles, one could write:

DUTY_50: -- Start definition of pattern DUTY_50 0(2) -- Signal de-asserted (off) for two clock cycles 1(2) -- Signal asserted (on) for two clock cycles CLOCK_4: -- Start definition of pattern CLOCK_4 DUTY_50(25) -- Repeat pattern DUTY_50 25 times

The DUTY_50 pattern is four sys_clocks long, de-asserted for two clocks and asserted for two clocks. The CLOCK_4 pattern is 25 copies of the DUTY_50 pattern, for a total of 100 sys_clock cycles.

If you were to create a pattern of alternating 1's and 0's this clock-like signal would have a frequency equal to one half of the FIFO clocking frequency, sys_frequency. The normal sys_frequency for the VME LAT COMM I/O Board is 20 MHz, but an external clock source could also be used.

Using named patterns the user can create a palette of pattern fragments.