theory_of_operation.md

Theory of Operation

The pattern can be started (or halted) on either channel by setting the corresponding CTRL.ENABLE bit to 1 (or 0) for the desired channel. Once disabled, either channel can be configured independently. The channel parameters (i.e. clock divider ratio, clock polarity, pattern length, pattern data, and repetition count) can all be programmed on a per-channel basis. Enabling the pattern generator channel starts the pattern from the beginning.

Please note that writes to a channel's configuration registers have no effect while the channel is enabled. For operational simplicity, the configuration registers are only transferred into the internal finite state machines while a channel is disabled. Changes to the configuration registers only take effect once the channel has been disabled and re-enabled.

Block Diagram

Design Details

The design consists of two identical and independent finite state machines, each an instance of module pattgen_fsm. Each FSM is essentially three nested counters, with one counter to control the clock division, another to count out the sequence bits, and a third to keep count of the number of repetitions.

Each FSM consists of

• Inputs:
  • clk_io, reset, enable, clk_pre_divide, pattern, pattern_size, polarity, and n_repeats
• Outputs:
  • pda and pcl
• a single state variable with three states IDLE, ACTIVE, and END,
• a clock-divide counter, clk_div,
• a single clock-divide flop, clk_int, and
• two additional internal counters bit_ctr and repeat_ctr.

Each FSM is disabled when enable is low. Disabling the FSM is equivalent to an FSM reset, and both operations place the FSM in the IDLE state. While in IDLE, the other state machine registers assume their default states: The internal counters, the clock-divide, bit_ctr and repeat_ctr all reset to 0, as does clk_int.

Once the FSM is enabled, it transitions to the ACTIVE state. The clock-divide counter clk_div increments every cycle, except when it overflows matching the value applied to the clk_pre_divide input. Then clk_div resets to 0, toggling clk_int in the process. Two overflow events result in a complete clock cycle, resulting in an internal clock frequency of: $$f_{pclx}=\frac{f_\textrm{I/O clk}}{2(\textrm{CLK_RATIO}+1)}$$

The FSM clock output, pcl, is directly driven by clk_int, unless the polarity input is high, in which case pcl is inverted from clk_int.

The bit_ctr counter increments on every falling edge of clk_int, until it overflows at the pattern length based on the pattern_size input.

In the ACTIVE state, the FSM pda output is driven by a multiplexer, connected to the pattern input. The value of bit_ctr selects the bit value from the appropriate sequence position, via this multiplexor.

Finally whenever bit_ctr overflows and reverts to zero, the repeat_ctr increments, and the pattern starts again. Finally repeat_ctr overflows to zero as it reaches the input value n_repeats. When this overflow occurs, the FSM transitions to the END state. All counters halt, the pda data lines reset to zero, and an interrupt event is sent out to signal completion.

The entire sequence can be restarted either by resetting or disabling and re-enabling the FSM.

Interrupts

The pattern generator HWIP provides two interrupt pins, done_ch0 and done_ch1, which indicate the completion of pattern generation on the output channels. These interrupts can be enabled/disabled by setting/un-setting the corresponding bits of the INTR_ENABLE register. To clear the interrupts, bit 1 must be written the corresponding bits of INTR_STATE register