Readback

Readback is the name of a runlength-encoder core. This core is connected directly to the output of the PulsePins streamer (qout) for verification and testing purposes. It reads streamed symbols and RL compresses them. The results can be read through an Avalon stream interace. PulsePins comes with a suite of testing scripts that assert correct functionality of the streamer, including various corner cases, by matching the streamed out data with the reference sequences. The readback mechanism thus provides high assurance.

The sequence acquired takes the same format as used in the streamer (control, counter, value). Control register is all zeros; we assume that all data are BITLOAD updates.

The bus widths in ip/rl_encoder_if/rl_config.vh need to match those in ip/streamer/config.vh.

rl_encoder core

Inputs:

• qin: data signals; connects to qout of the streamer
• qin_valid: if 1, qin data is valid
• qin_clk: data are sampled when qin_clk is asserted
• activated: enable signal, sampling is only performed when this signal is high

The enable signal activated needs to be deasserted at the end of the sequence to indicate that the streaming has terminated, so that the RL encoder can add the last element to the acquired sequence.

rl_encoder_if interface

Provides an Avalon ST interface for reading the acquired sequence and an Avalon MM interface for control. The only purpose of the control interface is to reset the reader.

Software interface

The software interface is provided through class readback in the header file readhack.hh. The key member functions are:

• reset: perform the reset of the circuitry
• clear_fifo: read and discard all elements in the readback queue
• filled: returns true if there are elements to be read back
• check_fill_status: provides a report on the Avalon ST reading FIFO
• read: read a single element
• read_all: read indefinitely; this function never returns
• check: perform a comparison between the sequence that is read back and a reference Sequence object

The function check returns true if no errors are detected. A timeout argument can be provided; if there are no new elements received during the specified interval of time, an exception is raised. An exception is also raised if the reference sequence is exhaused and a new element is received from the encoder. A report is produced when the check is completed, including the number and ratio of errors, the difference in size (number of elements) and length (number of periods of strobed data).

Readback of external signals

If the output enable signal oe is low, the readback core is reading signals from the external pins, i.e., the 32 qout pins actually act as inputsq, and the streamer_qout_valid pin starts acting as the valid signal input port. The ppread tool can be used to read the generated data stream. One application of this tool is to troubleshoot another PulsePins board by connecting the qout and qout_valid ports together and clocking both devices from the same clock source.