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Control the LArPix chip

Project description

# larpix-control

Control the LArPix chip

[![Documentation Status](](

## Setup and installation

This code is intended to work on both Python 2.7+ and Python 3.6+, but it was designed in Python 3 and is not guaranteed to work in Python 2.

Install larpix-control from pip with

` pip install larpix-control `

To return your namespace to the pre-larpix state, just run pip uninstall larpix-control. If you’d prefer to download the code yourself, you can. Just run pip install . from the root directory of the repository.

### Tests

You can run tests to convince yourself that the software works as expected. After pip install`ing this package, you can run the tests from the repository root directory with the simple command `pytest.

You can read the tests to see examples of how to call all of the common functions. I imagine they will also come in handy when you’re confused about the bit order. (Also see the section on endian-ness below.)

## Tutorial

This tutorial runs through how to use all of the main functionality of larpix-control.

To access the package contents, use one of the two following import statements:

`python import larpix.larpix as larpix  # use the larpix namespace # or ... from larpix.larpix import *  # import all larpix classes into the current namespace `

### Endian-ness

We use the convention that the LSB is sent out first and read in first. The location of the LSB in arrays and lists changes from object to object based on the conventions of the other packages we interact with.

In particular, pyserial sends out index 0 first, so for bytes objects, index 0 will generally have the LSB. On the other hand, bitstrings treats the _last_ index as the LSB, which is also how numbers are usually displayed on screen, e.g. 0100 in binary means 4 not 2. So for BitArray and Bits objects, the LSB will generally be last.

Note that this combination leads to the slightly awkward convention that the least significant bit of a bytestring is the last bit of the first byte. For example, if bits[15:0] of a packet are 0000 0010 0000 0001 ( = 0x0201 = 513), then the bytes will be sent out as b’x01x02’.

### Creating a LArPix Chip

The Chip object represents a single LArPix chip and knows about everything happening on the chip regarding configuration, data sent in, and data read out. To create a Chip, just provide the chip ID number (hard-wired into the PCB) and the index for the IO Chain (daisy chain) that the chip is part of:

`python myChip = Chip(100, 0) `

The Chip object uses these ID values when it creates data packets to ensure that the packet reaches the correct chip. And other objects use the ID values to ensure that received data from the physical chip makes its way to the right Chip object.

The chip’s configuration register is represented by the myChip.config attribute, which is an instance of the Configuration object.

### The Configuration object

The Configuration object represents all of the options in the LArPix configuration register. Each row in the configuration table in the LArPix datasheet has a corresponding attribute in the Configuration object. Per-channel attributes are stored in a list, and all other attributes are stored as a simple integer. (This includes everything from single bits to values such as “reset cycles,” which spans 3 bytes.) Warning: there is currently no type checking or range checking on these values. Using values outside the expected range will lead to undefined behavior, including the possibility that Python will crash _or_ that LArPix will be sent bad commands.

Configuration objects also have some helper methods for enabling and disabling per-channel settings (such as csa_testpulse_enable or channel_mask). The relevant methods are listed here and should be prefixed with either enable_ or disable_:

  • channels enables/disables the channel_mask register
  • external_trigger enables/disables the external_trigger_mask
  • testpulse enables/disables the csa_testpulse_enable register
  • analog_monitor enables/disables the csa_monitor_select register

Most of these methods accept an optional list of channels to enable or disable (and with no list specified acts on all channels). The exception is enable_analog_monitor (and its disable counterpart): the enable method requires a particular channel to be specified, and the disable method does not require any argument at all. This is because at most one channel is allowed to have the analog monitor enabled.

The machinery of the Configuration object ensures that each value is converted to the appropriate set of bits when it comes time to send actual commands to the physical chip. Although this is not transparent to you as a user of this library, you might want to know that two sets of configuration options are always sent together in the same configuration packet:

  • csa_gain, csa_bypass, and internal_bypass are combined into a single byte, so even though they have their own attributes, they must be written to the physical chip together
  • test_mode, cross_trigger_mode, periodic_reset, and fifo_diagnostic work the same way

Similarly, all of the per-channel options (except for the pixel trim thresholds) are sent in 4 groups of 8 channels.

Configurations can be loaded by importing larpix.configs and running the load function. This function searches for a configuration with the given filename relative to the current directory before searching the “system” location (secretly it’s in the larpix/configs/ folder). This is similar to #include “header.h” behavior in C.

Configurations can be saved by calling chip.config.write with the desired filename.

Once the Chip object has been configured, the configuration must be sent to the physical chip. This is accomplished with the Controller object, which we’ll discuss next.

### Communicating with the physical LArPix chip

Communication between the computer and the physical LArPix chip is handled by the Controller object and uses a Serial interface. (The interface specification is given in the fpga_interface.txt file. It’s based on RS-232 8N1.) To initialize a Controller object, simply provide the port you’d like to communicate over. For the envisioned normal application (with an FTDI chip as USB-serial bridge), this will likely be something like /dev/ttyUSB0.

`python controller = Controller('/dev/ttyUSB0') `

An important attribute of the Controller object is chips, which is a list of Chip objects controlled by the particular Controller. Add a single Chip object to the list with controller.chips.append(myChip), or add a whole list with controller.chips.extend(list_of_chips).

You might want to change the following attributes at some point, but their defaults should work in most cases:

  • baudrate: default = 1000000 baud. Controls the number of bits per second, including RS-232 start and stop bits.
  • timeout: default = 1 second. Controls how long to wait before ending a read command
  • max_write: default = 8192 bytes. Controls the maximum number of bytes to send with a single write command. The limit is entirely due to the buffer capacity of the FTDI chip.

#### Sending data

The only data that LArPix can receive is configuration data. To send all of the configuration packets in write mode, simply call

`python myChip = Chip(chip_id, io_chain) # Edit the configuration # ... myController = Controller('/dev/ttyUSB0') myController.write_configuration(myChip) `

To send only a particular configuration register or list of configuration registers, pass the register or list of registers to the function:

`python register_to_update = 51 myController.write_configuration(myChip, register_to_update) # or pass a list ... registers_to_update = [0, 5, 42] myController.write_configuration(myChip, registers_to_update) `

There is currently not a way to specify which register to update by passing a string or other way of identifying the register by name.

Similar functionality exists to read the configuration data. This requires both sending data to and receiving data from the LArPix chip. To send the “read configuration” commands, call read_configuration exactly the same way you would call write_configuration. Read on to learn about receiving data from LArPix in more detail.

#### Receiving data

There are 3 reasons to receive data from LArPix: because it’s real data (ADC counts, etc.), because it’s configuration data that has been requested, or because it’s test data from either the UART test or the FIFO test.

The simplest way to receive data from LArPix is to just listen for a certain amount of time and save all the packets received. This is accomplished with the run method:

`python  # listens for 10 seconds `

This method makes sense for physics runs or any special runs that aren’t provided by the following other methods.

To read configuration data, call read_configuration, as mentioned earlier.

To make it easy to run tests, the following methods will configure the chip, run the test, and record the data received: run_testpulse, run_fifo_test, and run_analog_monitor_test.

#### Accessing received data

Every method that reads data processes the data from a bytestream into a Packet object. The Packet objects are appended to the list stored in the reads attribute of the correct Chip object, as defined by the chipid returned by the Packet. It’s worth noting here that the Controller object is only aware of Chip objects listed in the controller.chips attribute.

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