Epc901-breakout

Introduction

The ESPROS epc901 is a CCD line sensor with a resolution of 1024 monochrome pixels.

Compared to other image sensors the epc901 is very easy to drive, requiring only one voltage, I2C, a few digital I/Os and an ADC. It is also available for $33 in single units from Digi-key. On the downside, the sensor comes in a BGA package, the evaluation kit is very expensive at $1400, and we haven't seen this sensor from any other distributors than Digi-key.

This repository contains design files to build hardware and software to interact with the sensor.

For Windows users, there's now also a version of g3gg0.de SpectrumPlotter adapted to work with my epc901 firmware.

@fmilburn3 posted a write-up of this project and a few spectroscopy experiments on element14.

Table of contents

Hardware

• Breakout board
  • Connector pinout
  • Jumpers
  • Config resistor array
  • Physical dimensions
  • Lens holder & lens
• Adapter board
  • Pin mapping
  • Jumpers
• Shared
• Spectroscope tube

Software

• Firmware
  • Shell
  • Capture logic
  • Trigger logic
  • Transfer format
• Python
  • epc901camera.py
  • example scripts

License

Hardware

All hardware in this project is designed with KiCad version 5.1.6.

The hardware consists of a breakout board for the image sensor, and an adapter board for vertically mounting the breakout board on an ST Nulceo-144 development board.

Breakout board

The folder ./hardware/breakout contains the KiCad project for the epc901 breakout board.

The image sensor comes in a 2x16, 0.5 mm pitch BGA package and looks very fragile. With careful handling and placement, we successfully assembled a few boards using a stainless steel solder stencil and an unmodified toaster oven for reflow. Reflow on a hotplate probably works as well.

In addition to the image sensor and related passives, the breakout board also integrates an ADC. Footprint and firmware are compatible with Texas Instruments ADCS7476 and Analog Devices AD7476 which are 1 MSPS 12-bit ADCs with SPI-like interface.

The PCB can be ordered from OSH Park using this link.

Connector pinout

Jumpers

Config resistor array

These resistors control the configuration of the image sensor on power-up. For details, see sections 4, 7.1 and 12 in the epc901 datasheet.

Physical dimensions

The size of the board is 61 x 36 mm. The mounting holes in the corners are spaced 55 x 30 mm and have a diameter of 2.2 mm, designed for M2 screws.

The mounting holes for the lens holder are located 29 mm from the left board edge and 8 mm from top and bottom board edge. They have a diameter of 2.2 mm, designed for M2 screws.

Lens holder & lens

The breakout board has space and mounting holes for a lens holder. The spacing between the mounting holes is 20mm. For full coverage, the lens should support 1/2.3" or larger sensors.

For our prototypes, we chose a M12 metal lens holder w/ 20mm hole spacing and a 25mm M12 lens for 1/2" sensors. In the US, a wide selection of lens holders and lenses can be found at M12 Lenses Inc.

For use in a spectrograph, a lens may not be required. However we recommend protecting the sensor against accidental physical damage with some sort of bracket or baffel.

Adapter board

The folder ./hardware/adapter-shield contains the KiCad project for an adapter to mount the epc901 breakout board on the NUCLEO evaluation board.

The layout of the adapter board follows the Arduino Shield standard. This means you could mount the adapter and sensor breakout board on any compatible MCU board. Though you will likely have to adapt the firmware to make it work.

The PCB can be ordered from OSH Park using this link.

Pin mapping

Jumpers

Shared

The folder ./hardware/shared contains custom KiCad symbols, footprints and 3d models that can be reused across KiCad projects in this repository.

Spectroscope tube

For spectroscopy experiments we created a tube to mount an optical slit and difraction grating. The 3D model is available in the folder ./hardware/spectroscope-tube.

Software

Firmware

The folder ./software/breakout contains the firmware to control the epc901 breakout board with an STM32L4 using the NUCLEO-L496ZG evaluation board. This specific MCU and evaluation board were chosen because we had one laying around.

The firmware is developed and built with the latest version of STM32CubeIDE (version 1.4.2 as of October 2020). To import the project into STM32CubeIDE, clone this repository, open STM32CubeIDE and import the project with File > Open Project From File System.., and select the folder ./software/breakout. Follow the standard procedure for building the firmware and programming the NUCLEO evaluation board.

Shell

The firmware exposes a command shell over serial. The parameters of the serial connection are 115200 8N1.

Preceding a command with @ will suppress local echo and command prompt, which is useful when developing an application that interacts with the shell.

Most commands that configure an aspect of the firmware return the current setting if no value is provided. For example exposure will return the currently configured exposure time.

Capture logic

The capture command will start the image capture process based on the configuration of the camera.

First the firmware checks if there's enough space in the frame buffer for the configured number of frames (1 or burst frames). An error will be returned if there is not enoug space.

Next, if trigger is enabled, the camera waits until the configured trigger condition is met.

Next, image capture starts. If burst is enabled, it will capture the configured number of frames. All frames captured are stored in a frame buffer.

After the capture process is complete, images can be read from the frame buffer with the transfer command.

The shell will respond with BUSY if the camera is waiting for a trigger or a single image capture or burst is in progress. At any time, the command capture abort will return the camera back to idle state.

Trigger logic

External

The external trigger source is wired to pin D35/PB11 and the USER button (B1).

Trigger direction rising triggers when the signal changes from low to high, or when the button is pressed down. Trigger direction falling triggers when the signal changes from high to low, or when the button is released.

Level

The level trigger source uses live sensor data to trigger. When starting capture, the firmware will continously capture images until the trigger condition is met or capture is aborted.

Trigger direction rising triggers when a pixel is on or above the configured trigger level. Trigger direction falling triggers when no pixel is on or above the configured trigger level.

Zone

The zone trigger source uses live sensor data to trigger. When starting capture, the firmware will continously capture images until the trigger condition is met or capture is aborted.

Trigger direction rising triggers when a pixel is inside the configured trigger zone. Trigger direction falling triggers when no pixel is inside the configured trigger zone.

Transfer format

The transfer command reads images from the frame buffer and sends them over serial. Each image is sent in two chuncks, each terminated with a linefeed.

1. Frame metadata seperated with commas: frame number, time in milliseconds since first frame, exposure time in microseconds, number of pixels in frame
2. Frame data: a continous string with one 3-character hexadecimal number per pixel representing the brightness value

Python

The folder ./software/python contains a Python library that wraps the shell of the firmware, and a few useful Python scripts that use the library.

epc901camera.py

The Python library epc901camera.py wraps the shell exposed by the firmware into an API that's easy to use from Python.

from epc901camera import Camera
camera = Camera()
camera.open("my serial port")
camera.setExposure(1000)
camera.capture()
pixels = camera.getPixels()
camera.close()

You can use sendCommand() to directly interact with the shell, which can be useful for operations that are not supported by the library yet. The following snippet writes to I2C registers of the sensor to increase the gain.

camera.sendCommand("regwrite 2 0x99")  # turn on config via regs
camera.sendCommand("regwrite 0 0x19")  # 4x gain, read-out direction same as set by resistor

See example scripts for more details.

example scripts

There are three scripts to interact with the breakout board:

• live.py displays output of the image sensor in realtime
• snap.py captures one image
• burst.py captures a series of images in a burst

The scripts are controlled with shared and a few unique command line arguments.

License

Unless stated otherwise, all software in this project is licensed under the terms of MIT License.

The main exception are files imported from the STM32 SDK (./software/breakout/Drivers), and a few files auto-generated by STM32CubeIDE. See source code for applicable license terms.

All hardware in this project is licensed under the terms of CERN Open Hardware Licence Version 2 - Permissive.

See LICENSE-SW and LICENSE-HW for additional details.