072-0300
Alphasense User Manual
OPC-N2 Optical Particle Counter
Issue 5
Alphasense Ltd Page 7 of 34
December 2015
Sensor Technology House, 300 Avenue West, Skyline 120, Great Notley. Essex.CM77 7AA. UK
Tel: +44 (0) 1376 556700 - Fax: +44 (0) 1376 335899
Email: [email protected] - Web: www.alphasense.com
Connecting and Operating The OPC N2
7
Connecting power and taking readings
The OPC-N2 is shipped pre-calibrated. There are no user serviceable parts. Power and data
communications are provided via the SPI socket. Firmware uploading and SD card downloading
are via the micro-USB socket. Use of the SD card will require internal firmware updating if the
firmware version is prior to version 0015d.
Connection to the OPC-N2 for real-time data transfer can be made via the SPI direct to your own
circuit’s internal bus using your own or the SPI interface provided by Alphasense. The SPI interface
supplied by Alphasense requires a USB A-B lead to connect to the USB port of a computer. The
green LED shows that power is supplied to the OPC-N2 and the red LED flashes when the PC and
OPC-N2 are communicating. The micro-USB socket can be used for updating the internal firmware
and downloading the SD card measured data, it is not possible to download real time data via this
socket.
SPI Connection
The SPI socket is a Molex 'Pico Clasp' 6 way Housing, Part Number 501330-0600. Pins are
assigned in table 2. Pin 1 on the OPC-N2 is closest to the USB micro connector.
Pin
Function
1
Vcc
2
SCK
3
SDO
4
SDI
5
/SS
6
GND
Table 2. SPI pin assignments
Please note that it is not possible to put the MISO (SDO) line into a high impedance mode (tri-state).
This means that it is not possible to use the OPC on a shared SPI bus.
OPC power requirement
The OPC-N2 requires a 4.8 to 5.2 Volt DC supply with minimum electrical noise, this is then stepped
down to a 3v3 supply (via the SPI-ISS adapter) for the SPI logic lines. The logic lines are NOT 5
volt tolerant.
The OPC N2 requires 175 mA continuous (95 mA continuous with fan off and laser at low power)
with a short one amp current surge at switch-on. Care must be taken that the current limit is not
exceeded if multiple units are operated from of a single computer
It is recommended to allow 1 second for the OPC to respond to the first SPI command after power-
on and >0.5s after any control system code. (Note this is longer than on previous versions)
Software interface configuration
The following interface rules will help you to make a reliable connection with the OPC.
1. The SPI mode is a Mode 1 device, where the idle state for the clock is low and data is
transmitted on the leading edge (transition from idle to active clock) and is received on the
trailing edge of the signal.
2. SPI clock acceptable frequency range: 300-750kHz. (Other frequencies may also be acceptable,
but are untested).
