HID Hi-O iCLASS Installation Guide - Manual (Page 32)

iCLASS OEM75 Integration Guide, 3141-907, Rev. B.0
Page 32 of 49 January 2014
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6.8 I²C Implementation
To enable I²C operation and set a Slave Address, first present a configuration card to the OEM75.
For the OEM75, use the Inter-Integrated Circuit (I²C) as the underlying protocol for message
transfers. I²C is a master/slave multi-drop serial data link invented by Philips. It uses two signal
lines, SDA (Serial Data) and SCL (clock). SCL is driven by the master to control transfers.
The OEM75 processor is the I²C slave. Any host communicating with the OEM75 must operate as
an I²C master and use the 7 bit address space reference design. For details on this communication
method, see the following URL: http://en.wikipedia.org/wiki/I²C .
6.8.1 General Recommendations
For I²C operation, an external 10k pull-up resistor is required on the I²C-SCK signal.
Note: The OEM75 already contains internal 10k pull-up resistors on the I²C SDA signal.
CAUTION: The pull-up resistors voltage source (+3.3V Read) on the I²C inputs is switched OFF
during Ultra Low Power Mode. Do not hold I²C input signals high during Ultra Low Power Mode.
Doing so causes erratic operation.
6.8.2 I²C Baud Rates and Clock Stretching
I²C Write Function is when the external host master I²C is sending messages to the OEM75 slave
I²C. I²C Read Function is when the external host master I²C is querying for messages from the
OEM75 slave I²C.
The OEM75 slave I²C is tested with a master CPU driving the clock, resulting in a clock rate of
115.2 K Baud. The OEM75 has a crystal frequency of over 27 MHz, and therefore significantly
higher baud rates (230.4 K Baud, 460.6 K Baud) are attainable. However, these higher baud rates
have not been tested.
Clock stretching is implemented on the OEM75. This allows the OEM75 to stop the transmission
clock while processing the I²C interrupt. While higher burst speeds of greater than 115.2 K Baud are
possible for individual bytes, there may be clock-stretching activity occurring on the OEM75 that
slows down the overall effective baud rate of a multi-byte transmission.
6.8.3 I²C Message Transmission
An I²C Write Function is when the external host master I²C is sending messages to the OEM75
slave I²C. I²C Read Function is when the external host master I²C is querying for messages from
the OEM75 slave I²C.
Host I²C Master Write Function
1. The master CPU issues an I²C bus Start.
2. The master CPU sends the OEM75 address with the least significant bit set to zero. This
indicates the address is a write address.
3. Send a byte to the OEM75 indicating the number of bytes in the pending message.
4. Send all message bytes.
5. The master CPU issues an I²C bus Stop.