Introduction

Each I²C bus consists of two lines (signals): a clock line (SCL) and a data line (SDA).  The devices on the I²C bus are either masters or slaves. The master is always the device that drives the clock line. The slaves are the devices that respond to the master. Only the master can initiate a transfer over the I²C bus. Each slave has a unique address.

Known issues

Using Neonode TSM with Raspberry Pi and I²C

Broadcom BCM2835, used in several versions of Raspberry Pi, has a bug in its I²C implementation that it doesn't support clock stretching properly. Neonode TSM utilizes clock stretching and depending on the setup and configuration of Neonode TSM and RPi I²C, communication might be unstable.

When the problem occurs, the I²C master (RPi) and slave (Neonode TSM) disagree on the number of clock cycles due to a too short clock cycle sent by master (RPi). Therefore the data, as interpreted by the slave and the data as the master (RPi) intended, differ. As a consequence, data corruption occurs.

Links that describe the problem and discuss workarounds etc:

• https://www.advamation.com/knowhow/raspberrypi/rpi-i2c-bug.html
• https://forums.raspberrypi.com/viewtopic.php?f=44&t=13771
• https://github.com/raspberrypi/linux/issues/254
• https://github.com/raspberrypi/linux/issues/4884

No universal work-around seems to be available. One option is to use a software I²C implementation instead of the Broadcom HW I²C. Availability of, and how to setup, such a software I²C driver differs between operating system used on the RPi.

The DataReady signal

The Touch Sensor Module uses a signal called DataReady (DR) to inform the host that there is data to read and that a read operation can be initialized.  Refer to Electrical Integration for more information.

 Module Address

The slave I²C address of the sensor module is 0x50 (7 bit). The address itself consists of 7 bits but 8 bits are always sent. The extra bit informs the slave if the master is reading from or writing to it. If the bit is 0 the master is writing to the slave. If the bit is 1 the master is reading from the slave.

The resulting address bytes are 0xA1 (read) and  0xA0 (write).

 Syntax

The I²C Transport Protocol is simple and the syntax is identical in both directions (read or write):

 Examples:

1. The payload is 8 bytes long. The transmission reads:

   EE 08 [payload]

2. The payload is 25 bytes long. The transmission reads: 

   EE 19 [payload]

Sending Data

To send data to the Touch Sensor Module, the host initiates a write operation for the sensor module's address and writes the full payload. 

(Do not confuse the I²C FrameStart constant with the type byte that indicates that a serialized message is a request. The value is EE for both bytes, but the meaning is completely different.)

Example:  Sending a request with an ENABLE command to the sensor module:

EE 0B EE 09 40 02 02 00 65 03 81 01 00

Receiving Data

The sensor module triggers the DataReady signal when there is data for the host to receive. To maximize the performance and minimize the load on the I²C bus, the host is expected to read data in a certain sequence:

1. The sensor module asserts DataReady
2. The host initiates a two bytes I²C read operation for the I²C 7-bit address 0x50.
3. The sensor module fills in the first two bytes, FrameStart and DataSize.
4. The host initiates an I²C read operation for address 0x50 and reads XX bytes (as indicated by DataSize).
5. The sensor module desserts DataReady.

BootComplete

When the sensor module has finished booting, and hardware such as the I²C module have been configured, the command BootComplete is put into the send queue. The DataReady signal is triggered and the host needs to read the data to acknowledge that the module is ready to operate. If the sensor module powers on before the host system does, the host system needs to check if the DataReady signal is active, in which case it needs to read the data.