Full Answer
What is the I2C reference voltage?
With the I2C specification 2.0 released 1998 the possible I2C reference voltage was decreased to 2 volt. Since the I2C bus lines are used bidirectionally, interfacing I2C devices with different voltages is not straightforward, special level-shifting devices are necessary for this purpose.
What should I consider when working with low voltage I2C systems?
Things to consider when working with low voltage I2C systems. When I2C was first introduced the typical electrical VCC level of electronics was 5V and the initially specified speed was a maximum of 100 kHz. With these parameters it was possible to operate over a wide range of electrical parameters in terms of bus termination and capacitance.
What is the voltage of a low voltage IC?
Low Voltage IC families normally run off of 3.3 volts [3.6 volts], 2.5 volts, or 1.8 volts. The graph shows 5 volt CMOS, TTL, and mixed CMOS/TTL IC devices, and 3.3 volt LVTTL and LVCMOS IC devices.
What is the maximum I2C frequency for SCL and SDA signals?
Spikes in SCL and SDA signals must be filtered up to a certain amount (only for full speed I2C). Setup and hold times; this includes a specified maximum SCL clock rate (100 kHz for normal speed, 400 kHz for full speed).
What are voltage levels for I2C?
I2C uses only two bidirectional open-collector or open-drain lines: serial data line (SDA) and serial clock line (SCL), pulled up with resistors. Typical voltages used are +5 V or +3.3 V, although systems with other voltages are permitted.
Is I2C 3.3 or 5V?
Both of it's I2C interfaces run at 3.3v.
What logic level is I2C?
In the beginning the I2C bus focussed on 5 volt logic. With the I2C specification 2.0 released 1998 the possible I2C reference voltage was decreased to 2 volt.
What is maximum I2C speed?
Data on the I2C-bus can be transferred at rates of up to 100 kbit/s in the Standard-mode, up to 400 kbit/s in the Fast-mode, up to 1 Mbit/s in Fast-mode Plus, or up to 3.4 Mbit/s in the High-speed mode. The bus capacitance limits the number of interfaces connected to the bus.
What voltage is SPI?
The supply voltage for the analog front-end is 5V, the low-power µP has a supply of 2.5V. The MAX3390E level shifts the SPI signals from 2.5V to 5V and from 5V to 2.5V for the DOUT signal. Figure 5. The MAX3390E level translator in an SPI-bus application.
How do I choose I2C pullup resistor?
Once the minimum and maximum value of the pullup resistor has been selected, the decision for the value of resistor can be made based on trade-off between the speed and power budget. A smaller resistor will give a higher speed because of smaller RC delay, and a larger resistor will give lower power consumption.
How much current does I2C use?
The I2C bus specification and user manual also sets a maximum of 0.4V for the low-level output voltage, because it indicates that a maximum sink current of 3mA is flowing across the transistor.
How do you check I2C lines?
You can start the testing process by verifying each of the following features on the I2C bus: START and STOP condition generation. A start condition is generated when the serial data (SDA) line switches from high voltage to low voltage before the serial clock (SCL) line switches from high to low.
Why does I2C need pull-up resistor?
Why is a pull-up resistor used in I2C? Because there are only 2 logic level outputs from I2C, Low and open circuit. The default condition is all chips on the line are in open circuit mode, and the pull-up resistor makes the line high. The master chip can send an address signal to one of the chips on the line.
How Slow can You Run I2C?
Theoretically minimum I2C speed is 0Hz as per I2C specification.
What is I2C baud rate?
Standard baud rates for I2C are: Baud Rate. Description. 100 kHz. Original speed.
How is I2C speed calculated?
Assuming that my i2c clock is operating at 400 kHz, I will be able to transfer 1 bit every clock cycle, and hence 400,000 bits in a second. Thus no. of times I can read all sensor values every second (RPS): RPS = 400,000 / 1016 = 393.7 assuming my sensors are ready to supply data at that rate.
How is data transferred on I2C?
Data on the I2C bus is transferred in 8-bit packets (bytes). There is no limitation on the number of bytes, however, each byte must be followed by an Acknowledge bit. This bit signals whether the device is ready to proceed with the next byte. For all data bits including the Acknowledge bit, the master must generate clock pulses. If the slave device does not acknowledges transfer this means that there is no more data or the device is not ready for the transfer yet. The master device must either generate Stop or Repeated Start condition.
What is an I2C master?
All I2C master and slave devices are connected with only those two wires. Each device can be a transmitter, a receiver or both. Some devices are masters – they generate bus clock and initiate communication on the bus, other devices are slaves and respond to the commands on the bus.
What happens when the SDA signal is low but it should be high?
If the SDA signal on the bus is low but it should be high, then this master has lost arbitration. Master I2C device that has lost arbitration can generate SCL pulses until the byte ends and must then release the bus and go into slave mode. The arbitration procedure can continue until all the data is transferred.
What is high speed mode?
High-speed mode uses signals called SCLH and SDAH to emphasize the higher speed. These signals are usually separated from standard SDA and SCL lines. High-speed mode introduces also few differences (or improvements) in the specifications: 1 Improved data and clock line output drivers 2 Schmitt trigger and spike suppression circuits on data and clock inputs 3 Clock synchronization and arbitration is not used 4 Clock signal has 1 to 2 high/low ratio
How many masters can I2C have?
I2C can have more than one master and each can send commands. Arbitration. A process to determine which of the masters on the bus can use it when more masters need to use the bus. Synchronization. A process to synchronize clocks of two or more devices.
When does SDA signal change?
For each clock pulse one bit of data is transferred. The SDA signal can only change when the SCL signal is low – when the clock is high the data should be stable.
How are SCL and SDA connected?
Both signals (SCL and SDA) are bidirectional. They are connected via resistors to a positive power supply voltage. This means that when the bus is free, both lines are high. All devices on the bus must have open-collector or open-drain pins. Activating the line means pulling it down ( wired AND ). The number of the devices on a single bus is almost unlimited – the only requirement is that the bus capacitance does not exceed 400 pF. Because logical 1 level depends on the supply voltage, there is no standard bus voltage.
The Physical Bus
I²C itself comprises two wires. One I²C line transmits data, the other the clock signals that synchronize the conversation between devices. The data line is called ‘SDA’, the clock line ‘SCL’.
Controllers And Peripherals
The I²C bus separates devices into ‘controllers’ and ‘peripherals’. Only one device can send out timing pulses on the SCL line at a time, and that’s the one chosen to be the controller. All the others synchronize their timings to the controller, and are thus considered peripherals.
Addressing
In order for one I²C device to communicate with another on a one-to-one basis, both devices need to be uniquely identifiable. This identity is the device’s I²C address. I²C addresses are usually 7-bit numbers, so a bus can comprise up to 127 devices in all.
Signalling
The I²C bus’ controller uses a peripheral’s 7-bit address to identify the component it wants to talk to. In fact, the signalling is more complex than that, but fortunately all the details are handled by the imp so that you need only supply the address as an 8-bit value.
Timing
The standard clock speed for I²C communications is 100kHz — 100,000 SCL pulses per second. It’s possible to go faster, up to 400kHz. Some devices may not be able to support this speed; check the datasheet that accompanies the device you want to connect to your imp.
Example Code
The following code works with the TAOS TSL2561 visible and infrared light sensor, a 3.3V device that uses I²C to communicate with its host microcontroller. The chip’s datasheet can be downloaded from the Adafruit website. Adafruit sells the chip on a low-cost breakout board which includes suitable pull-up resistors on the power pin, VCC.
What The Code Does
The agent code responds to an incoming HTTP request by notifying the device that it requires a reading. For simplicity, the reading is simply displayed in the log.
