🚀 go-pugleaf

I've been using the I2C bus (or Inter IC bus as it used to be called when it came out in the early 80's) for over a
decade now.
From some of the communication I've seen on newsgroups I see there is a lot of interest in it and yet a certain
amount of misunderstanding about it.
Because of this and my own experience with the bus I feel I may be able to help answer some questions and set
matters straight.


__________________
OTHER BUSES
	MICROWIRE BUS
	Requires clock, data in, data out, and individual chip selects.
	Better than a parallel bus for certain devices but only the first step in serial control.

	SPI BUS
	Fast if hardware is built into the micro but inherently point-to-point so limited to master+slave.
	Great though for two micros to communicate with one another directly.
__________________
I2C BUS

LICENSE
The I2C was created by Philips but to the best of my knowledge any chipmaker that uses it only has to comply with
the standard, they do not have to pay Philips for the privilege. This way the I2C standard remains a standard.

SPEED
I2C can be slower than micro-wire or SPI or any other dedicated point-to-point 'bus'. It's maximum speed is limited
to 400K bps. You can perform a random read fom a 24C04 in about 80us.

MULTIDROP
Only I2C offers true multidrop and multimaster capability. All chips on the I2C bus are addressable.
No special chip select signals are needed, it is truely a 2 wire bus (plus ground of course).

CONTENTION
Most configurations require only a single master so there is no need to have complicated software to handle
contentions. Otherwise, it is available for those who require it.

DRIVE
I2C bus lines are 'open-drain' type with an external pull-up required usually back at the processor end.
If you are using single-master mode (that is, only one processor controlling the bus) the clock pin only needs to
be an output (no pullup required) as it never needs to relinquish control.
If you are trying to drive the I2C bus from an output only port such as a PC printer port you only need insert a
diode such as a BAT85 and a 4K7 pull-up resistor in the SDA line.

output ------------------ SCL
output --|--|<|----|----- SDA
         |-/\/\/---|
input  ------------|

DEVICES AVAILABLE
Besides the ability to interface processors to the bus the real advantage is being able to interface 'dumb' chips
to the bus.
Some of the more common chips available are;

PCF8574		16 pin	8-bit I/O, 30 ma sink passive pullup
PCF8591		16 pin	4 channel 8-bit ADC + 8-bit DAC
PCF8583		8 pin	RTC + 256 bytes RAM
24CXX		8 pin	Range of serial EEPROMs
SAA1064		24 pin	4-digit LED driver
TDA8444		16 pin	octal 6-bit DAC

MY OWN EXPERIENCE
I always like to leave two I/O lines available on any design I do and run them out to a 4-pin strip on the edge
of the board. The number of times a customer has required that extra little widget and I've been able to
accommodate it using the I2C bus for expansion.
My favourite chip for this is the PCF8574 which is an 8-bit I/O chip with weak pull-ups.
With a single chip I can interface directly to an LCD module, or a 4X4 keypad, LEDs, etc.

Most drivers I write are rarely more than 90 lines of assembler to create a byte write, byte read, start, and stop
subroutines. If you want some examples or how-to then just ask me.

MY CONNECTOR (MOSTLY)
A single 4 pin 0.1" header.
	1 SDA
	2 SCL
	3 GND
	4 V+

CORRECTIONS WELCOME

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\/	Peter Jakacki                                     \/
/\	Cybertronics                                      /\
\/	Brisbane, Australia                               \/
/\	Email:  cyb@ozemail.com.au, cybert@aai.com.au     /\
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