I2C transaction with re-start

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

Colleagues,

I’ve searched the forum on this subject, but I didn’t find anything. Also, I’m relatively new to using PICs as I2C slaves.

Here’s what I want to do. The slave PIC has handful of 1-byte data fields that master PIC can read over I2C. Master sends over I2C a 1-byte command that identifies the data field, then it reads the data field over I2C. Some I2C A/Ds follow this scenario, where master picks a channel, then reads the results of the A/D conversion. Similar thing with EEPROMs, where master first selects the address, then reads the contents at that address.

Matro · Guest

A restart is almost the same thing as a stop and start again. The PIC will operate the same, i.e. close the pending transaction and open a new one.
The interest of the restart is that in multi-master mode the bus isn't released and so another master can't take control of the bus.
If you replace it by a stop then a start, another master should be able to take control of the bus between the stop and the start. So you have to wait to read. But the worst is that the other master should for exemple change the register value or the register pointer value of the same slave. And so the read you will do after waiting will have no signification.
That's why restart exists.

Matro.

Ttelmah · Guest

A 'restart', is part of the I2C specifications. It is called a 'repeated start'.
In most devices, it doesn't differ from a stop/start transaction. As Matro says, In a multi-master setup, arbitration is not allowed between a repeated start, and a stop. In the normal examples, the usage, is simply to send an address, then reverse direction, but in fact this is a 'sub example', of this general use, where you can send a start, address, several bytes of data, then change direction, with a repeated start, and carry on receiving data (combined format). However on some systems, a 'stop', resets the internal counters, so on these a repeated start is a required transaction, if addressing from a specific location is required.

Best Wishes