Hi Will,
Having control over when the STOP happens is important since different
devices will behave differently, especially if you are performing a
series of events like read/write/read or something similar. Some devices
expect the STOP to happen in different places in a command sequence.
My vote would be a less attractive AP where the stop is a flag. Since we
apparently aren't using _begin to set the START we may as well drop the
STOP being handled in _end, but I'm obviously curious what other people
think here.
Sending multiple STARTS is perfectly valid BTW:
http://www.i2c-bus.org/repeated-start-condition/
K.
On 18/10/16 21:58, will sanfilippo wrote:
Hello:
I am new to i2c so please bear with me. Hopefully this makes sense to the i2c
experts out there.
In adding i2c to the nordic platforms I ran across an issue that I am not sure
how to solve. Well, I know one way to solve it but it would require changes to
the API. Here is the issue:
In order to get the nordic HW to generate a NACK bit (well, the opposite of an
ACK) on the 9th bit after a read, you need to do something special to the HW
prior to reading the last received character. When you do this, the HW will
generate the correct 9th bit and will also put a STOP condition on the line.
While it is possible to get the HW to generate a STOP independently, I dont see
how I can get it to generate the correct 9th bit unless you know it is the last
character you want to read.
Another issue that is bothersome is the nordic SDK itself: when you are done
with the read it automatically generates the STOP. That is fine in and of
itself; the SDK could be modified, but the issue is that dang 9th bit.
Our API has the following functions:
hal_i2c_master_begin(): Not exactly sure what this does on all platforms but
currently is a NOOP on the nordic platforms.
hal_i2c_master_write(): generates a start but no stop.
hal_i2c_master_read(): generates a start but no stop.
hal_i2c_master_end(): generates a stop
At first glance, this seems to be a fine API and is easy to see a
“transaction”: a begin, writes and/or reads chained together, and an end.
Unfortunately, I could not see how to do this on the nordic platforms.
One way to do this would be to get rid of begin/end and have a flag in the
read/write API. The flag would be “generate stop”. This way users can chain
reads/writes together and end them with a STOP whenever they want. The only con
to this is that it is not so easy to look at the code to see that transactions
have a stop.
Given that I dont have alot of experience wth many i2c devices, I dont know if
having the ability to skip START and/or address when the read/write APIs are
called is useful. So far in my limited experience the start/address condition
between the read/writes is not an issue.
So to summarize, here is what I suggest:
* Remove the begin/end API.
* Add a flag to write/read that will cause a STOP to be generated at the end of
the read write.
Thanks! Comments greatly appreciated.
