I've developed a product with a PIC18F2221 as an I2C slave. Since it 
must handle START and REPEATED START conditions differently, it needs 
to generate an interrupt on START and STOP conditions. The SSPCON 
register can be configured to do this; however, I have been unable to 
reliably detect them since SSPIF can be set for a variety of reasons. 
It also doesn't help that the S and P bits in the SSPSTAT register 
can't be reset in software. To get around this, I added extra 
hardware (an inverter and two flip-flops) connected from SCL and SDA 
to the INT1 and INT2 pins. We now have a working product which is 
currently being manufactured.

In order to reduce the cost, my client wants to figure out a way to 
get rid of the extra START/STOP detection hardware, even if it means 
using a microcontroller not made by Microchip. We looked at the 
PIC24FJ16GA002, but it doesn't generate START/STOP interrupts when 
configured as an I2C slave. (The datasheet sounded like it MIGHT do 
this, but experimentation proved otherwise.)

Has anyone been able to reliably detect START/STOP interrupts on a 
PIC18F or any other microcontroller in the same price range?

microchip.com lists the following budgetary prices:
PIC24FJ16GA002 - $1.66
PIC18F2221 - $1.84



Steve Rapinchuk


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