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Software Implementation of I²C Bus Master
Amar Palacherla
This application note describes the software implementation of I 2 C interface routines for the PIC16CXXX family of devices. Only the master mode of I 2 C interface is implemented in this application note. This implementation is for a single master communication to multiple slave I 2 C devices. Some PIC16CXXX devices, such as the PIC16C64 and PIC16C74, have on-chip hardware which implements the I 2 C slave interface, while other PIC16CXXX devices, such as the PIC16C71 and PIC16C84, do not have the same on-chip hardware. This application note does not describe the I 2 C Bus specifications and the user is assumed to have an understanding of the I 2 C Bus. For detailed information on the bus, the user is advised to read the I 2 C Bus Specification document from Philips/Signetics (order number 98-8080-575). The I 2 C Bus is a two-wire serial bus with multiple possible masters and multiple possible slaves connected to each other through two wires. The two wires consists of a clock line (SCL) and a data line (SDA) with both lines being bi-directional. Bi-direc-tional communication is facilitated through the use of wire and connection (the lines are either active-low or passive high). The I 2 C Bus protocol also allows collision detection, clock synchronization and hand-shaking for multi-master systems. The clock is always generated by the master, but the slave may hold it low to generate a wait state. In most systems the microcontroller is the master and the external peripheral devices are slaves. In these cases this application note can be used to attach I 2 C slaves to the PIC16CXXX (the master) microcontroller. The multi-master system is not implemented because it is extremely difficult to meet all the I 2 C Bus timing spec-ifications using software. For a true slave or multi-master system, some interface hardware is necessary (like START & STOP bit detection). In addition to the low level single master I 2 C routines, a collection of high level routines with various message structures is given. These high level macros/routines can be used as canned routines to interface to most I 2 C slave devices. As an example, the test program talks to two Serial EEPROMs (Microchip?s 24LC04 and 24LC01).
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