PIC18CXX2
DS39026C-page 128
2001 Microchip Technology Inc.
14.4
MSSP I2C Operation
The MSSP module in I2C mode, fully implements all
master and slave functions (including general call sup-
port) and provides interrupts on START and STOP bits
in hardware to determine a free bus (multi-master func-
tion). The MSSP module implements the standard
mode specifications, as well as 7-bit and 10-bit
addressing.
Two pins are used for data transfer. These are the RC3/
SCK/SCL pin, which is the clock (SCL), and the RC4/
SDI/SDA pin, which is the data (SDA). The user must
configure these pins as inputs or outputs through the
TRISC<4:3> bits.
The MSSP module functions are enabled by setting
MSSP enable bit SSPEN (SSPCON<5>).
FIGURE 14-7:
MSSP BLOCK DIAGRAM
(I2C MODE)
The MSSP module has six registers for I2C operation.
These are the:
MSSP Control Register1 (SSPCON1)
MSSP Control Register2 (SSPCON2)
MSSP Status Register (SSPSTAT)
Serial Receive/Transmit Buffer (SSPBUF)
MSSP Shift Register (SSPSR) - Not directly
accessible
MSSP Address Register (SSPADD)
The SSPCON1 register allows control of the I2C oper-
ation. Four mode selection bits (SSPCON<3:0>) allow
one of the following I2C modes to be selected:
I2C Master mode, clock = OSC/4 (SSPADD +1)
I2C Slave mode (7-bit address)
I2C Slave mode (10-bit address)
I2C Slave mode (7-bit address), with START and
STOP bit interrupts enabled
I2C Slave mode (10-bit address), with START and
STOP bit interrupts enabled
I2C Firmware controlled master operation, slave
is idle
Selection of any I2C mode with the SSPEN bit set,
forces the SCL and SDA pins to be open drain, pro-
vided these pins are programmed to be inputs by set-
ting the appropriate TRISC bits.
14.4.1
SLAVE MODE
In Slave mode, the SCL and SDA pins must be config-
ured as inputs (TRISC<4:3> set). The MSSP module
will override the input state with the output data when
required (slave-transmitter).
When an address is matched or the data transfer after
an address match is received, the hardware automati-
cally will generate the Acknowledge (ACK) pulse and
load the SSPBUF register with the received value cur-
rently in the SSPSR register.
There are certain conditions that will cause the MSSP
module not to give this ACK pulse. These are if either
(or both):
a)
The buffer full bit BF (SSPSTAT<0>) was set
before the transfer was received.
b)
The overflow bit SSPOV (SSPCON<6>) was set
before the transfer was received.
In this case, the SSPSR register value is not loaded
into the SSPBUF, but bit SSPIF (PIR1<3>) is set. The
BF bit is cleared by reading the SSPBUF register, while
bit SSPOV is cleared through software.
The SCL clock input must have a minimum high and
low for proper operation. The high and low times of the
I2C specification, as well as the requirement of the
MSSP module, are shown in timing parameter #100
and parameter #101.
Read
Write
SSPSR reg
Match Detect
SSPADD reg
START and
STOP bit Detect
SSPBUF reg
Internal
Data Bus
Addr Match
Set, Reset
S, P bits
(SSPSTAT reg)
RC3/SCK/SCL
RC4/
Shift
Clock
MSb
SDI/
LSb
SDA