PIC18FXX20
17.4.2
OPERATION
17.4.3.1
Addressing
Once the MSSP module has been enabled, it waits for
a START condition to occur. Following the START con-
dition, the 8-bits are shifted into the SSPSR register. All
incoming bits are sampled with the rising edge of the
clock (SCL) line. The value of register SSPSR<7:1> is
compared to the value of the SSPADD register. The
address is compared on the falling edge of the eighth
clock (SCL) pulse. If the addresses match, and the BF
and SSPOV bits are clear, the following events occur:
The MSSP module functions are enabled by setting
MSSP Enable bit, SSPEN (SSPCON<5>).
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 = (FOSC / 4) x (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
1. The SSPSR register value is loaded into the
SSPBUF register.
• I2C Slave mode (10-bit address), with START and
STOP bit interrupts enabled
2. The buffer full bit BF is set.
3. An ACK pulse is generated.
4. MSSP interrupt flag bit, SSPIF (PIR1<3>), is set
(interrupt is generated, if enabled) on the falling
edge of the ninth SCL pulse.
In 10-bit Address mode, two address bytes need to be
received by the slave. The five Most Significant bits
(MSbs) of the first address byte specify if this is a 10-bit
address. Bit R/W (SSPSTAT<2>) must specify a write
so the slave device will receive the second address
byte. For a 10-bit address, the first byte would equal
‘11110 A9 A8 0’, where ‘A9’ and ‘A8’ are the two
MSbs of the address. The sequence of events for 10-bit
address is as follows, with steps 7 through 9 for the
slave-transmitter:
• 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 inputs by setting
the appropriate TRISC bits. To ensure proper operation
of the module, pull-up resistors must be provided
externally to the SCL and SDA pins.
17.4.3
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).
The I2C Slave mode hardware will always generate an
interrupt on an address match. Through the mode
select bits, the user can also choose to interrupt on
START and STOP bits
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
currently in the SSPSR register.
Any combination of the following conditions will cause
the MSSP module not to give this ACK pulse:
1. Receive first (high) byte of Address (bits SSPIF,
BF and bit UA (SSPSTAT<1>) are set).
2. Update the SSPADD register with second (low)
byte of Address (clears bit UA and releases the
SCL line).
3. Read the SSPBUF register (clears bit BF) and
clear flag bit SSPIF.
4. Receive second (low) byte of Address (bits
SSPIF, BF, and UA are set).
5. Update the SSPADD register with the first (high)
byte of Address. If match releases SCL line, this
will clear bit UA.
6. Read the SSPBUF register (clears bit BF) and
• The buffer full bit BF (SSPSTAT<0>) was set
before the transfer was received.
clear flag bit SSPIF.
7. Receive Repeated START condition.
• 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.
8. Receive first (high) byte of Address (bits SSPIF
and BF are set).
9. Read the SSPBUF register (clears bit BF) and
clear flag bit SSPIF.
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.
DS39609A-page 170
Advance Information
2003 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
