PIC18F2420/2520/4420/4520
17.4.2
OPERATION
17.4.3.1
Addressing
The MSSP module functions are enabled by setting the
MSSP Enable bit, SSPEN (SSPCON1<5>).
The SSPCON1 register allows control of the I2C
operation. Four mode selection bits (SSPCON1<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 addressing)
• I2C Slave mode (10-bit addressing)
Once the MSSP module has been enabled, it waits for
a Start condition to occur. Following the Start condition,
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:
• I2C Slave mode (7-bit addressing) with Start and
Stop bit interrupts enabled
• I2C Slave mode (10-bit addressing) with Start and
Stop bit interrupts enabled
• I2C Firmware Controlled Master mode, slave is
Idle
Selection of any I2C mode, with the SSPEN bit set,
forces the SCL and SDA pins to be open-drain,
provided 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.
1. The SSPSR register value is loaded into the
SSPBUF register.
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 Addressing 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 Addressing mode is as follows, with steps 7
through 9 for the slave-transmitter:
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
1. Receive first (high) byte of address (bits, SSPIF,
BF and UA (SSPSTAT<1>), are set).
2. Update the SSPADD register with second (low)
byte of address (clears UA bit and releases the
SCL line).
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.
3. Read the SSPBUF register (clears BF bit) 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.
Any combination of the following conditions will cause
the MSSP module not to give this ACK pulse:
6. Read the SSPBUF register (clears BF bit) and
clear flag bit, SSPIF.
• The Buffer Full bit, BF (SSPSTAT<0>), was set
before the transfer was received.
7. Receive Repeated Start condition.
• The overflow bit, SSPOV (SSPCON2<6>), was
set before the transfer was received.
8. Receive first (high) byte of address (bits, SSPIF
and BF, are set).
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.
9. Read the SSPBUF register (clears BF bit) 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.
DS39631E-page 174
© 2008 Microchip Technology Inc.
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