PIC16C72 Series
1998 Microchip Technology Inc.
Preliminary
DS39016A-page 47
8.4
SSP I2C Operation
The SSP module in I2C mode fully implements all slave
functions, except general call support, and provides
interrupts on start and stop bits in hardware to facilitate
rmware implementations of the master functions. The
SSP module implements the standard mode specica-
tions 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
congure these pins as inputs or outputs through the
TRISC<4:3> bits.
The SSP module functions are enabled by setting SSP
Enable bit SSPEN (SSPCON<5>).
FIGURE 8-7:
SSP BLOCK DIAGRAM
(I2C MODE)
The SSP module has ve registers for I2C operation.
These are the:
SSP Control Register (SSPCON)
SSP Status Register (SSPSTAT)
Serial Receive/Transmit Buffer (SSPBUF)
SSP Shift Register (SSPSR) - Not directly acces-
sible
SSP Address Register (SSPADD)
The SSPCON register allows control of the I2C opera-
tion. Four mode selection bits (SSPCON<3:0>) allow
one of the following I2C modes to be selected:
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 inputs by setting
the appropriate TRISC bits.
Additional information on SSP I2C operation may be
found in the PICmicro Mid-Range MCU Reference
Manual, DS33023.
8.4.1
SLAVE MODE
In slave mode, the SCL and SDA pins must be cong-
ured as inputs (TRISC<4:3> set). The SSP 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
then load the SSPBUF register with the received value
currently in the SSPSR register.
There are certain conditions that will cause the SSP
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 overow 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.
Table 8-3 shows what happens when a data transfer
byte is received, given the status of bits BF and
SSPOV. The shaded cells show the condition where
user software did not properly clear the overow condi-
tion. Flag bit BF is cleared by reading the SSPBUF reg-
ister 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 specication as well as the requirement of the SSP
module is shown in timing parameter #100 and param-
eter #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