Datasheet

 2011-2013 Microchip Technology Inc. DS40001609C-page 197
PIC16(L)F1508/9
21.4.9 ACKNOWLEDGE SEQUENCE
The ninth SCLx pulse for any transferred byte in I
2
C is
dedicated as an Acknowledge. It allows receiving
devices to respond back to the transmitter by pulling
the SDAx line low. The transmitter must release con-
trol of the line during this time to shift in the response.
The Acknowledge (ACK
) is an active-low signal, pull-
ing the SDAx line low indicated to the transmitter that
the device has received the transmitted data and is
ready to receive more.
The result of an ACK
is placed in the ACKSTAT bit of
the SSPxCON2 register.
Slave software, when the AHEN and DHEN bits are
set, allow the user to set the ACK
value sent back to
the transmitter. The ACKDT bit of the SSPxCON2 reg-
ister is set/cleared to determine the response.
Slave hardware will generate an ACK
response if the
AHEN and DHEN bits of the SSPxCON3 register are
clear.
There are certain conditions where an ACK
will not be
sent by the slave. If the BF bit of the SSPxSTAT regis-
ter or the SSPOV bit of the SSPxCON1 register are
set when a byte is received.
When the module is addressed, after the eighth falling
edge of SCLx on the bus, the ACKTIM bit of the
SSPxCON3 register is set. The ACKTIM bit indicates
the acknowledge time of the active bus. The ACKTIM
Status bit is only active when the AHEN bit or DHEN
bit is enabled.
21.5 I
2
C Slave Mode Operation
The MSSP Slave mode operates in one of four modes
selected in the SSPM bits of SSPxCON1 register. The
modes can be divided into 7-bit and 10-bit Addressing
mode. 10-bit Addressing modes operate the same as
7-bit with some additional overhead for handling the
larger addresses.
Modes with Start and Stop bit interrupts operate the
same as the other modes with SSPxIF additionally
getting set upon detection of a Start, Restart, or Stop
condition.
21.5.1 SLAVE MODE ADDRESSES
The SSPxADD register (Register 21-6) contains the
Slave mode address. The first byte received after a
Start or Restart condition is compared against the
value stored in this register. If the byte matches, the
value is loaded into the SSPxBUF register and an
interrupt is generated. If the value does not match, the
module goes idle and no indication is given to the soft-
ware that anything happened.
The SSP Mask register (Register 21-5) affects the
address matching process. See Section 21.5.9
“SSPx Mask Register” for more information.
21.5.1.1 I
2
C Slave 7-bit Addressing Mode
In 7-bit Addressing mode, the LSb of the received data
byte is ignored when determining if there is an address
match.
21.5.1.2 I
2
C Slave 10-bit Addressing Mode
In 10-bit Addressing mode, the first received byte is
compared to the binary value of ‘1 1 1 1 0 A9 A8 0’. A9
and A8 are the two MSbs of the 10-bit address and
stored in bits 2 and 1 of the SSPxADD register.
After the acknowledge of the high byte the UA bit is set
and SCLx is held low until the user updates SSPxADD
with the low address. The low address byte is clocked
in and all eight bits are compared to the low address
value in SSPxADD. Even if there is not an address
match; SSPxIF and UA are set, and SCLx is held low
until SSPxADD is updated to receive a high byte
again. When SSPxADD is updated the UA bit is
cleared. This ensures the module is ready to receive
the high address byte on the next communication.
A high and low address match as a write request is
required at the start of all 10-bit addressing communi-
cation. A transmission can be initiated by issuing a
Restart once the slave is addressed, and clocking in
the high address with the R/W
bit set. The slave hard-
ware will then acknowledge the read request and pre-
pare to clock out data. This is only valid for a slave
after it has received a complete high and low address
byte match.
21.5.2 SLAVE RECEPTION
When the R/W
bit of a matching received address byte
is clear, the R/W
bit of the SSPxSTAT register is
cleared. The received address is loaded into the
SSPxBUF register and acknowledged.
When the overflow condition exists for a received
address, then not Acknowledge is given. An overflow
condition is defined as either bit BF of the SSPxSTAT
register is set, or bit SSPOV of the SSPxCON1 register
is set. The BOEN bit of the SSPxCON3 register modi-
fies this operation. For more information see
Register 21-4.
An MSSP interrupt is generated for each transferred
data byte. Flag bit, SSPxIF, must be cleared by soft-
ware.
When the SEN bit of the SSPxCON2 register is set,
SCLx will be held low (clock stretch) following each
received byte. The clock must be released by setting
the CKP bit of the SSPxCON1 register, except
sometimes in 10-bit mode. See Section 21.2.3 “SPI
Master Mode” for more detail.