2001 Microchip Technology Inc.
DS30292C-page 75
PIC16F87X
TABLE 9-2:
DATA TRANSFER RECEIVED BYTE ACTIONS
9.2.1.3
Slave Transmission
When the R/W bit of the incoming address byte is set
and an address match occurs, the R/W bit of the
SSPSTAT register is set. The received address is
loaded into the SSPBUF register. The ACK pulse will
be sent on the ninth bit, and the SCL pin is held low.
The transmit data must be loaded into the SSPBUF
register, which also loads the SSPSR register. Then,
the SCL pin should be enabled by setting bit CKP
(SSPCON<4>). The master must monitor the SCL pin
prior to asserting another clock pulse. The slave
devices may be holding off the master by stretching the
clock. The eight data bits are shifted out on the falling
edge of the SCL input. This ensures that the SDA sig-
nal is valid during the SCL high time (Figure 9-7).
An SSP interrupt is generated for each data transfer
byte. The SSPIF flag bit must be cleared in software
and the SSPSTAT register is used to determine the sta-
tus of the byte transfer. The SSPIF flag bit is set on the
falling edge of the ninth clock pulse.
As a slave-transmitter, the ACK pulse from the master
receiver is latched on the rising edge of the ninth SCL
input pulse. If the SDA line is high (not ACK), then the
data transfer is complete. When the not ACK is latched
by the slave, the slave logic is reset and the slave then
monitors for another occurrence of the START bit. If the
SDA line was low (ACK), the transmit data must be
loaded into the SSPBUF register, which also loads the
SSPSR register. Then the SCL pin should be enabled
by setting the CKP bit.
FIGURE 9-6:
I2C WAVEFORMS FOR RECEPTION (7-BIT ADDRESS)
Status Bits as Data
Transfer is Received
SSPSR
SSPBUF
Generate ACK
Pulse
Set bit SSPIF
(SSP Interrupt occurs
if enabled)
BF
SSPOV
0
Yes
1
0
No
Yes
1
No
Yes
0
1
Yes
No
Yes
Note:
Shaded cells show the conditions where the user software did not properly clear the overflow condition.
P
9
8
7
6
5
D0
D1
D2
D3
D4
D5
D6
D7
S
A7 A6 A5 A4 A3 A2 A1
SDA
SCL
12
3
4
5
6
7
8
9
12
3
4
56
7
89
12
3
4
Bus Master
Terminates
Transfer
Bit SSPOV is set because the SSPBUF register is still full.
Cleared in software
SSPBUF register is read
ACK
Receiving Data
D0
D1
D2
D3
D4
D5
D6
D7
ACK
R/W=0
Receiving Address
SSPIF
BF (SSPSTAT<0>)
SSPOV (SSPCON<6>)
ACK
ACK is not sent.
Not
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