OPTION_REG datasheet(157/222 Pages) MICROCHIP

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DS22331A-page 157

MCP19111

27.4.4

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, and an ACK pulse is

sent by the slave on the ninth bit.

Following the ACK, slave hardware clears the CKP bit

and the SCL pin is held low (see Section 27.4.7

“Clock Stretching” for more detail). By stretching the

clock, the master will be unable to assert another clock

pulse until the slave is done preparing the transmit

data.

The transmit data must be loaded into the SSPBUF

SCL pin should be released by setting the CKP bit of

the SSPCON1 register. The eight data bits are shifted

out on the falling edge of the SCL input. This ensures

that the SDA signal is valid during the SCL high time.

The ACK pulse from the master-receiver is latched on

the rising edge of the ninth SCL input pulse. This ACK

value is copied to the ACKSTAT bit of the SSPCON2

transfer is complete. In this case, when the not ACK is

latched by the slave, the slave goes Idle and waits for

another occurrence of the Start bit. If the SDA line was

low (ACK), the next transmit data must be loaded into

the SSPBUF register. Again, the SCL pin must be

released by setting bit CKP.

An MSSP interrupt is generated for each data transfer

byte. The SSPIF bit must be cleared by software and

the SSPSTAT register is used to determine the status

of the byte. The SSPIF bit is set on the falling edge of

the ninth clock pulse.

27.4.4.1

Slave Mode Bus Collision

A slave receives a Read request and begins shifting

data out on the SDA line. If a bus collision is detected

and the SBCDE bit of the SSPCON3 register is set, the

BCLIF bit of the PIR register is set. Once a bus collision

is detected, the slave goes Idle and waits to be

addressed again. User software can use the BCLIF bit

to handle a slave bus collision.

27.4.4.2

7-bit Transmission

A master device can transmit a read request to a

slave, and then clock data out of the slave. The list

below outlines what software for a slave will need to

accomplish

standard

transmission.

Figure 27-10 can be used as a reference to this list.

Master sends a Start condition on SDA and

SCL.

S bit of SSPSTAT is set; SSPIF is set if interrupt

on Start detect is enabled.

Matching address with R/W bit set is received by

the Slave setting SSPIF bit.

Slave hardware generates an ACK and sets

SSPIF.

SSPIF bit is cleared by user.

Software reads the received address from SSP-

BUF, clearing BF.

R/W is set so CKP was automatically cleared

after the ACK.

The slave software loads the transmit data into

SSPBUF.

CKP bit is set releasing SCL, allowing the

master to clock the data out of the slave.

10. SSPIF is set after the ACK response from the

master is loaded into the ACKSTAT register.

11. SSPIF bit is cleared.

12. The slave software checks the ACKSTAT bit to

see if the master wants to clock out more data.

13. Steps 9-13 are repeated for each transmitted

byte.

14. If the master sends a not ACK; the clock is not

held, but SSPIF is still set.

15. The master sends a Restart condition or a Stop.

16. The slave is no longer addressed.

Note 1:If the master ACKs the clock will be

stretched.

2: ACKSTAT is the only bit updated on the

rising edge of SCL (9th) rather than the

falling.

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