FMS9874 Datasheet, PDF (15/25 Pages) Fairchild Semiconductor – Graphics Digitizer - 3x8-Bit, 108Ms/s Triple Video A/D Converter with Clamps

FMS9874
PRODUCT SPECIFICATION
SDA
SCL
t_BUFF
t_STAH
t_DHO
t_DAL
t_DSU
t_STASU
t_DAH
Figure 18. Serial Bus: Read/Write Timing
t_STOSU
SDA
bit 7
bit 6
bit 5
bit 4
bit 3
bit 2
bit 1
bit 0
ACK
SCL
Figure 19. SerialBus: Typical Byte Transfer
SDA
A6
A5
A4
A3
A2
A1
A0
R/W\
ACK
SCL
Figure 20. Serial Bus: Slave Address with Read/Write Bit
There are five steps within an I²C/SMBus cycle:
1. Start signal
2. Slave address byte
3. Pointer register address byte
4. Data byte to read or write
5. Stop signal
When the Serial Bus interface is inactive, SCL = H and SDA
= H. Communications are initiated by sending a start signal
(Figure 14, left waveform) that is a HIGH-to-LOW transition
on SDA while SCL is HIGH. A start signal alerts all slaved
devices that a data transfer sequence is imminent.
After a start signal, the first eight bits of data comprise a seven
bit slave address followed a single R/W bit (Read = H, Write
= L) to set the direction of data transfer: read from; or write
to the slave device. If the transmitted slave address matches
the address of the FMS9874 which set by the state of the
ADD pin, the FMS9874 acknowledges by pulling SDA
LOW on the 9th SCL pulse (see Figure 16). If the addresses
do not match, the FMS9874 does not acknowledge.
For each byte of data read or written, the MSB is the first bit
of the sequence.
Data Transfer via Serial Interface
If a slave device, such as the FMS9874 does not acknowl-
edge the master device during a write sequence, SDA
remains HIGH so the master can generate a stop signal. Dur-
ing a read sequence, if the master device does not acknowl-
edge by bringing SDA = L, the FMS9874 interprets SDA =
H as “end of data.” SDA remains HIGH so the master can
generate a stop signal.
To write data to a specific FMS9874 control register, three
bytes are sent:
1. Write the slave address byte with bit R/W = L.
2. Write the pointer byte.
3. Write to the control register indexed by the pointer.
After each byte is written, the pointer auto-increments to
allow multiple data byte transfers within one write cycle.
Data is read from the control registers of the FMS9874 in a simi-
lar manner, except that two data transfer operations are required:
1. Write the slave address byte with bit R/W = L.
2. Write the pointer byte.
3. Write the slave address byte with bit R/W = H
4. Read the control register indexed by the pointer.
REV. 1.5 11/10/00
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