Rastergraf Topaz, User Manual

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Rastergraf
5-20 Programming On-board Devices and Memories
5.6 Talk To Me Through I
2
C
The SM731 chip has a control register that is used to implement the I
2
C
protocol, a 2 wire serial bus designed by Philips Semiconductor. The
SM731 is the I
2
C master and it controls the bus through the DDC control
register in the SM731 chip. The I
2
C bus supports specific “start”, “stop”
and “acknowledge” states, so it is possible to probe for these devices and
determine whether they exist.
I
2
C is used to control the following devices:
CY22150 clock (Topaz, Garnet, Duros)
THC63DV164 DVI digital video encoder/transmitter,
LM75 thermal sensor,
AT24C02 2 Kbit serial EEPROM, and
the Display Monitor,
An I
2
C device is determined by a combination of device internal bits (bits
4-7) and (usually) three pins that are wired by the board designer (bits 1-3)
Bit 0 is used to denote a Read (1) or Write (0) operation.
The LM75 must be read in 2 byte increments, otherwise it will hang the
I
2
C bus. Since most vendors combine the R/W bit with the actual I
2
C
address (e.g., write @ 0x88, read @ 0x89), the following table uses that
convention.
Table 5-3 I
2
C Device Addresses
Device R/W Hex I2C Bus
W 0x68 1
CY22150
Reference Clock
R 0x69 1
W 0x70 1
THC63DV164
R 0x71 1
W 0x9C 1
LM75
R 0x9D 1
W 0xA8 1
AT24C02
R 0xA9 1
W 0xA8 0
Display Monitor
R 0xA9 0