7.5.2 Data Rates

The two-wire bus operates in one of three speed modes:

• Standard: allows a clock frequency of up to 100kHz;
• Fast: allows a clock frequency of up to 400kHz; and
• High-speed mode (also called Hs mode): allows a clock frequency of up to 3.4MHz.

The BUF20800-Q1 is fully compatible with all three modes. No special action is required to use the device in Standard or Fast modes, but High-speed mode must be activated. To activate High-speed mode, send a special address byte of 00001xxx, with SCL = 400kHz, following the START condition; xxx are bits unique to the Hs-capable master, which can be any value. This byte is called the Hs master code. (Note that this is different from normal address bytes—the low bit does not indicate read/write status.) The BUF20800-Q1 will respond to the High-speed command regardless of the value of these last three bits. The BUF20800-Q1 will not acknowledge this byte; the communication protocol prohibits acknowledgment of the Hs master code. On receiving a master code, the BUF20800-Q1 will switch on its Hs mode filters, and communicate at up to 3.4MHz. Additional high-speed transfers may be initiated without resending the Hs mode byte by generating a repeat START without a STOP. The BUF20800-Q1 will switch out of Hs mode with the next STOP condition.