4.1.3 Data Input and Output Bit Frames

Communication with the AT21CS01/AT21CS11 is conducted in time intervals known as bit frames, each lasting t_BIT in duration. Each bit frame contains a single binary data value. Input bit frames are used to transmit data from the host to the AT21CS01/AT21CS11 and can either be a logic ‘0’ or a logic ‘1’. An output bit frame carries data from the AT21CS01/AT21CS11 to the host. In all input and output cases, the host initiates the bit frame by driving the SI/O line low. Once the AT21CS01/AT21CS11 detects the SI/O being driven below the V_IL level, its internal timing circuits begin to run.

The duration of each bit frame is allowed to vary from bit to bit as long as the variation does not cause the t_BIT length to exceed the specified minimum and maximum values (see AT21CS01/AT21CS11 AC Characteristics). The t_BIT requirements will vary depending on whether the device is set for Standard Speed or High-Speed mode. For more information about setting the speed of the device, refer to Setting the Device Speed.