4.1.3.4 Data Output Bit Frame

A data output bit frame is used when the host is to receive communication back from the AT21CS01/AT21CS11. Data output bit frames are used when reading any data out as well as any ACK or NACK responses from the device. Just as in the input bit frame, the host initiates the sequence by driving the SI/O line below the V_IL threshold, which engages the internal timing generation circuit of the AT21CS01/AT21CS11.

Within the output bit frame is the critical timing parameter, t_RD, which is defined as the amount of time the host must continue to drive the SI/O line low after crossing the V_IL threshold to request a data bit from the AT21CS01/AT21CS11. Once the t_RD duration has expired, the host must release the SI/O line.

If the AT21CS01/AT21CS11 responds with a logic ‘0’ (for either a ‘0’ data bit or an ACK response), it will begin to pull the SI/O line low concurrently during the t_RD window and continue to hold it low for a duration of t_HLD0, after which it will release the line to be pulled back up to V_PUP (see Figure 4-6). Thus, when the host samples SI/O within the t_MRS window, it will see a voltage less than V_IL and decode this event as a logic ’0’. By definition, the t_HLD0 time is longer than the t_MRS time and therefore, the host is ensured to sample while the AT21CS01/AT21CS11 is still driving the SI/O line low.

If the AT21CS01/AT21CS11 intends to respond with a logic ‘1’ (for either a ‘1’ data bit or a NACK response), it will not drive the SI/O line low at all. Once the host releases the SI/O line after the maximum t_RD has elapsed, the line will be pulled up to V_PUP. Thus, when the host samples the SI/O line within the t_MRS window, it will detect a voltage greater than V_IH and decode this event as a logic ‘1’.

The data output bit frame is shown in greater detail in Figure 4-7.