FM25Q32
11.2.11 Fast Read Dual I/O (BBh)
The Fast Read Dual I/O (BBh) instruction allows for improved random access while maintaining
two IO pins, IO0 and IO1. It is similar to the Fast Read Output (0Bh) instruction but with the
capability to input the Address bits (A23-0) two bits and output data two bits per clock. This
reduced instruction overhead may allow for code execution (XIP) directly from the Dual SPI in
some applications.
The Fast Read Dual I/O instruction can further reduce instruction overhead through setting the
Mode bits (M7-0) after the input Address bits (A23-0), as shown in figure 11a. The upper nibble of
the Mode (M7-4) controls the length of the next Fast Read Dual I/O instruction through the
instruction or exclusion of the first byte instruction code. The lower nibble bits of the Mode (M3-0)
are don’t care (“X”), However, the IO pins should be high-impedance prior to the falling edge of the
first data out clock.
If the Mode bits (M7-0) equal “Ax” hex, then the next Fast Dual I/O instruction (after /CS is raised
and then lowered) does not require the BBh instruction code, as shown in figure 11b. This reduces
the instruction sequence by eight clocks and allows the address to be immediately entered after
/CS is asserted low. If Mode bits (M7-0) are any value other “Ax” hex, the next instruction (after /CS
is raised and then lowered) requires the first byte instruction code, thus returning to normal
operation. A Mode Bit Reset instruction can be used to reset Mode Bits (M7-0) before issuing
normal instructions (See 11.2.25 for detailed descriptions).
Figure 11a. Fast Read Dual Input/Output Instruction Sequence Diagram (M7-0 = 0xh or NOT Axh)
preliminary(Aug.18.2010) 25