W25Q20BW
7.
FUNCTIONAL DESCRIPTION
7.1 SPI OPERATIONS
7.1.1 Standard SPI Instructions
The W25Q20BW is accessed through an SPI compatible bus consisting of four signals: Serial Clock
(CLK), Chip Select (/CS), Serial Data Input (DI) and Serial Data Output (DO). Standard SPI instructions
use the DI input pin to serially write instructions, addresses or data to the device on the rising edge of
CLK. The DO output pin is used to read data or status from the device on the falling edge CLK.
SPI bus operation Modes 0 (0,0) and 3 (1,1) are supported. The primary difference between Mode 0 and
Mode 3 concerns the normal state of the CLK signal when the SPI bus master is in standby and data is
not being transferred to the Serial Flash. For Mode 0 the CLK signal is normally low on the falling and
rising edges of /CS. For Mode 3 the CLK signal is normally high on the falling and rising edges of /CS.
7.1.2 Dual SPI Instructions
The W25Q20BW supports Dual SPI operation when using the “Fast Read Dual Output (3Bh)” and “Fast
Read Dual I/O (BBh)” instructions. These instructions allow data to be transferred to or from the device at
two to three times the rate of ordinary Serial Flash devices. The Dual SPI Read instructions are ideal for
quickly downloading code to RAM upon power-up (code-shadowing) or for executing non-speed-critical
code directly from the SPI bus (XIP). When using Dual SPI instructions, the DI and DO pins become
bidirectional I/O pins: IO0 and IO1.
7.1.3 Quad SPI Instructions
The W25Q20BW supports Quad SPI operation when using the “Fast Read Quad Output (6Bh)”, “Fast
Read Quad I/O (EBh)”, “Word Read Quad I/O (E7h)” and “Octal Word Read Quad I/O (E3h)” instructions.
These instructions allow data to be transferred to or from the device six to eight times the rate of ordinary
Serial Flash. The Quad Read instructions offer a significant improvement in continuous and random
access transfer rates allowing fast code-shadowing to RAM or execution directly from the SPI bus (XIP).
When using Quad SPI instructions the DI and DO pins become bidirectional IO0 and IO1, and the /WP
and /HOLD pins become IO2 and IO3 respectively. Quad SPI instructions require the non-volatile Quad
Enable bit (QE) in Status Register-2 to be set.
7.1.4 Hold Function
For Standard SPI and Dual SPI operations, the /HOLD signal allows the W25Q20BW operation to be
paused while it is actively selected (when /CS is low). The /HOLD function may be useful in cases where
the SPI data and clock signals are shared with other devices. For example, consider if the page buffer
was only partially written when a priority interrupt requires use of the SPI bus. In this case the /HOLD
function can save the state of the instruction and the data in the buffer so programming can resume
where it left off once the bus is available again. The /HOLD function is only available for standard SPI and
Dual SPI operation, not during Quad SPI.
To initiate a /HOLD condition, the device must be selected with /CS low. A /HOLD condition will activate
on the falling edge of the /HOLD signal if the CLK signal is already low. If the CLK is not already low the
/HOLD condition will activate after the next falling edge of CLK. The /HOLD condition will terminate on the
-9-
Publication Release Date: January 25, 2011
Preliminary - Revision B
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