M45PE16 datasheet(10/45 Pages) STMICROELECTRONICS

10 / 45 page

SPI modes

M45PE16

10/45

SPI modes

These devices can be driven by a microcontroller with its SPI peripheral running in either of

the two following modes:

●

CPOL=0, CPHA=0

●

CPOL=1, CPHA=1

For these two modes, input data is latched in on the rising edge of Serial Clock (C), and

output data is available from the falling edge of Serial Clock (C).

The difference between the two modes, as shown in Figure 4, is the clock polarity when the

bus master is in Stand-by mode and not transferring data:

●

C remains at 0 for (CPOL=0, CPHA=0)

●

C remains at 1 for (CPOL=1, CPHA=1)

Figure 3.

Bus master and memory devices on the SPI bus

The Write Protect (W) and Hold (HOLD) signals should be driven, High or Low as appropriate.

Figure 3 shows an example of three devices connected to an MCU, on an SPI bus. Only one

device is selected at a time, so only one device drives the Serial Data Output (Q) line at a

time, the other devices are high impedance.

The pull-up resistor R (represented in Figure 3) ensures that no device is selected if the Bus

Master leaves the S line in the high impedance state.

In applications where the Bus Master might enter a state where all inputs/outputs SPI lines

are in high impedance at the same time (for example, if the Bus Master is reset during the

transmission of an instruction), the clock line (C) must be connected to an external pull-

down resistor so that, if all inputs/outputs become high impedance, the C line is pulled Low

(while the S line is pulled High). This ensures that S and C do not become High at the same

time, and so, that the tSHCH requirement is met.

AI12836b

SPI Bus Master

SPI Memory

Device

SDO

SDI

SCK

CQD

SPI Memory

Device

CQD

SPI Memory

Device

CQD

CS3

CS2 CS1

SPI Interface with

(CPOL, CPHA) =

(0, 0) or (1, 1)

HOLD

VCC

VSS

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