FM25C160B
the data bus. A device on the SPI bus is activated using the CS
pin.
Functional Overview
The FM25C160B is a serial F-RAM memory. The memory array
is logically organized as 2,048 × 8 bits and is accessed using an
industry standard serial peripheral interface (SPI) bus. The
functional operation of the F-RAM is similar to serial flash and
serial EEPROMs. The major difference between the
FM25C160B and a serial flash or EEPROM with the same pinout
is the F-RAM's superior write performance, high endurance, and
low power consumption. It also differs from Cypress’s 25160 by
supporting SPI mode 3 and the industry standard 16-bit
addressing protocol. This makes the FM25C160B a drop-in
replacement for most 16-Kbit SPI EEPROMs that support modes
0 & 3.
The relationship between chip select, clock, and data is dictated
by the SPI mode. This device supports SPI modes 0 and 3. In
both of these modes, data is clocked into the F-RAM on the rising
edge of SCK starting from the first rising edge after CS goes
active.
The SPI protocol is controlled by opcodes. These opcodes
specify the commands from the bus master to the slave device.
After CS is activated, the first byte transferred from the bus
master is the opcode. Following the opcode, any addresses and
data are then transferred. The CS must go inactive after an
operation is complete and before a new opcode can be issued.
The commonly used terms in the SPI protocol are as follows:
Memory Architecture
SPI Master
When accessing the FM25C160B, the user addresses 2K
locations of eight data bits each. These eight data bits are shifted
in or out serially. The addresses are accessed using the SPI
protocol, which includes a chip select (to permit multiple devices
on the bus), an opcode, and a two-byte address. The upper 5 bits
of the address range are 'don't care' values. The complete
address of 11 bits specifies each byte address uniquely.
The SPI master device controls the operations on a SPI bus. An
SPI bus may have only one master with one or more slave
devices. All the slaves share the same SPI bus lines and the
master may select any of the slave devices using the CS pin. All
of the operations must be initiated by the master activating a
slave device by pulling the CS pin of the slave LOW. The master
also generates the SCK and all the data transmission on SI and
SO lines are synchronized with this clock.
Most functions of the FM25C160B are either controlled by the
SPI interface or handled by on-board circuitry. The access time
for the memory operation is essentially zero, beyond the time
needed for the serial protocol. That is, the memory is read or
written at the speed of the SPI bus. Unlike a serial flash or
EEPROM, it is not necessary to poll the device for a ready
condition because writes occur at bus speed. By the time a new
bus transaction can be shifted into the device, a write operation
is complete. This is explained in more detail in the interface
section.
SPI Slave
The SPI slave device is activated by the master through the Chip
Select line. A slave device gets the SCK as an input from the SPI
master and all the communication is synchronized with this
clock. An SPI slave never initiates a communication on the SPI
bus and acts only on the instruction from the master.
The FM25C160B operates as an SPI slave and may share the
SPI bus with other SPI slave devices.
Note The FM25C160B contains no power management circuits
other than a simple internal power-on reset circuit. It is the user’s
responsibility to ensure that VDD is within datasheet tolerances
to prevent incorrect operation. It is recommended that the part is
not powered down with chip enable active.
Chip Select (CS)
To select any slave device, the master needs to pull down the
corresponding CS pin. Any instruction can be issued to a slave
device only while the CS pin is LOW. When the device is not
selected, data through the SI pin is ignored and the serial output
pin (SO) remains in a high-impedance state.
Serial Peripheral Interface – SPI Bus
Note A new instruction must begin with the falling edge of CS.
Therefore, only one opcode can be issued for each active Chip
Select cycle.
The FM25C160B is a SPI slave device and operates at speeds
up to 15 MHz. This high-speed serial bus provides
high-performance serial communication to a SPI master. Many
common microcontrollers have hardware SPI ports allowing a
direct interface. It is quite simple to emulate the port using
ordinary port pins for microcontrollers that do not. The
FM25C160B operates in SPI Mode 0 and 3.
Serial Clock (SCK)
The Serial Clock is generated by the SPI master and the
communication is synchronized with this clock after CS goes
LOW.
SPI Overview
The FM25C160B enables SPI modes 0 and 3 for data
communication. In both of these modes, the inputs are latched
by the slave device on the rising edge of SCK and outputs are
issued on the falling edge. Therefore, the first rising edge of SCK
signifies the arrival of the first bit (MSB) of a SPI instruction on
the SI pin. Further, all data inputs and outputs are synchronized
with SCK.
The SPI is a four-pin interface with Chip Select (CS), Serial Input
(SI), Serial Output (SO), and Serial Clock (SCK) pins.
The SPI is a synchronous serial interface, which uses clock and
data pins for memory access and supports multiple devices on
Document Number: 001-86150 Rev. *C
Page 4 of 20
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