FM25CL64 - Automotive Temp.
Overview
Serial Peripheral Interface – SPI Bus
The FM25CL64 is a serial FRAM memory. The
memory array is logically organized as 8,192 x 8 and
is accessed using an industry standard Serial
Peripheral Interface or SPI bus. Functional operation
of the FRAM is similar to serial EEPROMs. The
major difference between the FM25CL64 and a serial
EEPROM with the same pinout is the FRAM’s
superior write performance.
The FM25CL64 employs
a
Serial Peripheral
Interface (SPI) bus. It is specified to operate at speeds
up to 16 MHz. This high-speed serial bus provides
high performance serial communication to a host
microcontroller. 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
FM25CL64 operates in SPI Mode 0 and 3.
Memory Architecture
The SPI interface uses a total of four pins: clock,
data-in, data-out, and chip select. It is possible to
connect the two data pins together. Figure 2
illustrates a typical system configuration using the
FM25CL64 with a microcontroller that offers an SPI
port. Figure 3 shows a similar configuration for a
microcontroller that has no hardware support for the
SPI bus.
When accessing the FM25CL64, the user addresses
8,192 locations of 8 data bits each. These data bits
are shifted serially. The addresses are accessed using
the SPI protocol, which includes a chip select (to
permit multiple devices on the bus), an op-code, and
a two-byte address. The upper 3 bits of the address
range are ‘don’t care’ values. The complete address
of 13-bits specifies each byte address uniquely.
Protocol Overview
Most functions of the FM25CL64 either are
controlled by the SPI interface or are handled
automatically by on-board circuitry. The access time
for 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 an EEPROM, it is not necessary to poll the
device for a ready condition since writes occur at bus
speed. So, by the time a new bus transaction can be
shifted into the device, a write operation will be
complete. This is explained in more detail in the
interface section.
The SPI interface is a synchronous serial interface
using clock and data pins. It is intended to support
multiple devices on the bus. Each device is activated
using a chip select. Once chip select is activated by
the bus master, the FM25CL64 will begin monitoring
the clock and data lines. The relationship between the
falling edge of /CS, the clock and data is dictated by
the SPI mode. The device will make a determination
of the SPI mode on the falling edge of each chip
select. While there are four such modes, the
FM25CL64 supports modes 0 and 3. Figure 4 shows
the required signal relationships for modes 0 and 3.
For both modes, data is clocked into the FM25CL64
on the rising edge of SCK and data is expected on the
first rising edge after /CS goes active. If the clock
begins from a high state, it will fall prior to beginning
data transfer in order to create the first rising edge.
Users expect several obvious system benefits from
the FM25CL64 due to its fast write cycle and high
endurance as compared with EEPROM. In addition
there are less obvious benefits as well. For example
in a high noise environment, the fast-write operation
is less susceptible to corruption than an EEPROM
since it is completed quickly. By contrast, an
EEPROM requiring milliseconds to write is
vulnerable to noise during much of the cycle.
The SPI protocol is controlled by op-codes. These
op-codes specify the commands to the device. After
/CS is activated the first byte transferred from the bus
master is the op-code. Following the op-code, any
addresses and data are then transferred. Note that the
WREN and WRDI op-codes are commands with no
subsequent data transfer.
Note that the FM25CL64 contains no power
management circuits other than a simple internal
power-on reset. It is the user’s responsibility to
ensure that VDD is within datasheet tolerances to
prevent incorrect operation.
Important: The /CS pin must go inactive after an
operation is complete and before a new op-code
can be issued. There is one valid op-code only per
active chip select.
Rev. 3.0
May 2007
Page 3 of 13
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
