Ramtron
FM25C160
Overview
Serial Peripheral Interface – SPI Bus
The FM25C160 is a serial FRAM memory. The
memory array is logically organized as 2,048 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 FM25C160 and a serial
EEPROM with the same pin-out relates to its
superior write performance. It also differs from
Ramtron’s 25160 by supporting SPI mode 3 and the
industry standard 16-bit addressing protocol. This
makes the FM25C160 a drop-in replacement for most
16Kb SPI EEPROMs that support modes 0 & 3.
The FM25C160 employs a Serial Peripheral Interface
(SPI) bus. It is specified to operate at speeds up to 5
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
FM25C160 operates in SPI Mode 0 and 3.
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 lines together. Figure 2
illustrates a typical system configuration using the
FM25C160 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.
Memory Architecture
When accessing the FM25C160, the user addresses
2,048 locations each with 8 data bits. 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 5 bits of the address
range are ‘don’t care’ values. The complete address
of 11-bits specifies each byte address uniquely.
Protocol Overview
The SPI interface is a synchronous serial interface
using clock and data lines. 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 FM25C160 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
FM25C160 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 FM25C160
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.
Most functions of the FM25C160 either are
controlled by the SPI interface or are handled
automatically by on-board circuitry. The access time
for memory operation essentially is 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. That is, by the time a new bus transaction can
be shifted into the part, a write operation will be
complete. This is explained in more detail in the
interface section below.
Users expect several obvious system benefits from
the FM25C160 due to its fast write cycle and high
endurance as compared with EEPROM. However
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 nois e during much of the cycle.
The SPI protocol is controlled by op-codes. These
op-codes specify the commands to the part. 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.
Certain op-codes are commands with no subsequent
data transfer. The /CS 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.
Note that the FM25C160 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 data sheet tolerances to
prevent incorrect operation.
23 October 2000
3/14
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