FM33256/FM3316 SPI Companion w/ FRAM
timer, 16-bit nonvolatile event counter,
comparator for early power-fail detection or other
purposes, and a 64-bit serial number.
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Overview
The FM33xx devices combine a serial nonvolatile
RAM with a real-time clock (RTC) and a processor
companion. The companion is a highly integrated
peripheral including a processor supervisor, analog
comparator, a nonvolatile counter, and a serial
Processor Supervisor
Supervisors provide a host processor two basic
functions: Detection of power supply fault conditions
and a watchdog timer to escape a software lockup
condition. Both FM33xx devices have a reset pin
(/RST) to drive a processor reset input during power
faults, power-up, and software lockups. It is an open
number.
The
FM33xx
integrates
these
complementary but distinct functions under
a
common interface in a single package. Although
monolithic, the product is organized as two logical
devices. The first is a memory and the second is the
companion which includes all the remaining
functions. From the system perspective they appear
to be two separate devices with unique op-codes on
the serial bus.
drain output with a weak internal pull-up to VDD
.
This allows other reset sources to be wire-OR’d to
the /RST pin. When VDD is above the programmed
trip point, /RST output is pulled weakly to VDD. If
VDD drops below the reset trip point voltage level
(VTP), the /RST pin will be driven low. It will remain
low until VDD falls too low for circuit operation
which is the VRST level. When VDD rises again above
VTP, /RST continues to drive low for at least 50 ms
(tRPU) to ensure a robust system reset at a reliable VDD
level. After tRPU has been met, the /RST pin will
return to the weak high state. While /RST is asserted,
serial bus activity is locked out even if a transaction
occurred as VDD dropped below VTP. A memory
operation started while VDD is above VTP will be
completed internally.
The memory is organized as a standalone nonvolatile
SPI memory using standard op-codes. The real-time
clock and supervisor functions are accessed under
their own op-codes. The clock and supervisor
functions are controlled by 30 special function
registers. The RTC/alarm and some control registers
are maintained by the power source on the VBAK
pin, allowing them to operate from battery or backup
capacitor power when VDD drops below a set
threshold. Each functional block is described below.
Memory Operation
Table 1 below shows how bits VTP(1:0) control the
trip point of the low-VDD reset. They are located in
register 18h, bits 0 and 1. The reset pin will drive
low when VDD is below the selected VTP voltage, and
the SPI interface and FRAM array will be locked out.
Figure 2 illustrates the reset operation in response to
The FM33xx products are available in memory sizes
of 16Kb and 256Kb. The two devices are software
compatible; that is, both versions use consistent two-
byte addressing for the memory device. This makes
both devices the same as its standalone memory
counterparts, such as the FM25L16.
a low VDD
.
Table 1.
VTP Setting VTP1 VTP0
Memory is organized in bytes, for example the
256Kb memory is 32,768 x 8. The memory is based
on FRAM technology. Therefore it can be treated as
RAM and is read or written at the speed of the SPI
bus with no delays for write operations. It also offers
effectively unlimited write endurance unlike other
nonvolatile memory technologies. The SPI protocol
is described on page 18.
2.6V
0
0
1
1
0
1
0
1
2.75V
2.9V
3.0V
VDD
VTP
The memory array can be write-protected by
software. Two bits in the Status Register control the
protection setting. Based on the setting, the protected
addresses cannot be written. The Status Register &
Write Protection is described in more detail on page
20.
tRPU
RST
Figure 2. Low VDD Reset
Processor Companion
A watchdog timer can also be used to drive an active
reset signal. The watchdog is free-running
programmable timer. The timeout period can be
software programmed from 60 ms to 1.8 seconds in
In addition to nonvolatile RAM, the FM33xx devices
incorporate a real-time clock with alarm and highly
integrated processor companion. The companion
includes a low-VDD reset, a programmable watchdog
a
Rev. 1.0
Dec. 2006
Page 4 of 28
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
