FM33256B SPI Companion w/ FRAM
Pin Configuration
Pin Name
/CS
Function
Chip Select
VDD
ACS
SCK
SI
CS
SO
1
2
14
13
SCK
SI
SO
Serial Clock
Serial Data Input
Serial Data Output
Power Fail Input
Power Fail Output (NMI)
Event Counter Input
Alarm/Calibration/SqWave
Reset Input/Output
External Crystal Connections
(optional)
3
4
12
11
CNT
VBAK
X2
PFI
PFO
CNT
ACS
/RST
X1, X2
5
6
7
10
9
PFO
RST
PFI
X1
8
VSS
VDD
VBAK
VSS
Supply Voltage
Battery-Backup Supply
Ground
Pin Descriptions
Pin Name
Type
Pin Description
/CS
Input
Chip Select: This active low input activates the device. When high, the device enters low-
power standby mode, ignores the SCK and SI inputs, and the SO output is tri-stated.
When low, the device internally activates the SCK signal. A falling edge on /CS must
occur prior to every op-code.
SCK
SI
Input
Input
Serial Clock: All I/O activity is synchronized to the serial clock. Inputs are latched on the
rising edge and outputs occur on the falling edge. Since the device is static, the clock
frequency may be any value between 0 and 16 MHz and may be interrupted at any time.
Serial Input: All data is input to the device on this pin. The pin is sampled on the rising
edge of SCK and is ignored at other times. It should always be driven to a valid logic level
to meet IDD specifications. The SI pin may be connected to SO for a single pin data
interface.
SO
Output Serial Output: This is the data output pin. It is driven during a read and remains tri-stated
at all other times. Data transitions are driven on the falling edge of the serial clock. The
SO pin may be connected to SI for a single pin data interface.
CNT
ACS
Input
Event Counter Input: This input increments the counter when an edge is detected on this
pin. The polarity is programmable and the counter value is nonvolatile or battery-backed,
depending on the mode. This pin should be tied to ground if unused.
Output Alarm/Calibration/SquareWave: This is an open-drain output that requires an external
pullup resistor. In normal operation, this pin acts as the active-low alarm output. In
Calibration mode, a 512 Hz square-wave is driven out. In SquareWave mode, the user
may select a frequency of 1, 512, 4096, or 32768 Hz to be used as a continuous output.
The SquareWave mode is entered by clearing the AL/SW and CAL bits in register 18h.
X1, X2
/RST
PFI
I/O
32.768 kHz crystal connection (see Crystal Type section for suggestions). See AN407
app. note for details on how to connect external oscillator.
Reset: This active-low output is open drain with weak pull-up. It is also an input when
used as a manual reset. This pin should be left floating if unused.
Early Power-fail Input: Typically connected to an unregulated power supply to detect an
early power failure. This pin must be tied to ground if unused.
I/O
Input
PFO
Output Early Power-fail Output: This pin is the early power-fail output and is typically used to
drive a microcontroller NMI pin. PFO drives low when the PFI voltage is <1.5V.
Supply Backup supply voltage: A 3V battery or a large value capacitor. If no backup supply is
used, this pin should be tied to VSS and the VBC bit should be cleared. The trickle charger
is UL recognized and ensures no excessive current when using a lithium battery.
Supply Supply Voltage.
VBAK
VDD
VSS
Supply Ground
This product conforms to specifications per the terms of the Ramtron
standard warranty. The product has completed Ramtron’s internal
qualification testing and has reached production status.
Ramtron International Corporation
1850 Ramtron Drive, Colorado Springs, CO 80921
(800) 545-FRAM, (719) 481-7000
www.ramtron.com
Rev. 3.0
Aug. 2012
Page 2 of 28
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
