that are sold pre-programmed, with a
bootstrap loader, by Revolution
Education Ltd (REL). This proprietary
code allows a PICAXE to be repeatedly re-programmed in Basic using a
PC-based editor (download it free at,
The REL version of Basic supports
many dedicated interface commands
(readadc, serout, writei2c, and
readi2c, etc.) that greatly simplify the
development of sensor interface software. The PICAXE-18X can execute
approximately 20,000 Basic commands a second and store about 600
lines of code onboard internal Flash
memory. In-circuit programming is
normally performed using a simple
three-wire interface to a PC serial port.
A Ramtron FM24CL256 non-volatile FRAM device is used to store
flight data. The low power CL version
of this 32,768 x 8 bit memory
operates over a supply range of 2.7V-
3.6V and has an I2C interface that can
operate at a maximum bus frequency
of 1 MHz. The flight recorder can
write data to memory at a rate of over
500 samples per second, continuously for about a minute.
This performance would normally
be sufficient for observing a complete
model rocket flight from lift-off to
touch-down. Stored data can be downloaded to a PC at 9.6K baud via a two-pole jack socket (J1). Because the flight
recorder operates from a 3V supply, a
transistor (Q1) is included to increase
the serial output level above the
RS-232 threshold. This is achieved by
powering the collector pull-up resistor
(R4) from an external 9V battery supply.
An important consideration when
designing a lightweight rocketry payload is the choice of power source.
For this application, a small PCB-mounted 15 mAh battery proved
more than capable of supporting a
number of flights before re-charging.
At the flight recorder’s modest 5 mA
consumption, the voltage from the
Ni-MH battery remains flat over most
BY MIKE BESSANT
Ferroelectric Random Access
Memory (FRAM) chips are pin compatible with their more established
EEPROM and Flash counterparts,
but use a completely different type of
non-volatile storage technology that
offers a number of operational advantages. These include an increased
( 10 billion) read/write cycle life,
together with higher write speeds and
noise immunity. Compared with
using EEPROM or Flash storage, the
amount of software code needed to
interface with a FRAM device is less
because data does not have to be
formatted into blocks before writing.
FRAM can be used as simply as
non-volatile SRAM but without the
complication of battery back-up.
Ramtron International Corp. (www.
ramtron.com) offers a wide range of
parallel and serial FRAM products.
A leading example is the eight-pin
FM25L512 that has storage capacity
of 512Kb and an SPI bus speed of
■ FIGURE 2.
August 2007 39