Building Your Own PICAXE Cable by Tommy N. Tyler

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BUILD IT YOURSELF
BUILDING YOUR OWN
PICAXE DOWNLOAD
CABLE
By Tommy N. Tyler
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july2014_Tyler.
■ FIGURE 1.
AXE027 PICAXE
download cable.
PICAXE products offer an attractive alternative to novices who lack experience using
microprocessors and would like to learn more about them without a major
investment in development tools and software training. PICAXE chips are Flashprogrammed PIC processors containing a proprietary interpreter that offers powerful,
easy-to-use, BASIC-like instructions. The chips are inexpensive, and the editing
software for writing and testing the program is a free download. Ample info and tips
are available in forums and published manuals. (And don’t forget about Nuts & Volts
bi-monthly column, the PICAXE Primer!) What else could you want?
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■ FIGURE 2. Download cable using a converter module that can provide inverted RX and TX.
M
any hobbyists are reluctant to venture into
new technology because of startup costs. A
PICAXE provides a low cost way to get
started with microprocessors. There is one
catch, though. You may have assembled your first circuit
on a solderless breadboard using only a few dollars in
parts and written and debugged a program with the fun
and exciting simulator built in to the free programming
editor, only to discover that transferring the program from
your PC into the PICAXE chip required a special USB
download cable that costs $20 to $30. This article
addresses that issue.
The PICAXE system uses a serial comm cable to send
a program from the program editor in a PC directly into
the chip. No other programming equipment is required.
PICAXE's AXE027 download cable has a USB-to-TTL RS232 converter circuit on a small printed circuit board
(PCB), which is embedded within a molded USB Type A
connector at one end and a 3.5 mm miniature stereo plug
at the other end. The PCB is a bit of an overkill, with top,
bottom, and intermediate ground plane layers. Figure 1
shows details of the PCB assembly removed from a
recently acquired AXE027 cable and its schematic. This
assembly is similar to an older design shown in PICAXE
documents, but it has a few minor differences.
The cable uses an FT232Rx series converter — a chip
found in many USB-to-serial converters because of its
versatility, as well as the availability of drivers for a wide
variety of operating systems. One reason PICAXE chose
this chip was because of its compatibility with the
company's original serial comm system. FTDI's converter
chips allow you to invert the TX and RX signals so they are
active low, just like standard RS-232 signals.
When you look at the chip's configuration EEPROM
with FTDI's utility program (FTProg 2.6.8), the descriptive
data for the AXE027 is:
Chip Type: 'FT232R'
Vendor ID: 0403
Product ID: BD90
Product Description: 'AXE027 PICAXE USB'
Under hardware-specific invert RS-232 signals, FTProg
shows that TXD, RXD, RTS#, and CTS# were all inverted.
(Output designations are followed by a # symbol if they
are active low by default.) Note that RTS and CTS are
jumpered together through R1 on the board. Inverting
TXD and RXD is absolutely necessary, but other cables I
have tested seem to work just fine with or without
inversion of RTS# and CTS# — regardless of whether they
are connected. Also, some PICAXE documentation shows
a 100 ohm resistor in series with RX — like R2 in series
with TX — but my purchased sample has no such resistor
nor any place for it.
FTProg also shows that all five CBUS outputs of the
AXE027 cable were reconfigured from their FTDI factory
default configurations to TXDEN. I don't know why that
was done, but it has nothing to do with using the cable to
download PICAXE programs. In summary, there are ways
that PICAXE could have made it difficult to substitute thirdparty download cables, but to their credit there is no
evidence they have attempted to do that.
The requirements for a USB download cable for
PICAXE are few and simple: just three wires — serial in,
serial out, and ground. The key component is a USB-to-RS232 converter chip that can operate at 5 VDC and 4800
baud (the PICAXE comm frequency). There are at least
four major suppliers of these chips worldwide: FTDI,
Prolific, SI Labs, and Microchip; some can provide the
inverted RX/TX configuration. For the others, you can add
a couple of simple inverters. Purchasing a ready-made
converter module or breakout board can save you the
tedious job of soldering the small closely-spaced pins of a
converter chip.
Figure 2 shows a basic download cable circuit using a
module with an FTDI 232 series or Microchip MCP2200
chip, either of which allows you to invert the RX/TX
signals by modifying the internal configuration of the
EEPROM. I've shown a generic three-pin connector rather
than the stereo connector used by PICAXE. The 22K and
10K resistors cannot be part of the cable. They keep the
PICAXE serial input from floating around like an antenna
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■ FIGURE 3. Examples of Prolific and SI Labs USB modules.
when the download cable is disconnected, which could
result in noise pickup that might interfere with program
execution. Their importance is clearly explained in PICAXE
documentation.
If you want your download cable to be really small,
take a look at Jim Paris' FTXUSB serial breakout board
(www.tindie.com/products/jimparis/microftx-usb-serialbreakout-1/?pt=directsearch). At only 0.4" x 0.64" —
including a micro USB B connector — it's the smallest
cable currently being made. It uses a tiny FT230XQ chip
with RX and TX signals that you can invert with FTProg.
The cost is a little over $12 shipped, to which you must
■ FIGURE 4. Download
cable with transistor
inverters.
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July 2014
add a USB micro cable and an output cable.
The next largest category is the CRIUS FTDI basic
breakout board from DealExtreme (www.deal
extreme.com). At only $6.59 (including shipping), this high
quality FTDI board also doesn't need extra inverters. All
you would need to do would be to install input and
output cables and use FTProg to invert RX and TX.
Omega MCU Systems sells a board with an SI Labs
CP2103 chip and a couple of transistor inverters added
(www.onebytecpu.com/usermanuals/u2p_datasheet.pdf),
but it costs about $15 shipped, and you would still need
cables. It's also a little clunky.
You can buy a postage stamp-sized MCP2200
breakout board direct from the manufacturer (Microchip)
or from major electronics suppliers (Digi-Key, Mouser,
Newark, Farnell, etc.) for $15 plus shipping. Microchip
provides an EEPROM configuration utility that is similar to
FTProg that allows you to invert the RX and TX signals. All
you would need to add would be cables.
For the lowest cost download cable, you can use a
USB-to-TTL converter module based on a Prolific PL2303HX chip or an SI Labs CP-2102 or 2103 chip. These
are available from many sources in China, Taiwan, and
Hong Kong. Figure 3 shows some examples.
An easy way to recognize a board using a Prolific or
SI Labs chip is the 12 MHz crystal. (FTDI-based boards
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■ FIGURE 5. Download cable
with DIP IC inverters.
■ FIGURE 6. Download cable
with SMD dual inverter.
use a built-in oscillator, so no external crystal is needed.
The MCP2200 board uses a tiny 12 MHz ceramic
resonator.) Most modules have five output pins (RX, TX,
GND, +3.3V, and +5V), giving you a choice for powering
other circuits. Some boards (like the one on the left) only
have four pins, with a jumper block for selecting Vdd
(+3.3V or +5V).
There is no standard pinout sequence. You can
temporarily power your PICAXE circuit from the module,
but for a final design use something that works when the
download cable is unplugged.
Nearly all modules have one or more LEDs to indicate
power, transmit, receive, etc. All of them use a USB Type
A connector which eliminates the need for a separate USB
cable. My favorite is the board at the far right of Figure 3,
which is supplied with a protective outer wrapper of clear
heat shrink tubing. It uses a Prolific PL-2303HX chip, is
well built, reliable, and has good stable drivers.
If you search Google for CEG004200 or search eBay
for PL-2303HX, you'll find it widely available for as little as
$1 or $2 including shipping. None of these boards have
the capability of inverting RX and TX. You must add a
couple of inverters powered by the module's +5V output.
That's easily done in several ways.
Figure 4 shows how you can use discrete transistors
and resistors for the inverters. Any type of NPN transistors
can be used, and the resistor values are not critical. Note
that the 22K and 10K resistors from Figure 2 are still
necessary to prevent noise on the serial input when the
cable is disconnected.
Transistor inverters are inexpensive and parts are
readily available. However, if you want to simplify your
cable with just one component instead of six, you can use
almost any digital IC with two or more inverters, such as
the 74HCT04 hex inverter or the 74HCT00 quad NAND
shown in Figure 5.
There are many choices of similar ICs in a 14-pin DIP
package with through-hole leads, and many hobbyists
prefer this type of package because it is easy to work with
when using perfboard or a solderless breadboard.
If you can deal with surface-mount components
and want something elegant, a good choice is the
SN74LVC2GU04DBVR in a tiny SOT-23 package with just
six pins as shown in Figure 6.
To package the additional inverter circuitry, attach a
small perfboard extension to the end of the module board
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■ FIGURE 7.
AXE027
schematic.
with epoxy. For connections to the inverters, use tiny #30
gauge wire-wrap wire or strands from stranded hookup
wire. After anchoring a three-conductor cable and testing
the unit, cover the add-on board assembly with epoxy or
heat shrink. To test your completed cable — including the
inverters — jumper pin 1 to pin 2, and then use Windows
HyperTerminal (or any other serial comm utility software)
to perform a loopback test.
If you are using a PICAXE development board, you
can terminate the cable with a standard 3.5 mm stereo
plug or any suitable three-conductor connector. Since
most of the converter modules will operate up to 115 Kb,
you can use your cable for many computer interface
projects other than PICAXE downloading. Most other
applications for a serial comm cable use standard TTL
polarity signals, so you won't need the inverters. NV
Here’s a summary parts list that applies to the simplest versions of the cable. Select the
miscellaneous parts per the version you are building.
Prolific PL-2303-HX Converter Module
2N3904 NPN Transistor
Digi-Key Part Number 2N3904FS-ND
1/8W Resistor 1K
Digi-Key Part Number CF18JT1K00TR-ND
1/8W Resistor 10K
Digi-Key Part Number CF18JT10K0CT-ND
74HCT04 Hex Inverter
Digi-Key Part Number 296-1605-5-ND
74HCT00 Quad NAND
Digi-Key Part Number 296-1603-5-ND
SN74LVC2GU04DBVR Dual Inverter
Digi-Key Part Number 296-13276-1-N
PARTS
LIST
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July 2014
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