Many 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 RS-
232 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 third-party 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-RS-
232 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
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
■ FIGURE 2. Download cable using a converter module that can provide inverted RX and TX.
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