PICAXE OTA Programmer

Making a PICAXE OTA Controller

Michael Shaw, May 2019

In this article I’ll show you how to make a controller using the PICAXE chip that can accept over the air (OTA) program updates. This idea came to me after I mounted the weather station I designed onto the roof of our house.

Figure 1 Weather Station mounted on the peak of our house

What if I discovered a problem with the weather station program and wanted to make changes? What if I wanted to make improvements to the code over time? Do I have to go back up on the roof to reprogram it?

I use PICAXE chips in my projects because they are cheap and easy to use, have a great software platform and are very reliable. There’s just one problem, no one knows how the chip programming (except PICAXE) works. With this system that’s no problem.  We’ll still use the free PICAXE Programming Editor IDE, but updates will be transmitted to the remote unit through the air using a pair of XBEE wireless transceivers, no cables necessary.

The PICAXE system stores programs using what is described as tokenized storage in memory. When you try to print out or decipher a PICAXE program, it’s just gibberish and cannot be understood. Believe me, I’ve tried. But you don’t need to know the meaning behind each of the bytes, just transfer the whole program to the remote unit and reboot the chip!  This system requires the use of two PICAXE controllers, one acting as the base unit and the other as the remote unit. The base unit is used to capture the PICAXE program into an onboard Maxim 24LC256 256K EEPROM memory and then transmits that program verbatim to the remote unit’s onboard 24LC256 EEPROM memory using XBEE wireless transceivers. Then a command is sent to the remote unit to reboot the chip from the remote’s 24LC256 EEPROM onboard memory. 

This system does not work with just any PICAXE chips, however. The chips I use are the PICAXE 20X2 which is a 20 pin PICAXE chip. The reason I use these is because they support the ability to boot the chip from an external EEPROM address using the BOOTI2C command and also supports the #SLOT number software compiler directive which works with the PICAXE Programming Editor Version 5.5.1. So here is a simplified diagram of the system I use to perform OTA program updates to my PICAXE weather station:

Figure 2 Block Diagram of OTA Programming System

Create the PICAXE remote program the way you normally would do using the PICAXE Programming Editor. In the programming directives area place these lines:

#revision 3                        ' increment this for remote updates

'#slot 4                              ' uncomment this when loading into base EEPROM

The first line keeps track of the remote program’s revision number. This is important because when the BOOTI2C command is executed, it looks at the revision number of the program in the 24LC256 EEPROM. If the program revision number is higher than the current program revision number, then it will copy the program from the 24LC256 EEPROM memory into the PICAXE internal memory and reboot the chip.  

The second line is used to direct the program download of the remote program into the base unit’s onboard 24LC256 EEPROM memory. This directive targets the i2c program slot #4 used by X2 parts. When a BOOTI2C 4 command is executed it will copy the contents from the 24LC256 EEPROM memory and load it into internal memory slot 0.

As mentioned earlier, this technique is being used with a solar powered weather station mounted on the roof of the house. The weather station uses the PICAXE chip to collect weather data such as wind speed, outdoor temperature, rain, lightning, light and solar panel and battery status. It transmits this stream of data using an XBEE transceiver to a unit located inside the house once every minute. Because the XBEE transceiver doesn’t need to be on all the time, only when sending the weather data, it is turned off when not in use using a solid state relay. This greatly increases the battery life for this solar powered weather station but presents a problem, though, when you want to send control messages to it. So at the end of every transmitted message, it waits for a few seconds for a command mode message. If it gets the message the XBEE transceiver remains on and commands can be sent to control the weather station or upload a new program.

 After the remote unit is placed in the command mode the upload process begins with the base unit sending the upload command. The remote unit responds by sending an acknowledgment and then waiting for the first 2 bytes from the base unit. After the first bytes are received, the remote again sends and acknowledgement and the process is repeated. If the remote unit fails to receive data from the base unit, it will wait for a specified period of time and then abort the upload. The transfer file size is set to 2000 bytes and the whole file is sent even though the program size (should be) less than that. The reason is there is no way to tell (at least at this moment) where the PICAXE program ends, so a definitive file size is set to insure everything is transferred correctly. To keep it simple, the only error checking is a checksum that the base and remote units calculate. If the checksums match at the end of the transfer then it’s safe to assume that the file was transferred successfully.

Figure 3 PICAXE Terminal showing Successful File Upload

Once the file has been uploaded then a “B” or boot command is sent to the remote which tells the remote unit to execute the BOOTI2C 4 command. This command copies the contents from the 24LC256 EEPROM memory and loads it into the PICAXE internal memory slot 0 and resets the PICAXE, provided that the program revision number is higher than the current program revision number. Otherwise the command will be ignored. 

     Figure 4 Inside the Base Unit

Figure 5 Inside the Weather Station Remote unit

This system works well as long as there is not a lot of electrical noise in the vicinity and  limiting the distance between the remote and base. Testing with other transceivers has not been performed, but could improve performance. The program was written using the PICAXE Programming Editor, Version 5.5.1 which supports downloading to slot 4 for the PICAXE 20X2 chip. 

 Figure 6 Base Unit Code Transmit Function

Figure 7 Remote Unit Code Receive Function