Last week we released the #twatch v2, an ethernet backpack for 20×4 character LCDs. This wasn’t the only version of the #twatch that we designed. V2 is based on the slick, inexpensive, single-chip ethernet microcontroller, the PIC 18F67J60. The biggest downside to the 18f67J60 is that it can only be programmed an average of 100 times. The limited program cycles make it really difficult to develop the firmware unless you’re a programming genie.
#twatch v1 is a development board for the #twatch firmware. It uses a PIC 24FJ64GA002 microcontoller with a million potential programing cycles, and an ENC28J60 for ethernet access. The design isn’t as advanced as v2, but it’s key to developing the #twatch firmware. We also use the v1 #twatch to diagnose new bugs, like the recent change in Twitter JSON.
You can buy an assembled #twatch v2 PCB for $30, including worldwide shipping. Keep reading for more about the v1 design.
#twatch v1 uses a 28pin SOIC PIC24FJ64GA002. This is one of our favorite chips, you might recognize it from the Bus Pirate. These 16bit chips are really easy to work with, and they have a ton of RAM for storing Tweets. Most importantly, it should last through a million program cycles, so it won’t burn out during development like the 18F67J60.
Since the 24FJ64GA002 doesn’t have a built-in ethernet transceiver, we used the 28pin SOIC ENC28J60 ethernet controller.
The 24FJ64GA002 doesn’t have enough 5volt tolerant pins to operate the LCD in 8bit parallel mode, so we used the 4bit mode instead. Most LCDs have a 4bit mode, but it cuts the refresh rate in half and makes the scrolling seem a bit jerky.
The LCD screen and ethernet transceiver can use a lot of current. Depending on the power supply, the voltage regulators may have to dissipate a bunch of heat. The v1 #twatch used surface mount SOT-223 regulators, but we decided to use larger TO-220 regulators on v2. Additionally, there’s some really poor routing and sizing of power traces on the v1 PCB.
V1 (image) and v1b (rendering) are slightly different versions of the same board. V1 uses a 10mm through-hole potentiometer, v1b uses a small 3mm surface mount version. V1b has cleaner routing around the backlight control, and uses a larger 1206 current-limiting resistor.
The PCBs and firmware for #twatch v1 are on the Google Code project page. If you want to build your own v1, Seeed has our extra PCBs, and we’ll be giving away free v1 and v2 #twatch PCBs for the next few Free PCB Sundays.
You can order an assembled v2 #twatch for $30, including worldwide shipping, until September 23rd, 2009. Your support makes our projects possible, thank you.