The next batch of Bus Pirates will be the ‘b’ revision of the v3 hardware. V3b has the same functions, size, and shape of v3a, and it takes the same firmware. We just made a few minor updates based on our experience manufacturing the v3a over the last few months. Our prototype is green, but the production boards will still be red.
The primary updates are:
- Updates to the LEDs
- Updates to the voltage regulator output capacitors
Click for a full size schematic image [PNG]. Download the v3b hardware files from the Google Code project page or the SVN. The schematic and PCB were made with the freeware version of Cadsoft Eagle. See the v3 article for a complete hardware overview, and check the Bus Pirate manual for links galore.
Updates to the LEDs
Many felt the indicator LEDs were too bright or uneven in brightness. All four indicator LEDs have been revised in v3b (top) to leave fewer opportunities for errors. All four LEDs are now connected to the same supply voltage (3.3volts), face the same direction, and use the same resistor value.
MODE and VREG
The MODE LED (LED2) is powered by 3.3volts coming from a PIC IO pin (LD1). The VREG LED (LED3) is powered by the 3.3volt regulator (VR3) that provides power to the IO header.
In v3a both LEDs had a 390ohm resistor (R31, R32), in v3b the resistor values were increased to 1100ohms. The new value reduces the brightness considerably, but it’s still perfectly visible on the prototype.
The v3a PWR LED (LED1) connects to the 4.75-5.25volt USB supply (left). In the interest of having all four LEDs run from same voltage with the same current limiting resistor, we connected the PWR LED to the 3.3volt PIC power supply (VR2) in revision b (right). This LED is also now a visual test of the PIC power supply regulator, not just the USB supply.
This LED already had a 1100ohm resistor (R2) when connected to 5volts. We left it the same so it matches the value of the updated MODE and VREG LEDs.
The USB LED was also powered by the ~5volt USB supply in the v3a circuit (left). In v3b we changed it to draw power from the 3.3volt PIC supply (VR2).
The new routing made it possible to rotate the USB LED so it’s pointing the same direction as the other LEDs, another opportunity for errors eliminated.
The USB LED already had a 1100ohm resistor (R3) in v3b, we left it the same so it matches the value of the updated MODE and VREG LEDs.
Voltage regulator output capacitor update
Each MIC5205 voltage regulator (VR2, VR3, VR4) requires a 1.0uF or greater output capacitor (C22, C23, C24). The v3a (bottom) has three 2.2uF tantalum capacitors, the v3b (top) has 1.0uF ceramic capacitors. This is moderately cheaper and uses less tantalum (a conflict mineral).
v3a used tantalum capacitors in a ‘size-A’ case, the smallest standard size. The ceramic capacitors used in v3b come in much smaller packages. We used 0805, but they also come in 0603, 0402, etc. This swap opens some room around the crowded power supply area and makes the board a bit cleaner.
The USB supply capacitor (C21) and the PIC internal voltage regulator capacitor (C20, not shown) use the same 10uF tantalum capacitors as the previous revision. The chip datasheets specify 10uF+ low-ESR tantalum capacitors.
Click for a full size board image [PNG]. The v3b PCB is the same size and shape as v3a, but we redefined the board dimensions in Eagle. Hopefully board shape will be a bit more consistent in the future.
|C1-6||0.1uF capacitor (0805)|
|C22, 23, 24||1uF ceramic capacitor (0805)||Updated VREG caps|
|C20, 21||10uF tantalum capacitor (SMC-A)|
|ICSP, IO||0.1″ pin header (3×05)|
|J1||USB MINI-B (SMD)|
|L1||1000ma+ ferrite bead (0805)|
|R1||2000 ohm resistor (0805)|
|R2,3, 31, 32||1100 ohm resistor (0805)||Same R for all LEDs|
|R10-17, 19-23||10000 ohm resistor (0805)|
|VR2,3||MIC5205 3.3volt regulator (SOT23-5)|
|VR4||MIC5205 5volt regulator (SOT23-5)|
Firmware and self-test
The v3b Bus Pirates should ship with v4.2 firmware and v4.2 bootloader, the first hardware to ship with the v4 bootloader. It’s had enough testing that we’re ready to use it in a production environment.
There were a number of manufacturing defects related to LED quality and placement in some batches. The v4.2 firmware includes an extended self-test that pauses for visual verification of the MODE and VREG LEDs. This should help catch any LED quality problems.
The v3b silk was updated with the new revision number and the Dangerous Prototypes logo.
Taking it further
What updates will be rolled into future Bus Pirate v3 revisions? We’re not sure yet.
- We’ve always wanted the IO header to be centered and shrouded
- Use an SMD header for faster production
- Correct the IO pin order to the logical order of v0 and v1a – MOSI-CLK-MISO-CS-AUX
- Route a few extra pins to the edge of the board
- Add an on-board EEPROM
Many of these updates would change the size and form factor of the Bus Pirate. That might require a version increment to avoid compatibility problems with existing v3.0 accessories like cases, etc.
Do you have any suggestions? Want to get involved? There’s always new Bus Pirate developments and support in the forum.