All trademarks are copyright of their respective owners.  Overview The PiFly board is designed for drones, high power rocketry, and RC airplanes. The board will also suit many other applications. Primarily designed for the Raspberry Pi Zero, it will work on any Raspberry Pi with a 40 pin connector or compatible. The physical form factor doesn’t conform to the Raspberry Pi HAT size, hence the HAT-ish reference.

figure 1

##Features • Power: Designed to run on 3 to 12 volts, it is intended to run on one to three LiPo cells, but any power supply will do. A single power supply powers the Raspberry Pi, the HAT board, and its USB device from a 5 volt 2 amp buck-boost switching power supply. (The four high current outputs require at least 4.5 volts; i.e. two or three LiPo cells)

• Raspberry Pi compatible: The PiFly is intended to connect to the Raspberry Pi Zero so that components of each board face away from the middle. A 40 pin socket could be soldered to the top of the board for use on the Raspberry Pi 3, or any 40 pin Raspberry Pi.

• RF Transmitter Amplifier and Filter: The board has a dual RF filter and 200mW amplifier. It supports an RF carrier from either GPIO14 or GPIO18. One filter is for the 144MHz HAM band and the other filter is for the 440MHz HAM band. 144MHz transmission can be supported by pifm, nbfm, and rpitx. 440MHz transmission may someday be supported by rpitx, but currently rpitx doesn’t seem to work on the Raspberry Pi Zero.

• Servos: Can control up to eight servos. (Standard 6 volt servos will require 2 LiPo cells, higher voltage tolerant servos can use 2 or 3 LiPo cells)

• High Current Outputs: Intended for Rocket upper stage igniters and parachutes, these high current, high side drivers have diagnostics and a redundant enable system to prevent false assertions. Connections use press-to-release terminal blocks.

• Support for headless operation: There is a shutdown button and a shutdown acknowledgment LED for safe headless shutdown. There is also a low battery comparator that can assert the shutdown request.

• Small size: The PiFly has the same width as Raspberry Pi Zero, but it is longer. Dimensions: 29.25mm by 150mm. It is intended to fit a high power rocket 38mm tube coupler, strap on the belly of a quad-copter, or in an RC airplane. Also small enough to be a smart handi-talkie with the addition of a USB software defined receiver. See PDF download below for application schematics.

• USB redirected: Cannot use the USB port in the 32mm width. The USB test points on a Raspberry Pi Zero can be soldered to test points on the HAT board for the USB type A connector and still fit in a 38mm tube coupler.

• GPS: The board uses the Skytrac Venus838 module. In binary mode this device can make 50 location updates per second. It has an SMA connector for an external antenna. This is required for the use of a helical, omni directional antenna. GPS data can be backed up with a super capacitor that has a connector for an external rechargeable coin cell. Currently there is only Raspberry Pi support for ASCII NMEA-0183 compatible output at 10 location updates per second. There is also a four pin connector for an external GPS if the board is built without the onboard GPS.

**• Most I/Os have ESD protection. • A/D support: Options for 8, 10, and 12 bit footprint compatible A/D converters. The default build uses the 10 bit TI ADS7957SDBTR A/D converter with 16 channels; some for internal measurement and diagnostics and some external channels. The external A/D connector is a 0.050” pitch connector. Two external channels can be set up for thermistors. Uses SPI interface.

• Keypad support: A connector for either a standard six-key keypad or a standard 12-key keypad connected through a resistor array to a A/D input. With an external resistor array 26 keys are possible.

• High G linear accelerometer: Uses NXP (Qualcom) MMA6555KW as on Altus Metrum’s TeleMega. Uses SPI interface.

• Barometric pressure sensor: Uses a Measurement Specialties (TE Connectivity) MS560702BA03-00 as on Altus Metrum’s TeleMega. Uses SPI interface.

• 9 axis inertial and magnetic platform: Uses an Invensense (TDK) MPS-9250 for 3-axis acceleration, 3-axis gyro, and 3-axis magnetometer. Mounted on board centerline. Uses I2C bus.

• Humidity sensor: Uses ST’s HTS221TR sensor. Uses I2C bus.

• Differential Pressure sensor: Can be used for drone/aircraft Pitot tube for airspeed. Measurement Specialties (TE Connectivity) 4525DO-DS5AI030DP on I2C bus.

• Audio Output: GPIO13 has PWM audio filter, amplifier, and connector.

• Microphone: Knowles SPH0645LM4H-B on the I2S bus.

• Time of Day Clock: Maxim DS3231S using the GPS Super Cap/Battery Backup.

• Maxim 1-Wire bus controller.

• Quad tachometer Input: compatible with Spektrum SPM1452 sensors. Uses an Analog Devices ADT7470.

####Power The PiFly board runs from a 5.0 volt to 12.0 volt maximum input on connector P1. The breakdown voltage of the input protection TVS is 12.2 volts and this voltage should never be exceeded. The input is protected from ESD to 15kV non-contact. If the high side drivers on J6 are not used, the input voltage can go as low as 3.0 volts. The power input connector is a Deans Ultra Plug; a small, lightweight, high current connector that is widely available. There is no battery charger on the PiFly.

The input power is converted to 5.1 volts through a buck-boost DC-DC converter. It is then further regulated to 3.3 volts with a linear regulator. The 5.1 powers the A/D converter analog part and the 3.3 volt powers most of the sensors.

The high side drivers on J6 are powered directly from the power input on P1. This is required since the high side driver can source as much as 42 amps. Care must be taken for high current applications if you use a LiPo battery. LiPo batteries have a built-in over-current protection. If your application exceeds this value, your battery will automatically disconnect and the PiFly and Raspberry Pi computer will lose power.

The servos that can connect to P2 and P3 can run directly off the power input or can run two diode drops below the power input. This is selected by W4. For all but the smallest servos, it is recommended that W4 be a soldered wire short, not a removable jumper due to current requirements. LiPo battery voltages and servo voltage options are listed below:

LiPo cell count		W4 position		Servo voltage
1 (3.0V - 3.7V)		1-2				1.8V – 2.7V
1 (3.0V - 3.7V)		2-3				3.0V - 3.7V
2 (6.0V – 7.4V)		1-2				4.8V – 6.4V
2 (6.0V – 7.4V)		2-3				6.0V – 7.4V
3 (9.0V – 11.1V)	1-2				7.8V – 10.1V
3 (9.0V – 11.1V)	2-3				9.0V – 11.1V

Servos could be powered independently by wiring an external power source to pin 2 on W4.

####Compatibility The PiFly is designed to be mated to a Raspberry Pi Zero, but is compatible with many Raspberry Pi 40-pin compatible computers. The intended connection to a Raspberry Pi Zero is soldering a dual row 40 pin header from the bottom side of the Raspberry Pi Zero to the ‘solder’ side of the PiFly at J1. This allows the boards to be 1.6mm apart for the tightest form factor. The I2C_0 bus has an EEPROM for storing a device tree, but there currently isn’t a device tree definition for the EEPROM. Below is a list of compatible boards.

https://www.raspberrypi.org/products/pi-zero/
https://www.raspberrypi.org/products/raspberry-pi-3-model-b/
https://www.pine64.org/?product=pine-a64-board-2gb
http://www.orangepi.org/orangepiplus2e/

All the connections to the host computer through the 40 pin connector have a small capacitor to prevent any RF ingress to the Host since the PiFly has a RF transmitter.

J8 is a reset pin that lines up with the reset pin on a Raspberry Pi Zero. This allows the reset to be kept low until adequate power is available to run. This can be useful for solar applications. This is not directly compatible with other boards but may be connected with hookup wire.

####RF Transmitter Amplifier & Filter The PiFly board has two RF outputs. Each one has its own filter for a specific RF band. The default build is for the 144MHz HAM band and the 434MHz HAM band. 144MHz transmission can be supported by pifm, nbfm, and rpitx. 440MHz transmission may someday be supported by rpitx on the Raspberry Pi Zero, but currently does not work. Rpitx works well on the Raspberry Pi 3. There has been no testing on other ‘compatible’ computers. Supports RF carrier from either GPIO4 or GPIO18 (also the I2S clock) selected with GPIO27. The on-board microphone can not be used with GPIO18 selected as the RF output. Below is a table for the multiplexor and the band.

	GPIO27			Selected Timer Output		Supporting Software
	Low				GPIO18 (shared with I2S)		rpitx
	High			GPIO4							pifm, nbfm, rpitx


	Band		TX Output Connector			Supporting Software
	144			J3							pifm, nbfm
	433			J2							rpitx

####Servo Outputs The PiFly has eight servo outputs on P2 and P3. Please read the above Power section to properly match your board power to your servo voltage requirements. The servo PWM pulses are generated by a Qualcomm (NXP) PCA9685PW. Only eight of the sixteen outputs are used, and the outputs are not in order with respect to the data sheet ordering. It is intended that a software driver should handle the assignments. There is no software overhead to keep a servo in a given position, as the PCA9685PW will continue generating the same pulse width until commanded to a new pulse width. The pulse width is clocked with an external 12.000MHz oscillator for consistent, temperature stable operation. It is recommended that the programmed ON time of each PWM is distributed evenly across the 4096 clock period to reduce large current spikes. Below is the setup requirements and servo PWM performance information.

Prescaler=49
	Update rate= 12000000/4096/49=59.79Hz
	Minimum adjustment=1/(12000000/49)=4.0833µs
	Full servo throw=1ms (+/- 0.5ms)
% adjustment=4.08333µs/1ms=0.4083%

Pulse width	Off time – On time
0.4ms		98
0.5ms		122
1.0ms		245
1.5ms		367
1.6ms		392

The servo PWM pins have ESD protection. See Appendix C for additional information.

####High Current Outputs The PiFly board has four 42 amp outputs. It is important to understand that these are not continuous 42 amp drivers. Any load over five amps must be momentary as there is not enough heat sinking on the high side driver. The load current of each output is monitored by the A/D converter allowing the software to protect against overloads. Each output is designed to drive a high current igniter like what is used for igniting an upper stage rocket motor or deploying a parachute. The output are also suitable for running a DC fan, lights, or any DC load that won’t over heat or exceed the inductive kick back capability of the VNQ5027AKTR-E.

At power up the high current outputs are not enabled. There is a flip-flop that is set to disable the outputs at power up. The only way to enable the outputs is to set all four of the Fire outputs to zero (inactive) and then make a low to high transition on the FireEnableEdge (GPIO25) output. This circuit prevents any single random write to the fire control lines from turning on an output. This protection circuit is designed for applications where only one set of outputs are active at a time, and then turned off, like rocket igniter applications. To keep this protection active between ignition events the turn-on, turn-off sequence must be:

	Turn on:		1) All Fire outputs (Fire A to Fire D) must be low
				2) Make a low to high transition on FireEnableEdge
				3) Set required Fire output high to provide output power to required outputs

	Turn off:		1) Set the FireEnableEdge low
				2) Make a low to high transition on FireEnableEdge, this will shut off the driver
				3) Set all Fire A to Fire D outputs low.
				4) This leaves the fire control immune to any single errant write of the GPIOs.

If protection is not required for your application, after enabling with FireEnableEdge, the individual Fire outputs may be activated or negated at will.

For A/D measurements of output current or voltage, see the A/D section. See Appendix C for additional information.

####Headless Operation The PiFly board has support for headless operation. There is a momentary push button on GPIO26 to request an orderly shutdown of the Raspberry Pi. There is also a LED that can be used to indicate that the software has received the request and is working on the shutdown. Here is a link to a tutorial on the Adafruit software for an external shutdown request. The software needs to be altered for using GPIO26 for the request and GPIO16 for the LED. In addition to the push button requesting a shutdown, there is a battery voltage comparator that can also request a shutdown if the input voltage gets too low. This is useful for remote applications such as solar powered installations or solar power recharged battery applications. If W5 is shorted, the comparator can request a shutdown when the input voltage falls below 3 volts. See Appendix C for additional information.

####USB Connector The PiFly board doesn’t have any USB electronics but it does allow the Raspberry Pi Zero’s USB port to be ‘re-directed’ to a type A connector that is mounted at the end of the PiFly board. This allows using a USB device in a narrow tube like a rocket body tube. Using this connector requires soldering three small wires from three test points on the Raspberry Pi Zero to three pads on the PiFly. This connection is not compatible with any other computer than the Raspberry Pi Zero. The USB connector has ESD protection.

####GPS The PiFly board has a GPS receiver and SMA connector for an external antenna. Antenna preamp power is provided (3.3V) so either an active or passive antenna may be used. The RF input has a bandpass filter to prevent de-sense to the RF front end from the local transmitter. Below is a list of omnidirectional helical antennas, The Richardson RF antenna was used for design.

Active: http://www.stepglobal.com/maxtena-m1227hct-a-l1-l2-gps-glonass-compact-active-helix-antenna https://www.radiall.com/antennas/standard-short-gps-l-1-active-25db.html

Passive: http://www.richardsonrfpd.com/Pages/Product-Details.aspx?productId=1142163 http://www.stepglobal.com/maxtena-m1516hct-p-sma-l1-gps-glonass-compact-passive-helical-antenna

The GPS receiver powers up in standard NMEA-0183 format supporting GGA, GLL, GSA, GSV, RMC, VTG, ZDA sentences. The baud rate may be increased to 115200 and the protocol changed to SkyTraq Binary, and the update rate may be set as high as 50 locations per second.

There are three options for back-up power for the almanac and ephemeris, a small SMT super capacitor (C2), a larger through-hole super capacitor (C58), or a external connection for a battery (P7). The Skytraq Venus638 may also used in this footprint by changing R74 & R108.

The board may be assembled without the SkyTraq GPS and have the serial port available for another purpose, or an external GPS by using connector P6. See Appendix C for additional information.

####Analog to Digital Converter The PiFly has a 16 channel A/D converter. The design is compatible with 8, 10, and 12 bit versions of the A/D converter. The default build is with the 10 bit version. The A/D driver should always read the converter so the data is left justified, putting DO-11 (bit position from the data sheet), the MSB, in the most significant bit of the processors register. The math will assume the binary point is just to the left of the most significant bit. This will make all data returned from the A/D converter a fraction from 0.0000 to, but not including, 1.0000. This will be true for any of the available resolutions. In the table below, <A/D> is always this fraction from 0.0000 to 1.0000.

Channel	Measurement			Conversion to Engineering Units
0		AnalogCH1, J7 pin 3		Application dependent
1		AnalogCH2, J7 pin 3		Application dependent
2		AnalogCH3, J7 pin 3		Application dependent
3		AnalogCH4, J7 pin 3		Application dependent
4		AnalogCH5, J7 pin 3		Application dependent, if thermistor, see appendix B
5		AnalogCH6, J7 pin 3		Application dependent, if thermistor, see appendix B
6		PiFly temperature		See appendix B
7		Keypad				See Keypad section below
8		Fire A current			<A/D> * 54.217 amps  (using 5V reference)
9		Fire B current			<A/D> * 54.217 amps  (using 5V reference)
10		Fire C current			<A/D> * 54.217 amps  (using 5V reference)
11		Fire D current			<A/D> * 54.217 amps  (using 5V reference)
12		Fire D output voltage		<A/D> * 30 volts (using 2.5 volt reference)
13		Fire C output voltage		<A/D> * 30 volts (using 2.5 volt reference)
14		Fire B output voltage		<A/D> * 30 volts (using 2.5 volt reference)
15		Fire A output voltage		<A/D> * 30 volts (using 2.5 volt reference)

This device is on the higher speed SPI bus so rapidly changing data can be captured accurately. The SPI address is SPI_ADDR1..0=0x2. See Appendix C for additional information.

There is a small breakout board for the A/D converter analog input connector. This adapts the 0.050” pitch connector to some 0.10” connectors and also provides a voltage reference for additional thermistors. See figure 2.

figure 2

####Keypad Support The PiFly board has a dedicated keypad connector. This connector supports a analog 1-wire keypad. There is a 7-resistor voltage divider that different keys short different combinations of resistors, to provide a unique voltage for each key. This connector has a resistor array that can be built two ways, first for a 6 key keypad with a single common, and second for a 3 x 4 matrix. For larger keypad requirements you will need to use an external resistor divider. Using 1% resistors keypads as large as 26 keys can be realized. Software to generate the keypad decode software can be found at:

	http://rau-deaver.org/1-wire_keyboard.html 

Below in figure 3 is a picture of the application with the A/D and resistor values filled in for the PiFly. Typically the application is used to take an A/D specification and keypad layout and give you resistor values, but that was done at the design of the PiFly. You just need to replace the values and generate your C code. One source for keypads with a compatible connector is XYMOX. See: http://www.xymox.com/stock-membrane-switches

figure 3

		figure 4									figure 5

For a 3x4 keypad like a XYMOX part number 12654: R65 no pop R77 populate with 1.33kΩ R69 change to 4.75kΩ R66 change to 1.33kΩ R67 change to 1.33kΩ

####High G linear accelerometer The PiFly board has an accelerometer for high acceleration application like rocketry. This is the same sensor used by Altus Metrum in their TeleMega board, the MMA6555. See: http://altusmetrum.org/TeleMega/ The device is mounted so –X acceleration is in the direction of the USB connector end of the board. This device is on the higher speed SPI bus so rapidly changing data can be captured accurately. The SPI address is SPI_ADDR1..0=0x1. See Appendix C for additional information.

####Barometric Pressure Sensor The PiFly board has a pressure sensor for the measurement of atmospheric pressure. This is the same sensor used by Altus Metrum in their TeleMega board, the MS560702BA03-00. See: http://altusmetrum.org/TeleMega/ This device is on the higher speed SPI bus so rapidly changing data can be captured accurately. The SPI address is SPI_ADDR1..0=0x0. See Appendix C for additional information.

####9 Axis Inertial and Magnetic Platform The PiFly board uses an Invensense (TDK) MPS-9250 for 3-axis acceleration, 3-axis gyro, and 3-axis magnetometer. Mounted on board centerline making it easier for the sensor to be on the centerline or your center of mass of your rocket/drone so you don’t have to mathematically back out roll accelerations. Mounted on board so +X acceleration is towards the GPS Antenna end of the board and +Y is towards the 40 pin Raspberry Pi connector. The magnetic coordinates are different from the acceleration reference frame. Uses I2C bus at address 0b1101001. See Appendix C for additional information.

####Humidity sensor Uses ST’s HTS221TR sensor as on Raspberry Pi’s Sensor HAT board. Uses I2C bus at address 0b1011111. See Appendix C for additional information.

####Differential Pressure Sensor For use with drone/aircraft Pitot tubes for airspeed. Measurement Specialties (TE Connectivity) 4525DO-DS5AI030DP. This is a through hole device that mounts over some SMT components. Uses I2C bus at address 0b0101000. See Appendix C for additional information.

####Audio Output There is a two-pin connector for headphone level audio output. See Appendix C for additional information.

####Microphone Knowles SPH0645LM4H-B on the I2S bus. The select pin is grounded so the microphone shows up on the Left channel. See Appendix C for additional information.

####Time of Day Clock To provide time for PiFlys built without a GPS, the Maxim DS3231S is an optional device. Battery Backup is provided by the same source as the GPS battery backup. Uses I2C bus at address 0b0101000.

####Quad tachometer input Four channel tachometer input compatible with Spektrum SPM1452 sensors. Uses an Analog Devices ADT7470. With resistor changes the ADT7470 also supports GPIOs and a digital temperature sensor bus that can optionally be enabled at the expense of tachometer channels. This footprint is also compatible with the ON Semiconductor ADT7460 that supports low voltage level tachometer inputs. Uses I2C bus at address 0b0101110. See Appendix C for additional information.