700
edits
Changes
Jump to navigation
Jump to search
Line 48:
Line 48: + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
no edit summary
== Schematics ==
== Schematics ==
A full gallery of all Oscar schematics can be seen below. This section discusses each sub-circuit separately.
=== Top Sheet ===
The top sheet displays the overarching hierarchy of the avionics board. Only perimeter Teensy pins were utilized, hence pins 24-33 and A10-A14 are NC. The GPS was connected to Hardware Serial 1 over pins 0/1. Pin 2 was driven low when SD card was detected (output of the Micro SD sheet). Pins 3-5 were not utilized. Pins 6-9 were used for RockBlock operation, although NETAV & SLEEP were not used for actual operation, and may be discarded in the future. Pins 10-13 were used to define the SPI Bus, and pin 10 was used as the Micro SD chip select. As seen in the bottom left, pinouts were supplied for I2C and SPI. VIN was supplied with +5V from the boost circuit. Two decoupling caps for +3.3 and +5 were sourced local to the MCU. Pins 22-20 were defined as chip selects for the thermocouple and BMP 280's (22 for thermocouple, 21/20 for BMP280). Pins 19/18 defined the I2C bus. The I2C bus had five total devices: Two MS5803's, One MPL3115A2, One MCP23017 multiplexer, and one RTC IC. Pin 16 connected to the battery protector (LTC4218) power good indicator. Pins 15 and 14 were used for current monitor (via 4218) and battery voltage detection.
=== Thermocouple ===
The thermocouple used the MAX3115K amplifier for thermocouple amplification and a K-type thermocouple. D11/D12, as well as R28/R27 were both included in the schematic and later discarded in the physical PCB. D11/D12 were shorted, while R28/R27 were open-circuited. These were included from the Adafruit breakout schematic, but exist purely for the purpose of shifty logic shifting, which was not necessary with a 3.3V MCU like the Teensy. Polarized Decoupling caps (which needn't be polarized) were included. A 1uF filter cap was placed between two signal conditioning ferrite beads (1kHz), which led to a 2 circuit Microfit connector.
=== Sensors ===
Both BMP280 sensors utilized SPI. A 100nF decoupling cap was provided for both. Both MS5803 sensors utilized I2C; addresses were set to 0x76 and 0x77 using the CSB address selection pin. 100nF decoupling caps were provided for both. The MPL3115A2 similarly utilized I2C -- no decoupling cap is provided; rather, a 1uF capacitor is provided to the CAP pin. 10 kOhm pullups are provided for the I2C bus.
=== RTC ===
A DS1307 RTC IC is utilized for the real-time clock. The Teensy 3.2 internal RTC is avoided due to past difficulties/problems. The DS1307 is powered by 5V (no 3.3V variety exists). A 32.768kHz 12.5pF time-keeping crystal X1 is used for the RTC, and a keystone 12mm coin cell holder provides backup power for the RTC. A 100 nF decoupling cap is provided.
=== RockBlock ===
The RockBlock uses a 2x3 vertical Molex microfit connector. It is provided +5V, GND, Serial TX/RX lines, as well as connections for the SLEEP & NETAV pins, which are not utilized.
=== Battery Protector ===
The LTC4218 multi-function IC is used for battery pack protection. A resistive network in the order of VBAT->18k->6.2k->91.k->GND is implemented to specify an overvoltage threshold of 4.52V and an undervoltage threshold of 2.3V. A current limit of 5 amps is implemented utilizing R23/RSENSE as defined by the 4218 data sheet. A shut-off FET Q7 is implemented to turn off the avionics in fault cases. A PG state is provided to the MCU.
=== Pinouts ===
Pinouts are provided for SCL, SDA, MOSI, MISO, SCLK, GND, +3.3, +5, and +VIN. I2C bus pinouts are given 6 pins, SPI bus pinouts are given 4 pins, and power pinouts are given 12 pins.
=== Multiplexer ===
The MCP23017 is used as a 16 I/O I2C multiplexer. A2/A1/A0 are grounded for default address selection. Reset is pulled high to disable reset functionality. Pins 0 - 7 are used to drive indicator LEDs, while pins 8 - 13 are used for binary operation of custom FETs.
=== Micro SD ===
The Micro SD connector is connected to the SPI bus and provided power. A card detect state (pulled low when detected) is connected to the MCU.
=== LEDs ===
Eight LED's are controlled by the multiplexer, with 220 ohm current limiting resistors. The LED's are green.
=== GPS ===
A 4 pin precidip socket is used to connect the external NEO-6M GPS breakout. Notably, The TX & RX lines are incorrect and should be swapped.
=== FETs ===
FET1 & FET2 include an INA169 I2C current monitor (which is later removed prior to flight because it was not being used and to remove possible I2C bus problems). Current monitors have addresses selected via pins A0/A1. An 0.1 ohm current sense resistor is used to calibrate the INA169 according to the data sheet. 2 pin vertical Molex Mirofit connectors are used for the custom FETs. All FETs are powered from raw battery voltage for high current applications. FET3-FET6 do not use current monitors. Notably, all six FET's have an incorrectly connected LED which does not function on the actual PCB.
=== 5V Boost ===
An LTC3428 is used for the 5V boost circuit. All relevant passives are determined from the LTC3428 datasheet. A two circuit molex microfit connector is used for the battery input.
<gallery widths=200px heights=200px>
<gallery widths=200px heights=200px>