BBB-GVS

Beaglebone Black cape which provides 5V GVS connections
We love the Beaglebone Black. With all of its I/O connections it offers exciting potential for embedded/connected devices. It even has real mounting holes which let the board be used for real-world applications.

When we got our first Beaglebone Black we were excited to try it with some of our Arduino GVS (Ground/Voltage/Signal) sensors and GVS output devices but couldn't since they are all 5V sensors. Sure we could cobble together some level shifters on a breadboard but in the end we wouldn't have something that could be deployed in a real application What we really wanted was a 5V Sensor shield like the one we use for our Arduino. But there were none out there. So we designed one.

Introducing the BBB-GVS cape.

Features
Extensive I/O connections:


 * (5) 5V GPIOs: J14, J15, J26, J27, J28
 * (2) 5V UART connections (auto direction detection): J12, J16
 * (1) 5V I2C bus connections (auto direction detection): J13
 * (1) 5V PWM: J25
 * (4) 5V Timers: J29-J32
 * (1) 3.3V UART: J11
 * (7) 3.3V GPIOs: J9, J10, J17, J19, J20, J21, J22
 * (1) 3.3V SPI_D1: J8
 * (1) 3.3V PWM: J18
 * (7) 1.8V analog GVS connections with analog reference voltage/ground: J1-J7
 * 5V pins are all ESD protected pins with 15 kV of protection
 * Fuses on 5V and 3.3V power protect the Beaglebone from miswired sensors
 * Selectable 5V source (SYS_5V or VDD_5V) - J22
 * Cape configuration EEPROM with address select jumpers J24
 * Beaglebone Black form factor

Technical Overview
In a nutshell, the 3.3V digital signals of the Beaglebone Black need to be changed into 5V digital signals. There are a number of discrete ways to convert 3.3V bus to 5V signals. Some of them use MOSFETs and a couple of resistors. These work OK but there are some powerful chips out there that can do the conversion even better. We looked around and found what we think is the best 3.3V<>5V data conversion chip, Texas Instrument's TXS0108. The TXS0108 is an 8-bit, bi-directional buffer with automatic direction detection. Each and every pin can transmit and receive independently and even at the same time. The part supports both open drain and push-pull operation. The part can run as fast as 60 Mb/s in push-pull operation and 2 Mb/s in open drain. This speed is fast enough enough for microprocessor GPIO pin or even the fastest serial interfaces that a microprocessor can throw at it.

What is GVS?
GVS is great for prototyping or deploy-able products. The beauty of the GVS connection is that power and ground are provided with each I/O signal. That provides the power needed to power external sensors and output devices. Otherwise splitting the one or two power pins into separate cables ends up being a real mess.

GVS stands for Ground, Voltage and Signal. It's a 3-pin unofficial standard. It uses 0.1" pitch pins. There are a large number of GVS sensors (inputs) and devices (output) parts on ebay. The sorts of GVS sensors include:


 * Buttons
 * Switches
 * Temperature sensors

The sorts of GVS output devices include:


 * Relay modules (not always wired as GVS, but they typically require 5V).
 * Buzzers
 * Solenoids
 * Servos (require PWM output pin)

To connect a GVS sensor to a GVS card, just use a 3-wire cable.

Other Connections

The Beaglebone Black also has I2C and UART connections. These allow various devices to be connected:


 * I2C Displays
 * I2C accelerometers, gyroscopes
 * UART GPS modules

The Design
We wanted a sensor shield like the ones use for the Arduino. In the Arduino's case, there's a limited amount of I/O lines The Beaglebone has many more I/O lines than the Arduino. We wanted to convert as many of the signals to 5V as we could possible fit on a board the size of a mint can. In particular, we wanted to convert the I2C bus as well as the UART lines to 5V. Also, we wanted to have a mix of 3.3V and 5V GVS signals. We also wanted to allow external connection to the analog inputs as we providing the analog reference voltage to run the analog input sensors.

That translated into 2 parts which handle 8 I/O lines each. Putting these parts along with all the GVS connections we could possibly add resulted in our design.

Voltage Translators
The BBB-GVS board uses Texas Instrument TXS0108 voltage translators.

Voltage Translators Features

 * No Direction-Control Signal Needed
 * Max Data Rates
 * 60 Mbps (Push Pull)
 * 2 Mbps (Open Drain)
 * 1.2 V to 3.6 V on A Port and 1.65 V to 5.5 V on
 * B Port (VCCA ≤ VCCB)
 * No Power-Supply Sequencing Required –
 * Either VCCA or VCCB Can Be Ramped First
 * Latch-Up Performance Exceeds 100 mA Per JESD 78, Class II
 * ESD Protection Exceeds JESD 22 (A Port)
 * 2000-V Human-Body Model (A114-B)
 * 150-V Machine Model (A115-A)
 * 1000-V Charged-Device Model (C101)
 * IEC 61000-4-2 ESD (B Port)
 * ±6-kV Air-Gap Discharge
 * ±8-kV Contact Discharge

Voltage Translators Architecture
Datasheet

The TXS0108E can be used in level-translation applications for interfacing devices or systems operating at different interface voltages with one another. The TXS0108E is ideal for use in applications where an open-drain driver is connected to the data I/Os. The TXS0108E can also be used in applications where a push-pull driver is connected to the data I/Os, but the TXB0104 might be a better option for such push-pull applications. The TXS0108E device is a semi-buffered auto-direction-sensing voltage translator design is optimized for translation applications (e.g. MMC Card Interfaces) that require the system to start out in a low-speed open-drain mode and then switch to a higher speed push-pull mode.



To address these application requirements, a semi-buffered architecture design is used and is illustrated above (see Figure 1). Edge-rate accelerator circuitry (for both the high-to-low and low-to-high edges), a High-Ron n-channel pass-gate transistor (on the order of 300 Ω to 500 Ω) and pull-up resistors (to provide DC-bias and drive capabilities) are included to realize this solution. A direction-control signal (to control the direction of data flow from A to B or from B to A) is not needed. The resulting implementation supports both low-speed open-drain operation as well as high-speed push-pull operation.

When transmitting data from A to B ports, during a rising edge the One-Shot (OS3) turns on the PMOS transistor (P2) for a short-duration and this speeds up the low-to-high transition. Similarly, during a falling edge, when transmitting data from A to B, the One-Shot (OS4) turns on NMOS transistor (N2) for a short-duration and this speeds up the high-to-low transition. The B-port edge-rate accelerator consists of one-shots OS3 and OS4, Transistors P2 and N2 and serves to rapidly force the B port high or low when a corresponding transition is detected on the A port.

When transmitting data from B to A ports, during a rising edge the One-Shot (OS1) turns on the PMOS transistor (P1) for a short-duration and this speeds up the low-to-high transition. Similarly, during a falling edge, when transmitting data from B to A, the One-Shot (OS2) turns on NMOS transistor (N1) for a short-duration and this speeds up the high-to-low transition. The A-port edge-rate accelerator consists of one-shots OS1 and OS2, Transistors P1 and N1 components and form the edge-rate accelerator and serves to rapidly force the A port high or low when a corresponding transition is detected on the B port.

J1-J7
Analog signals must be between 0 and 1.8V/

J1-J7 pin 1 = P9-34 = GND_ADC

J1-J7 pin 2 = P9-32 = BDD_ADC



J8-J11
3.3V digital



J12-J16
5V digital



J17-J22
3.3V digital



J23
Jumper 1-2 to power board from SYS_5V.

Jumper 2-3 to power board from VDD_5V.



J24
EEPROM address A0, A1 selection jumper selectable.



J25-J29
5V digital



J30-J32
5V digital



Revision X3

 * Missing OE on U2
 * Add wire U2-2 to U2-10
 * Ref des have gaps = need resequencing
 * Need a write enable jumper