Difference between revisions of "BBB-GVS"

Revision as of 11:55, 25 January 2020

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

(5) 5V GPIO GVS connections (auto direction detection)
(4) 5V GPIO/Timer GVS connections (auto direction detection)
(2) 5V UART Tx/Rx connections (auto direction detection)
(1) 5V PWM GVS connection
(1) 5V I2C bus 4-pin connections (auto direction detection)
5V pins are all ESD protected pins with 15 kV of protection
Cape configuration EEPROM
(7) 3.3V GPIO connections
(1) 3.3V UART Tx/Rx connection
(1) 3.3V PWM GVS connection
(7) 1.8V analog GVS connections
Resettable Fuses on 5V and 3.3V power
Selectable 5V source
3.3V and 5V power status LEDs
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.

EEPROM connections

Beaglebone Black GVS Board Layout

Connectors

J1-J7 Analog Inputs

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

Connector	Pin	Function
J1-3	P9_40	AIN1
J2-3	P9_38	AIN3
J3-3	P9_36	AIN5
J4-3	P9_33	AIN4
J5-3	P9_39	AIN0
J6-3	P9_37	AIN2
J7-3	P9_35	AIN6

J8-J11 GPIO, UART1 - 3.3V

3.3V digital

Connector	Pin	Function	GPIO
J8-3	P9_30	SPI_D1	GPIO3_16
J9-3	P9_27	GPIO3_19	GPIO3_19
J10-3	P9_23	GPIO1_17	GPIO1_17
J11-3	P9_24	UART1_TXD	GPIO0_15
J11-4	P9_26	UART1_RXD	GPIO0_14

J12-J16 - UART2, I2C1, GPIOs, UART4 - 5V

5V digital

Connector	Pin	Function	GPIO
J12-3	P9_21	UART2TX	GPIO0_3
J12-4	P9_22	UART2RX	GPIO0_2
J13-3	P9_18	I2C1SDA	GPIO0_12
J13-4	P9_17	I2C1SCL	GPIO0_13
J14-3	P9_15	GPIO1_16	GPIO1_16
J15-3	P9_12	GPIO1_28	GPIO1_28
J16-3	P9_13	UART4TX	GPIO0_31
J16-4	P9_11	UART4RX	GPIO0_30

J17-J22 GPIOs, PWM2A, 3.3V

3.3V digital

Connector	Pin	Function	GPIO
J17-3	P8_26	GPIO1_29	GPIO1_29
J18-3	P8_19	EHRPWM2A	GPIO0_22
J19-3	P8_18	GPIO2_1	GPIO2_1
J20-3	P8_17	GP1O0_27	GPIO0_27
J21-3	P8_16	GPIO1_14	GPIO1_14
J22-3	P8_15	GPIO1_15	GPIO1_15

J23 - 5VSEL

Jumper 1-2 to power BBB-GVS board from SYS_5V. (default)

Jumper 2-3 to power board from VDD_5V. VDD_5V is 5V with a maximum current of 1000mA, but only works if the BeagleBone is powered by an external power source

BBB-GVS-X3-J23.PNG

J24 - EEPROM address selection jumpers

EEPROM address A0, A1 selection jumper selectable.

Default = both installed.

J25-J29 PWM2B, GPIOs, TIMERs - 5V

5V digital

Connector	Pin	Function	GPIO
J25-3	P8_13	EHRPWM2B	GPIO0_23
J26-3	P8_14	GPIO0_26	GPIO0_26
J27-3	P8_11	GPIO1_13	GPIO1_13
J28-3	P8_12	GPIO1_12	GPIO1_12
J29-3	P8_9	TIMER5	GPIO2_5

J30-J32 - TIMERs - 5V

5V digital

Connector	Pin	Function	GPIO
J30-3	P8_8	TIMER7	GPIO2_3
J31-3	P8_7	TIMER4	GPIO1_3
J32-3	P8_10	TIMER6	GPIO2_4

J33 - EEPROM Write Enable

Install jumper to enable writes. Remove jumper to disable writes (default).

Assembly Sheet/Parts List

BBB-GVS Rev X4 Assembly Sheet