HVAC Controller Design Using AM335x Based System in Package

Published On: March, 14, 2019 By: Calvin Slater | Updated: July 17, 2019 by Greg Sheridan

Part 1: Design Requirements and Baseboard Overview

Building Automation System Controllers are flexible, multi-purpose, freely programmable embedded devices that can be found in a variety of HVAC Controller, lighting, and security applications within a building. These devices perform a variety of tasks including monitoring and control of chiller and boiler plants, pumps, valves, fans, lights, indoor air quality, electric metering, occupancy detection and access control. In many cases these devices are not application specific. A Building Automation System Controller being used on one piece of equipment can be reconfigured or reprogrammed to control something else that is completely different! The OSD335x family of System-in-Package products featuring the AM335x provides an excellent foundation for the design of an Open-Hardware HVAC Controller. This is the first of four in a series of technical application notes for building a reference design using the OSD335x-SM. For more introductory background, please see the blog article Open-Hardware Building Automation System Controller Reference Design Using AM335x Based System in Package

Table of Contents

1.Introduction
2.Background
3.Idea for Open Hardware HVAC Controller
4.Board Features
5.Design Objectives
6.Board Layout
6.1 Eliminating the Need for a Separate SOM
6.2 Using the Octavo Systems OSD3358-SM-RED as a Reference
7.About our Guest Author
8.Revision History

A PDF version of this App Note can be found here.

Notice

The information provided within this document is for informational use only. Octavo Systems provides no guarantees or warranty to the information contained.

2       Background

Most HVAC Controllers feature a quantity of two to sixteen configurable remote sensor inputs that can accept a variety of industry standard signals including resistance, relay contact, pulse, 4-20mA current signaling, and 0-10VDC scaled inputs. Control of equipment can be achieved through a mixture of on-board relay outputs or analog transmitters. Control applications are created in a visual block programming environment (similar to Node-Red) See Figure 2. Because these devices are freely programmable (even at runtime in some cases), they often possess a good amount of processing, memory, and storage resources. The amount of computing resources is typically much more than a comparable application specific preprogrammed device.

Historically these devices have been constructed using microcontrollers with 1MB or less of RAM. Typical microcontrollers have on-chip RAM and Flash built into a single piece of silicon. This is the limiting factor with regard to available volatile memory, because both RAM and flash must be implemented using the same silicon process on the device. Recently due to the several factors including increased performance demands (such as cloud connectivity and web services) and lower processor costs, newer generations of HVAC controllers are beginning to emerge that are System on Chip (SOC)/Microprocessor (MPU) based.

3       Idea for HVAC Controller

When examining the layout of these controllers it is observed that the System on Module (SOM) or Core Board module (the separate board containing the SOC, PMIC, RAM and Flash) is very similar to the layout of popular microprocessor hobbyist Single Board Computers (SBC).  The only major difference between a hobbyist SBC and a production HVAC controller is that SBCs do not have a Baseboard that contains a power supply, connectors, transceivers, and passive devices that make the board ruggedized and suitable for industrial environments.

There are many Capes, Shields, or Hats out there that have some or many of these features, but not all of them. The idea here would be to integrate these system components into a single simple, relatively inexpensive, yet ruggedized, single PCB Baseboard that a user could instantly have the hardware (and much of the software) to make their own Open-Hardware HVAC Controller design. Assuming that the board can be made for a few hundred dollars this situation would be preferable to a production controller that can cost a significant amount more.

4       HVAC Controller Board Features

With this background in mind, the next step is to define the specifications for an open-hardware HVAC controller.  The AM335x Based OSD335x-SM is a great platform for my controller. It integrates many of the components that would normally be on the SOM board into a single IC device.  Figure 4 shows a conceptual board layout image.  Below is a list of features needed to integrate into the board:

1.     Control Board will be a (4) layer board and have OSD335x-SM (OSD3358-512-BSM) SiP mounted directly to Baseboard PCB to save space, simplify design, and reduce overall cost.
2.      (1) Fully isolated RS-485 transceiver port module each with (3) position pluggable right-angle industrial type screw connectors supporting up to 115k BAUD rate communications. Isolation with reference ground is critical for reliable communication across networks spanning long distances (up to 4000ft).
3.     (1) CAN Bus transceiver port module for addition of high speed I/O expander board(s). Connector shall be a (3) position pluggable right-angle industrial type screw connector.
4.     (6) Analog Universal Input modules (UI-Basically a voltage divider and/or pull-up circuit with simple low-pass RC filtering), supporting Thermistor/Dry Contact, Pulse, 4-20mA, and 0-10VDC, industry standard input signals (jumper selectable).
5.     (5) 10A Form A relay Outputs, each with pluggable right-angle industrial style screw connectors.
6.     (1) Fused 24VAC half-wave-rectified power supply with (2) position pluggable right-angle industrial type screw connector. Half-wave power is important to avoid grounding issues with other low voltage building automation devices on the same circuit.
7.     Dual rotary decimal based encoder switches and (1) four-position dip switch connected to HC165 shift registers. These are used for serial load of initial user defined device configuration settings at startup.
8.     Board edge connector varistor-type or similar Surge Protective Devices and passive EMI filtering where applicable.
9.     Battery Backup module for Hibernation/RTC support in the event of power loss.

5    Design Objectives

Overall, the design will also have the following considerations:

1.     Baseboard ruggedized for use in industrial environments including cost effective circuit protection and passive EMI filtering.
2.     Board width shall be 4.8 inches (122mm). This is to fit in various types of common industrial enclosures. This should be a good width providing enough space for all components while not being too wide. Board height should not exceed six inches.
3.    Consolidation of discrete components to minimize board footprint wherever possible because space is at a premium in control enclosures. The OSD335x-SM SiP helps out tremendously with this.
4.    Designed from the start to be ready for FCC, CE evaluation and compliance. Design should be ready, but actual submission and testing not in our scope and left to the user.
5.   RS-485 ports compatible with Debian-Linux dev/tty and Node-Ned Serial Nodes.
6.   Suitable to work with Node-Red software wherever possible as well as the Linux image from BeagleBoard.org®.  This Linux image is based on the Debian distribution of Linux.  The BeagleBoard.org® Foundation is a US-based non-profit existing to provide education in and collaboration around the design and use of open-source software and hardware in embedded computing.
7.   Total unit power consumption rated at 15VA or less.
8.   Modular sub-system design. Related components should be grouped close together on board as “modules” where possible. Design user should be able to add or remove system modules (i.e. add or remove Universal Input or Com modules from schematic) easily without greatly affecting existing board layout or signal routing. This approach will also aid separating incompatible signal domains such as digital and analog, or isolated-ground and unit-ground.
9.   Baseboard design only. Design of protective housing left to the user.
10. Board cost target: $300USD (DIY/Hacker/Maker low volume cost) or less.
11. Components should be largely capable of being hand soldered or baked at home (i.e. not too small).

6    Board Layout

The OSD3358-512-BSM/ISM eliminates the need for a two-board solution. It also saves the cost of board-to-board header connectors, which are costly and are a possible point of failure. On many existing designs, the processor, crystal(s), external RAM, Flash, and passives as well as other various peripherals are located on a single board often referred to as a System on Module (SOM). These smaller SOM boards are always more expensive regarding cost per unit area due to the fact they are usually more than four layers, feature finer trace width, and have a greater number of high-speed signals, especially with regard to routing of external memory.

6.1    Eliminating the Need for a Separate SOM

On most SOMs, signals are escaped from the board by being routed to edge, mezzanine, or pin-type, header connectors. Each style of connector choice comes with its own size, reliability, and cost impact. The OSD335x-SM System in Package eliminates these special connectors by replacing them with a rugged wide pitch Ball Grid Array (BGA) package that connects directly to the baseboard. The SiP is in essence a simplified and consolidated SOM with the only difference being that the EMMC flash is external to the System in Package on the OSD335x-SM.

The baseboard will be a four-layer design. This board, though slightly more expensive than a two-layer will help with signal integrity. The internal ground plane will help allow a matched signal return path for all traces. This will reduce the risk of the design radiating and not being capable of passing FCC testing. A four-layer board will also make it very easy to escape all necessary signals from the OSD335x-SM. The wide pitch of the OSD335x-SM BGA pins allows passing of two 6/6 mil traces between each ball gap making it possible to escape most signals on a single layer. However, having the extra layers available is very helpful. The dimensions of the board will be 4.80 inches (122mm) by 5.00 inches.

The width of the board will make it compatible with many widely available industrial enclosures known generically as UM122. It’s important that all of the board connectors are on either the right or left side edges of the board only, and that the width of the board is consistent across all future device designs. This is because these HVAC controllers are usually stacked vertically or horizontally in a row, inside controls enclosures with wire duct for the I/O on each side. See Figure 5. The form-factor should always be consistent as possible across the entire family of devices. Future designs may have varying heights to accommodate different peripherals but the width as well as the general locations of board edge connectors will remain the same. There will be no connections on the top or bottom so that adjacent boards do not interfere with any edge connectors.

6.2    Using the Octavo Systems OSD3358-SM-RED as a Reference

The board will retain many of the same components used on the Octavo Systems OSD3358-SM-RED Reference, Evaluation, and Development Board wherever possible. Unneeded peripherals from that design will be deleted. The only major difference will be that this design uses the 100BASE-T Ethernet Phy called the LAN8710A because Gigabit Ethernet was not required.

The general layout for the board will be such that the large heat producing components such as the regulators will be located on the top. This will help promote convective cooling for the rest of the board, provided that the proper enclosure is design is used to allow for good airflow.

Analog circuits such as the Universal Inputs will be grouped into the same area. This will be located away from digital circuitry. The analog portion of the board will contain its own dedicated ground plane AGND with an unobstructed return path to the OSD’s AGND pin. Likewise, the ISO_GND signal for the RS-485 port will have its own ground region.

The above description is a high-level view of this design only. In the next part of this series we will examine each of these topics separately in greater detail. See Part 2 of this application note where we will begin with an in-depth discussion of Communication ports and I/O Interfaces.

7    About our Guest Author

This article was guest written by Calvin Slater who is a senior controls engineer that has spent eight years in the Building Automation System Control Industry and is highly interested in embedded hardware as well as open-source building automation frameworks. He is also a contributing editor for AutomatedBuildings.com where he has authored a series of articles on the system edge controller.

8    Revision History