Saturday, February 20, 2016

Building WiNode 5 - Part 3

Some five years have passed since the original WiNode design, and WiNode 5 - consider it to be the Anniversary Edition, has tried to build upon some of the changes that have happened over the intervening years. There has been an explosion of low cost wireless modules onto the hobbyist market, WiFi, Bluetooth Low Energy (BLE) and upgraded <1ghz an="" and="" be="" devices="" ism="" latest="" may="" modules="" mounted.="" of="" offers="" onto="" open-platform="" p="" some="" these="" this="" version="" which="" winode="">
More than just an ATmega1284 Platform

WiNode comes towards the end of my involvement with 8-bit microcontrollers - as I move towards 16 bit and 32-bit devices. For this reason it was designed with future proofing in mind.

Whilst a surface mounted ATmega1284 may be soldered to the pcb for the basic board, there is also provision for a 40 pin DIL socket or pair of female header strips to be fitted down the centre of the pcb.  If the ATmega1284 is not fitted to the board, then any other mcu of up to about a 48 pin package - suitably mounted on a small carrier pcb that matches the 40 DIL pin-out,  may be plugged into the board. The 40 pin "socket" is based around the pin-out of the DIL ATmega1284 - so even one of those can be fitted if necessary.

This technique allows the use of a range of small processors, including ARM Cortex M, MSP430 and the like. I already have prototype designs for STM32Fxxx and MSP430 microcontroller variants.

With the upgrade in processor, comes an increase in clock frequency, increase in memory, improvement in peripheral features and speed and often an increase in ADC resolution.

ARM Cortex M4 Variant

For instance, the STM32F373 - a 72MHz ARM Cortex M4 in a 48 pin LQFP package comes with 3 UARTS, 2 SPI, 2 I2C,  up to 256Kbytes of Flash and up to 32Kbytes of RAM, plus a multichannel  12 bit ADC and 3 separate differential input Sigma Delta 16 bit ADCs. The Cortex M4F core comes with floating point and DSP operations. This is the ideal processor for high speed manipulation of analogue sampled data - for example 3 phase energy monitoring.  These processors start at £3.33 in 1 off.

MSP430 Variant

Another interesting processor is the ultra low power MSP430 FR series.  These are a 16 bit processor with 24MHz clock frequency, 12 bit ADC channels, 2 UARTs, SPI and I2C.  The unique feature of the FR series is that they use non-volatile ferroelectric FRAM memory for program and data storage. This allows exceptionally low power operation, retaining data after power down and capable of very fast write access - for high speed programming or datalogging.

Other Interesting Options

There is an increased interest in the use of FPGAs as a means of providing open source soft core processors.  One such option is the J1 Forth processor implemented on a low cost Lattice FPGA - using an open source toolchain.  The FPGA is available in an 84 QFN package just 7mm x 7mm which could be fitted onto the DIL 40 carrier board.

The WiNode 5 Hardware and Peripherals

Here I describe the hardware in a little more detail:

All of the hardware on WiNode 5 is designed to work at a 3.3V Vcc - thus keeping power requirements to a minimum.  Think carefully before you connect to any non- 3.3V shield!

The RFM69 wireless transceiver is still very much fundamental to the design, tracing it's roots to Jean Claude Wippler's JeeNode design - which used the ATmega328 and the RFM12B - and was probably about one of the first commercial wireless connected Arduino variants.

The RFM69 uses D10 for it's chip select, and the new INT2 interrupt that is available on the ATmega1284 - appearing on D8.

In addition to the encumbent RFM69, WiNode 5 also offers an un-committed "X-Bee" footprint (XB1)  This allows any additional wireless device to be added at a later date - based on a shield that conforms to the X-Bee pinout.

One of the biggest advances in the last 2 years is the emergence of very low cost WiFi modules based on the ESP8266 device. WiNode provides a 4x2 connector to allow an optional ESP-01 WiFi module to be plugged in directly so that it can communicate with the mcu using the additional UART 1. Note that WiFi support is optional and not central to the WiNode philosophy. Using a pre-built mass-produced plug-in module is the quickest and cheapest way to give it WiFi connectivity.

The original WiNode used a non-volatile 32Kbyte 23K256 SPI SRAM - backed up by a small super-capacitor.  This feature has also been retained with IC3 a SOIC-8 package, but now the device can be an SRAM of up to 128Kbytes or a ferrroelectric FRAM of up to 256Kbytes. Chip selection is by port B0.

Whilst novel when first introduced on my Nanode RF design of 2011, the micro SD is now common place on many development boards and SBCs.  I have retained the same basic uSD socket - soldered to the underside of the pcb and selected using port D9.

With lower power, yet more capable microcontrollers, there is frequently the need to make portable battery powered devices.  With the emergence of cheap Lithium polymer (LiPo) battery technology, WiNode 5 reflects this with support for an external battery.  This does not need to be a LiPo as an on-board boost converter, IC6, based on low cost MCP1640 allows a single cell to be boosted up to 3V3 so as to power the board.

One of the biggest bug-bears 5 years ago was the crippling cost of placing an FT232R device onto the board in order to allow connection to a PC via USB. Fortunately there are now several much lower cost alternatives available, and WiNode 5 uses the CH340G device in position IC7 - which is available for less that $0.50. It needs a 12MHz crystal  Y3 for correct USB timing.

The ATmega1284 provides a whole additional 8 bit port over and above what the ATmega328 supplies.  Six of these additional port lines are broken out to a header which is placed below the usual 6 pin Arduino "Power" header.  These additional pins are general purpose I/O, but also carry the signals associated with the JTAG interface.

As WiNode 5 uses several SPI devices, the additional port lines C0-C5 can be used to select additional devices on the SPI bus - this offers an easy route for expansion. More on expansion later.

Board Pin Out Details

WiNode 5 has been designed to act either as a stand alone, battery powered, wireless connected controller or monitor,  alternatively as part of a larger system.

//                    _________________________________
//      PC-RXD0 (D0) |o            A7  (D29)          o|A5  (D27)
//      PC-TXD0 (D1) |o            A6  (D28)          o|A5  (D26)
//   * ESP-RXD1 (D2) |o            B0  (D30)          o|A3  (D25)
//   * ESP-TXD1 (D3) |o            B1  (D31)          o|A2  (D24)
//     KBD_DATA (D4) |o                               o|A1  (D23)
//           PD (D5) |o                               o|A0  (D22)
//    MOUSE_CLK (D6) |o                                |
//   MOUSE_DATA (D7) |o                                |
//                   |                                o|VIN
//    RFM_INT2  (D8) |o                               o|0V
//      uSD_CS  (D9) |o                               o|0V
//      RFM_CS (D10) |o                               o|+3V3
//        MOSI (D11) |o                               o|+5V
//        MISO (D12) |o                               o|RES
//         SCK (D12) |o                               o|C5  (D21)
//          0V       |o                               o|C4  (D20)
//        AREF       |o         Open Hardware         o|C3  (D19)
//     I2C-SDA (D14) |o            WiNode 5           o|C2  (D18)
//     I2C-SCL (D15) |o                               o|C1  (D17)
//         ANT       |O                               o|C0  (D16)
//                   |_________________________________|

One of the first applications is that of a retro-computer system using the EVITA graphics driver board (above).  EVITA provides 1024x768 full 24bit colour graphics and an interface for PS/2 keyboard and mouse. It also allows a Wii Nunchuck controller to be plugged in.

With just WiNode 5 and EVITA, you have a complete retro-computing work/play station capable of driving a large screen monitor in just 2 small pcbs.  For portability you can use a Gameduino2 shield and a LiPo battery.

The slight conflict yet to be resolved here is when using the PS/2 keyboard with the ESP-01 WiFi. The keyboard generates a clock which needs to interrupt the mcu via INT1 (D3).

If used with a Gameduino 2 - INT 0 (D2) is also required. This kind of precludes the use of the ESP-01 WiFi - unless a work around is possible.

PCB Layout

WiNode was designed from the start to be smaller than the standard Arduino (Uno) - always considered to be too bulky and "Arty" rather than a simple rectangular board with sensible pin spacing and mounting holes. At 55x64mm it was intended to fit into a readily available plastic case with a battery enclosure. WiNode used predominantly through hod mounted components wherever possible.

WiNode 5 takes advantage of surface mounted components - as it was found that even the through hole parts presented too much of a soldering challenge to the average user.  This means that the circuitry can have a greater density - thus a smaller pcb and be cheaper to assemble - using pick and place machines and reflow soldering ovens.  It is really geared up for the modern electronics pcb assembly processes.  As a result, WiNode 5 packs a lot more functionality into a 50mm x 50mm pcb - some 71% of the board area of it's predecessor.

Whilst WiNode 5 can be hand soldered - it will take a few hours and you really need to be proficient at surface mount detailed soldering, have good light and good magnification available.

The board is based on a 50 mm x 50 mm standard footprint - with the aim of retaining Arduino header compatibility on the smallest pcb possible.   WiNode takes advantage of pre-built plug in modules - such as the ESP-01, the adxl 335 accelerometer module and the X-Bee module.  These can be bought on ebay cheaper than they can be built.

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