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University of Leicester PLUME Ref: PLM-OBDH-COMSInterf-218-1Date: 18/03/2010

OBDH – COMS Interface Document

David Marshall, Gareth Gould

Date Updated Reference Number change

[29/11/2009] [PLM-OBDH-COMSInterf-218-0] First draft[18/03/2010] [PLM-OBDH-COMSInterf-218-1] Updated information and

configuration parameters.

Table of Contents

1 Introduction.......................................................................................................................1

2 COMS subsystem overview..............................................................................................1

3 Interface overview.............................................................................................................2

4 Modem Commands...........................................................................................................2

5 Modem Initialisation..........................................................................................................5

6 Baud Rate Settings...........................................................................................................5

7 References........................................................................................................................7

1 Introduction

This document describes the interface between the PLUME system board and the various components of the COMS subsystem. The interface is tested using the modem prototype board which contains both a PIC (Peripheral Interface Controller) modem and an RF Datatech transceiver.

All documents mentioned are referenced in section 7.

2 COMS subsystem overview

The COMS subsystem will receive control signals sent to the satellite from the ground station, as well as transmit system status, payload, ADCS and camera data. The data will be temporarily stored in the SD card until it’s transmitted.

The satellite must be in a certain state before it attempts to power-up the payload. For this reason, the satellite will transmit its state to the ground station, and wait for the ground station to request that the payload be powered-up.

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The COMS system will be powered on in Recovery mode and transmit a beacon to allow the ground station to track the satellite. The antenna will be deployed after 30 minutes. Intervals will be left in the beacon transmission to allow the reception of the first contact command from the ground station.

Should the satellite be unable to receive data from the ground station for whatever reason, to avoid the mission failing completely the satellite will enter Autonomous mode, where the important ADCS and payload data will be continuously transmitted.

3 Interface overview

The COMS subsystem connects to our Pumpkin FM430 flight module using the USART (Universal Synchronous/Asynchronous Receive/Transmit) peripheral interface built into the board’s MCU, a Texas Instruments MSP430F1612. The USART is set to asynchronous mode (UART) and uses the address-bit multiprocessor asynchronous communication format.

The COMS subsystem consists of an RF DataTech PIC (Peripheral Interface Controller) 16C711 modem, an RF DataTech LRT470-1 transceiver and an antenna. The flight controller communicates with the COMS subsystem entirely through the modem.

The flight controller connects to the modem through the modem connections shown in table 3.1.

Table 3.1: Modem connections

Modem pin Description Board pinRADRXD Outputs serial data H1.39 TXD_MHXRADTXD Serial data input H1.40 RXD_MHXRSSI Voltage

Received signal strength indicator

-

RAD+ Transmitter power supply -+5V 5 Volt Supply -TXPSUEN 10 Volt Supply -0V Ground H2.32 GNDTXP Transmitted power -

The flight controller communicates with the modem through the modem’s RADRXD and RADTXD connections; aside from the ground, the other pins are not required.

The UART character format is shown in Fig.3.1 and consists of a start bit (ST) (low), eight data bits (ABCDEFGH) (high or low), an address bit (X) (high or low) and a stop bit (SP) (high). The bit period is defined by the selected baud rate.

Figure 3.1: Character Format

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Table 3.2 shows the bus pins required to operate the COMS subsystem, extra to those that directly connect to the modem.

Table 3.2: Required pins for COMS subsystem

Pin Name I/O DescriptionH1.10 P4.6 O Set low to enable +5V_SW powerH2.2 P6.6 O Set low to enable transceiver interface

4 Connecting to the modem prototype

The modem prototype uses a DE-9 connector for serial RS-232 communication, however only 3 of the 9 pins are used. A diagram of the connector mounted on the side of the modem is shown in Fig.4.1.

Figure 4.1: Modem DE-9 female connector.

The modem is connected to a laptop using a custom DE-9 cable with an integrated level shifter shown in Fig.4.2. The level shifter switch must be set to the ‘on’ position for the connection to work correctly.

Figure 4.2: Level shifter

The modem is connected to the board using a different modified cable, with a DE-9 connector at one end and three pins for connection to the board at the other shown in Fig.4.3. The pins are each labelled for convenience.

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Figure 4.3: Modem cable connected to the development board bus

The two modems must be powered using an external power supply. In flight this will be provided by the PSU subsystem, during testing a DC power supply unit is used. The power supply must be set to 5V and maximum current for the modems to operate correctly. The power supply connected to the two modems is shown in Fig.4.4.

Figure 4.4: Modems connected to the power supply.

To simulate the transmission of data using aerials, an attenuator is used to damp the signals passing between the modems. Each modem has a coaxial cable which must be connected to each end of the attenuator as shown in Fig.4.5. The right hand 20dB switch should be on and the other switches left off for testing.

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Figure 4.5: Attenuator connected to the modems.

The connections to the board have sections of bare wire for connecting to a logic analyser, an example of the signals going to and from the modem as monitored with a logic analyser is shown in Fig.4.5.

Figure 4.5: Signals to and from the modem during modem initialisation monitored with a logic analyser. The top and bottom rows show signals from the board and signals from the

modem respectively.

Specifics of operating the logic analyser are detailed in the document PLM-COMS-BenchTestingComs-129-3.

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5 Modem Commands

The ACK (0000,0000) character is returned following every sent character to indicate that the modem is ready to receive the next character. This applies to both data and control characters. The only exceptions are replies to WAKE and ENQ.

The modem can be put into a sleep mode to save power. However any commands it receives in this state will not be processed. To wake the modem the WAKE command should be sent, the modem will respond with WAKAK and is ready to receive new commands. Should the modem respond with WAKAK unexpectedly, then the modem was asleep and the last character should be sent again. Even if the modem is already awake, it will respond with WAKAK to the WAKE command.

Table 2: Modem commands

Send Description Add Binary Hex Receive Add Binary Hex

SLEEP System sleeps C 1000,0000 80 ACK C 0000,0000 00

WAKE System wakes C 1111,1111 FF WAKAK C 0000,0010 02

TXSTT+ Data

Transmit C 101E,DCBA+ Data

- ACK C 0000,0000 00

TXEND End transmit mode

C 0010,0000 20 ACK C 0000,0000 00

ENQ Enquire C 010E,DCBA - RQD+ Data

C 0000,0001+ Data

01

ENQ0 Enquire for status

C 0100,0000 40 RQD+ Data

C 0000,0001+ Data

01

ENQ1 Enquire for type

C 0100,0001 41 RQD+ Data

C 0000,0001+ Data

01

ENQ2 Enquire for version

C 0100,0010 42 RQD+ Data

C 0000,0001+ Data

01

TEST Enable test mode

C 011E,DCBA - ACK? C 0000,0000? 00?

TEST1 Test mode 1 C 0110,0001? 61 ACK? C 0000,0000? 00?




TEST0 Exit test mode C 0110,0000 60 ACK C 0000,0000 00

CFG Configuration C 000E,DCBA+ Data

- ACK C 0000,0000 00

Notes for modem commands with modifiable character bits follow.

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TXSTT - Transmit – Command - 101E,DCBA

The lead in delay is the amount of time before the data is transmitted; this allows time to raise TXP from 0V to 5V so the power is correct.

If E is 1, then bits DCBA give the lead in delay as a multiple of 80ms (e.g. 0001=80ms, 0010=160ms etc), range 0-1200ms.

If E is 0, then bits DCBA give the lead in delay as a multiple of 5ms (e.g. 0001=5ms, 0010=10ms etc), range 0-75ms.

Currently E,DCBA is set to the maximum 1,1111 for testing, giving a lead in delay of 1.2 seconds.

ENQ – Enquire – Command - 010E,DCBA

Returns the RQD character rather than ACK, this denotes that the next data byte will be the control data requested. The modem is ready to receive the next character once both bytes have been received.

The bits ABCDE control the format of the returned data byte.

ENQ0 - ABCDE = 00000 - 0100,0000

Returns status MLKJ,IHGF where -

F = 1 = channel busyG = 1 = data carrier detectedH = 1 = transmitter onI = 1 = test mode onJ = 1 = synchroniser out of lock (error)K = 1 = rx buffer overflow (error)L = 1 = tx buffer overflow (error)M = 1 = power on reset status (error) – Set when the modem is powered on, as a result of a watchdog reset on the radio, or if an incomplete configuration is performed on the modem.

ENQ1 - ABCDE = 00001 – 0100,0001

Returns an 8 bit identifier to signify the radio type e.g. UHF MPT1329.

ENQ2 - ABCDE = 00010 – 0100,0010

Returns the code version running in the radio.

Note: The radio identifier and code version are not described further in the manual. The formats of the returned characters are unknown.

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TEST – Test – Command – 011E,DCBA

Puts in the PIC into a test mode.

ABCDE control the test number to be run.

TEST0 – ABCDE = 00000 – 0110,0000

Restores normal PIC operation.

TEST1 – ABCDE = 00001 – 0110,0001 ?

Transmit continuous zero tone.

TEST2 – ABCDE = 00010 – 0110,0010 ?

Transmit continuous one tone.

TEST3 – ABCDE = 00100 – 0110,0100 ?

Transmit continuous one carrier.

TEST4 – ABCDE = 01000 – 0110,1000 ?

Transmit 50Hz square wave.

CFG – Configuration – Command – 000E,DCBA

Configures the PIC.

ABCDE control which parameter to set.

All 7 configuration parameters must be set in sequence from CFG0 to CFG6. Should the configuration not complete, the modem will not function correctly.

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6 Modem Initialisation

The modem requires configuration commands in a specific order to set it up. Each command returns ACK.

The parameters are provided by the COMS team.

Send Description Add Binary Hex Sets

CFG0 Configuration parameter 0

C 0000,0000 00

TXPLL0 msd of the synthesiser TX code

D 1100,1110 CE


C 0000,0001 01

TXPLL1 lsd of the synthesiser TX code

D 1010,0000 A0


C 0000,0010 02

RXPLL0 msd of the synthesiser RX code

D 1011,0010 B2


C 0000,0011 03

RXPLL1 lsd of the synthesiser RX code

D 1000,0000 80


C 0000,0100 04

OFFSET synthesiser offset D 0100,0000 40


C 0000,0101 05

FFBAUD FFSK baud rate D 1000,0000 80 Baud rate = 1200bd


C 0000,0110 06

FFMODE FFSK format D 0000,0100 04 8 bit, parity off, one stop bit

7 Baud Rate Settings

The mspgcc website has a Baud Rate calculator at http://mspgcc.sourceforge.net/baudrate.html. This can be used to calculate the required settings of the registers BR0, BR1 and MCTL for any given clock speed and required baud rate.

Information from the calculator for the correct clock rate is given below.

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http://mspgcc.sourceforge.net/baudrate.html


/* uart calculator: http://mspgcc.sourceforge.net/baudrate.html this program license is at: http://www.fsf.org/licenses/licenses.html#GPL this program is distributed WITHOUT ANY WARRANTY

clock: 762000Hz desired baud rate: 9600bps division factor: 79.4 effective baud rate: 9596.98bps maximum error: 0.6561us 0.63%

time table (microseconds): event desired effective error error% startbit->D0 104.17 103.67 +0.492 +0.47 D0->D1 208.33 208.66 -0.328 -0.31 D1->D2 312.50 312.34 +0.164 +0.16 D2->D3 416.67 417.32 -0.656 -0.63 D3->D4 520.83 521.00 -0.164 -0.16 D4->D5 625.00 624.67 +0.328 +0.31 D5->D6 729.17 729.66 -0.492 -0.47 D6->D7 833.33 833.33 +0 +0.00 D7->stopbit 937.50 937.01 +0.492 +0.47 end of stopb 1041.67 1041.99 -0.328 -0.31*/UBR00=0x4F; UBR10=0x00; UMCTL0=0x4A; /* uart0 762000Hz 9596bps */UBR01=0x4F; UBR11=0x00; UMCTL1=0x4A; /* uart1 762000Hz 9596bps */

8 Ground station controls

The ground station needs to be able to send control signals to the satellite which will cause the flight controller to perform a variety of tasks. The tasks are as follows.

a) Report statusb) Power-up payloadc) Power-down payloadd) Take photoe) Transmit data from the SD card.

9 References

http://cubesat.wikidot.com/local--files/transceiver-page/PLM-COMS-LRTManual-136-1.pdf

http://cubesat.wikidot.com/local--files/coms/PLM-COMS-DataInter-117-2.docx

http://cubesat.wikidot.com/local--files/coms/PLM-COMS-BenchTestingComs-129-3.docx

http://focus.ti.com/lit/ug/slau049f/slau049f.pdf

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http://focus.ti.com/lit/ug/slau049f/slau049f.pdf

http://cubesat.wikidot.com/local--files/coms/PLM-COMS-BenchTestingComs-129-3.docx

http://cubesat.wikidot.com/local--files/coms/PLM-COMS-DataInter-117-2.docx

http://cubesat.wikidot.com/local--files/transceiver-page/PLM-COMS-LRTManual-136-1.pdf

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