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ARINC 429 with a Host
Processor on an FPGA
2005 MAPLD International Conference
September 2005
Paper #236
Ian Land
Ryan Mohan
Introduction
Trends
 Technological advances in semiconductor size and speed,
coupled with the demand for custom logic, has led to the use of
FPGAs to implement several systems on one chip.
Electronic Bus Systems
 ARINC 429, MIL-STD-1553 and Ethernet-MAC are used in
systems with data link layer controller and host processor (uP)
 ARINC 429, a low-speed standard, requires no more than an 8bit host processor
An FPGA with integrated Data Link Controller and Host
 There are many considerations to be made when interfacing an
ARINC 429 bus Interface to a host processor, including
 Glue logic
 Application code
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ARINC 429 Overview
 ARINC 429 Standard
 ARINC 429 is a two-wire, point-to-point data bus that is applicationspecific for commercial and transport aircraft.
 Words are 32 bits in length and are transmitted at either 12.5 or 100
kbps to other system elements monitoring the 429 bus.
 ARINC 429 uses a unidirectional data bus standard - the Mark33 Digital
Information Transfer System with Tx & Rx on separate ports
 Actel ARINC 429
 Provides a complete Transmitter (TX) and Receiver (RX) solution for up
to 16 Tx and 16 Rx channels per the ARINC 429-16 specification
 Each channel has selectable data rate and label memory size
 Selectable system clock
 Programmable interrupt and FIFO generation
 Internal wrap-around testing
 Requires external (standard) ARINC 429 line drivers and line receivers
to interface to the ARINC 429 bus.
 An external host CPU is necessary to set up the core’s control
registers and initialize the label memory.
 Consists of three main blocks: Transmit, Receive, and CPU Interface.
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ARINC 429 IP Core
Bus Interface – a 32-Bit Data Word
 32nd Bit is the parity bit
 SSM is the Sign/Status Matrix
 ARINC Data formats are commonly encoded as Binary Data
(BNR) and Binary Coded Decimal (BCD)
 SDI, the Source Destination Identifier,
specifies the intended receiver
 The lower 8 Bits specify label word
 Varies depending on the particular
equipment and systems that
require interconnection
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ARINC 429 IP Core
 CPU Interface - direct CPU access to memory
 The Internal Registers can be read/written via a 9-Bit CPU address
 Channel Number can range from 0 to 15
 Tx/Rx Bit specifies whether a Tx or Rx register is being addressed
 Register Index specifies which Core429 internal register is being
addressed:
RX Registers:
0 – Data Register
TX Registers: 0 – Data Register
1 – Control Register
1 – Control Register
2 – Status Register
3 – Label Memory
2 – Status Register
3 – Unused
 Byte Index is used to control CPU read/writes depending on the CPU
datapath width (4 operations for 8-bit CPU, 2 for a 16-bit CPU, 1 for a 32-Bit
CPU)
 Easily interfaced to the Actel 8-bit 8051 uP
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8051 8-Bit Processor IP Core
Overview
 High performance, single-chip, 8-bit
microcontroller.
Features
 Some of the key features are:
 32-Bit I/O Ports
 Serial Port
 Two 16-Bit Timer/Counters
 Interrupt Controller – Four Priority
Levels with 13 Interrupt Sources
 Internal Data Memory Interface
 External Memory Interface
 Optional On-Chip Instrumentation
Debug Logic
 Optional Power-Saving Modes
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8051 Processor IP Core
 Internal Special Function Registers (SFRs) hold data
and control the timer, interrupts and serial ports
 Four separate memory regions accessible to the CPU
 DATA
 256 bytes of memory used for dynamic storage of program data such
as register, stack, and variable data. Typically, only the lower 128
bytes are directly addressable for program data. Addressing the
upper 128 bytes points to the SFR memory region.
 CODE
 64 KB of memory used for program storage and interrupt vectors.
 XDATA
 64 KB of memory used for storage of large data sets, customdesigned peripherals, and extended stack space if necessary.
 SFR
 Upper 128 bytes of DATA space is a combination of internal core
memory and external memory used for internal and external SFRs.
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ARINC 429 IP Interfaced to 8051 uP
System overview
 8051 IP is host CPU for ARINC 429 Bus Interface IP
 Implemented on one FPGA and integrated alongside other
Intellectual Property to create a single chip solution.
 Depends on the existence of easy to use interface logic
 The 8051 SFR Interface
 The 429 CPU Interface
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429 Interfaced to a 8051 uP
Interface Logic – 8051 SFR Interface
 Can service up to 101 External SFRs
 External SFRs can be used to interface with an off-core
peripheral, such as an ARINC 429 Bus Interface Core.
 Such a peripheral can use all addresses from the SFR address
space range 0x80 to 0xFF except those already implemented
within Core8051 as Internal SFRs.
 Although the SFR address space contains 8-bit addresses, the
external SFRs are addressed by a 7-bit address which corresponds
to the 8-bit internal address with the MSB removed.
External Special Function Register Interface
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429 Interfaced to a 8051 uP
Interface Logic – 8051 SFR Interface
 Due to the restricted memory map, an indirect addressing
capability was created
 Use memory-mapped registers (external SFRs) to hold the Core429 9-
Bit CPU address and the corresponding data.
 This 429-to-8051 system implements 4 external SFRs
 The 9-Bit CPU address requires two of the 8-Bit SFRs
 Leaves one SFR with 7 unused bits, providing a convenient means for
control/handshaking functions.
 One SFR is used for data written to the 429 Bus Interface IP from the
8051 Host and one SFR for data read by the Host 8051
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429 Interfaced to a 8051 uP
Interface Logic - ARINC 429 IP CPU Interface
 Allows the system CPU (8051 IP) to:
 Read/Write ARINC data to the ARINC 429 IP internal FIFO
 Access the 429 control and status registers
 Write to the 429 internal label memory
 This system requires that the 429 IP be configured with a
CPU_DATA_WIDTH of 8 Bits to interface to the 8-Bit 8051 CPU
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429 Interfaced to a 8051 uP
Interface Logic – Glue Logic
 Advantage: Only a small amount of glue logic is needed to integrate
the IP Cores if the 8051 and the ARINC 429 cores are in the same clock
domain
 This system (with 4 TX and 4 RX channels) is physically implemented
on an Actel Development Board where both cores operate off a
common 16 MHz clock.
CPU
I/F
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429 Interfaced to a 8051 uP
Interface Logic – Glue Logic
 Physically connects 429 IP to 8051 IP Host
 Enforces a communication protocol between the two cores
 In this Core429-Core8051 System, the 4 SFRs used (0x40, 0x41,
0x42, 0x43) are mapped to WRITE_ADDRESS0, WRITE_ADDRESS1,
WRITE_DATA, READ_DATA respectively
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429 Interfaced to a 8051 uP
Interface Logic – Glue Logic
 Implementation
 Decodes the sfraddr input from the 8051 Host by comparing against




the predefined memory-map
Controls the handshaking between the 8051 Host and 429 IP
Accumulates the 9-Bit CPU address from the 2 address SFRs and
sends it to Core429
Passes data between Core8051 and Core429 according to the
protocol discussed
The verilog implementation of this logic is available for viewing
8051 Application Code
 The system is able to execute user programs written in C and
cross-compiled into 8051 assembly code
 Application code that uses ARINC 429 functions must follow the
communication protocol discussed
 Sample application code is available for viewing
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Verification
 Verified cores individually
 Against datasheet and ARINC standard
 100% Code Coverage target
 Simulated the system
 Verify the interface between the cores
 HW/SW co-verification could help verification of ARINC data transfers
 Takes a substantial effort and long simulation run times
 Tested the hardware
 Efficient method was to program the FPGA and run the software
 Found no handshake between 8051 and ARINC 429 bus interface
 Reviewed the design after unsuccessful handshake
 Reviewed logic analyzer data, focused on 8051 SFR interface
 Changed communication protocol, re-designed glue-logic and changed
application code
 Handshake occurred
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Verification (p2)
 Application software tests Core429 to host CPU
 Early SW – write & read ARINC 429 data to individual registers
 Interface through RS-232 and hyper terminal on a PC
 More sophisticated software after handshake
 Loopback test and communication with 4 TX / 4 RX channels
 Once the handshake worked
1. Internal FPGA loop back test
2. Verify off-chip, but within the development platform
 Send signal from daughter card’s transmit to receive connections
3. Connect to ARINC 429 tester to debug, verify and validate
 With FPGA, we checked different ARINC 429 configurations
(loopback, etc.)
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Verification (p3)
Test against standard
 System vs. ARINC 429 data monitor
 TX- SW terminal interface via RS-232 to host CPU, which instructed
Core429 to send data to the tester via transmit channels
 RX – Tester sent ARINC data word to the bus which was received
by FPGA where we monitored RX FIFO with terminal
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429 IP Core Interfaced to a M68K
 System Overview
 A small amount of glue logic is needed to connect the 429
CPU Interface to the M68K address and data buses
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429 IP Core Interfaced to a M68K
 Several considerations for interfacing
 Match Clocks - ie. 20 MHz
 Match CPU_DATA_WIDTH on Core429 to 16-bit M68K data bus width
 Consider how the 429 Internal Registers and Memory are addressed
and accessed
 Directly mapping the 429 9-bit address to the relatively large M68K address
bus is an effective solution
 Requires that a 9-Bit address space in the M68K memory map be reserved
for communication with the 429 Bus Interface IP
 For example, 0x00000000-0x000001FF, could be reserved unless this address
space is pre-reserved by the M68K or another device
 Note that there can be up to 8 TX and 8 RX ARINC 429 channels implemented
while the MSB of the 9-Bit 429 CPU address will remain zero
 CPU Interface
 The 429 CPU Interface would perform the same function as in the 4298051 system
 Based on the variation of M68K used, the width of the cpu_dout and cpu_din
signals will be have to be adjusted accordingly
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429 IP Core Interfaced to a M68K
 Interface Logic – Glue Logic Block Diagram
 The glue logic block interfaces the 429 CPU Interface to
the M68K address and data bus.
 This block is required to follow a communication protocol that adheres
to the M68K bus communication protocol
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429 IP Core Interfaced to a M68K
Interface Logic – Glue Logic
 One possible communication protocol is shown below
Note: The above description is for a 16-Bit read/write. 32-Bit ARINC 429
data operations require two read/write operations with the appropriate
CPU address and data.
During 8-Bit operations (on the 429 control/status registers, and label
memory) only one of either UDS or LDS will be asserted at a time.
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429 IP Core Interfaced to a M68K
Interface Logic – Glue Logic/Application Code
 Another important consideration for an ARINC 429-to-Host
CPU system is its application
 Many applications are better implemented by using interrupts
to indicate to the CPU that an ARINC 429 event has occurred
 The 429-8051 system is part of a terminal interface that
continually waits for user commands, via a keyboard
 Thus the register polling approach is acceptable
 Most applications favor the use of interrupts over the register
polling approach used in the 429-8051 system
 Interrupts will free up the host CPU to perform other operations
and functions
 To completely specify the glue logic between the 429-to-M68K
system, the interrupt lines IPL0, IPL1, and IPL2 need to be
interfaced with the 429 Bus Interface IP Core
 If interrupts are used, the M68K assembly code used for 429
operations would reside in an Interrupt Subroutine (ISR)
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Lessons Learned
 Integrating cores is not a simple connect
 Multiple Clock Domains
 Single clock domain is the simplest, fastest and safest
 Address and Data Bus Widths
 How much address space does the core require relative to that available ?
 Does the core have the same data bus width as the processor ?
 What about performance
 At what rate does the processor need to access the core


Does it require direct access
Is slow polled access possible
 Application code must match the system and protocol
 System simulation is required
 You should always run system-based simulations
 It is quicker and better than trying to debug the FPGA in hardware
 Can try more configurations than on board (loopback, 4/4, 16/16)

Customers and local FAEs also verified 4/5, 4/8 and many other variations
 A design is not ‘integrated’ until it is hardware tested
 Core429/Core8051 would not communicate in HW
 Additional debugging of IP and application SW code for handshake
 Core429/68K is likely to need debugging, too.
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