Design and Implementation of AMBA ASB apb bridge

Proceedings of 2013 International Conference on Fuzzy Theory and Its Application
National Taiwan University of Science and Technology, Taipei, Taiwan, Dec. 6-8, 2013
Abstract—The 32 bit AMBA ASB APB Bridge provides an
interface between the Advanced System Bus (ASB) and the
Advanced Peripheral Bus (APB). It inserts wait states for
burst of read or write transfers when the ASB must wait for
the APB. The bridge is designed to respond to transaction
requests from the currently enabled ASB master. The ASB
transactions are converted into APB transactions. APB
peripherals do not need a clock input as the APB access is
timed with a strobe signal generated by the ASB to APB
bridge interface. The AMBA ASB APB Bridge is modeled
using Verilog HDL and validated on SPARTAN 3E and
results are visualized on ChipScope Pro.
Keywords:- AMBA, ASB, APB, Verilog, SoC
I. INTRODUCTION TO AMBA
The Advanced Microcontroller Bus Architecture
(AMBA) interconnect protocol is developed by ARM
and it is the de facto industry-standard on-chip inter-
connect specification that serves as a framework for SoC
designs, effectively providing the “digital glue” that
binds IP process together. It is also the backbone of
ARM’s design reuse strategy. AMBA specifies a hie-
rarchy of bus types, tailored to differing priorities found
across the interconnect structure of SoC designs.
A. AMBA Buses
Three distinct buses are defined within the AMBA
specifications [1]. Advanced High-performance Bus
(AHB), Advanced System Bus (ASB), Advanced Pe-
ripheral Bus (APB).
Figure 1 shows, Microprocessors, DMA controllers,
memory controllers and other higher performance blocks
are suited for connection to the AHB/ASB. Lower per-
formance blocks such as UARTs, General Purpose In-
put/Output (GPIO) and Timers are suited for connection
to the APB.
The AMBA specification has been derived to satisfy four
key requirements. 1) To facilitate the right-first-time
development of embedded microcontroller products with
one or more CPUs or signal processors. 2) To be tech-
nology independent and ensure that highly reusable pe-
ripheral and system macro cell can be migrated across a
diverse range of IC processes and be appropriate for
full-custom, standard cell and gate array technologies. 3)
To encourage modular system design to improve
Fig. 1. Advanced Microcontroller Bus Architecture [1]
processor independence, providing a development road
map for advanced cached CPU cores and the develop-
ment of peripheral libraries. 4) To minimize the silicon
infrastructure required to support efficient on-chip and
off-chip communication for both operation and manu-
facturing test.
II. AMBA ASB
The Advanced System Bus (ASB) specification de-
fines a high-performance bus that can be used in the de-
sign of high performance 16 and 32-bit embedded mi-
crocontrollers. AMBA ASB supports the efficient con-
nection of processors, on-chip memories and off-chip
external memory interfaces with low-power peripheral
macrocell functions. The bus also provides the test in-
frastructure for modular macrocell test and diagnostic
access. The ASB is a high-performance pipelined bus,
which supports multiple bus masters. The basic flow of
the bus operation is, the arbiter determines which master
is granted access to the bus, when granted a master in-
itiates transfers on the bus and the decoder uses the high
order address lines to select a bus slave and then the slave
provides a transfer response back to the bus master and
data is transferred between the master and slave.
There are three types of transfer that can occur on the
ASB: NONSEQUENTIAL: Used for single transfers or
for the first transfer of a burst.
SEQUENTIAL: Used for transfers in a burst. The ad-
dress of a sequential transfer is always related to the
previous transfer.
ADDRESS-ONLY: Used when no data movement is
required. The three main uses for address only transfers
are for idle cycles, for bus master handover cycles and
for speculative address decoding without committing to a
data transfer.
Manu B.N Prabhavathi P
M.Tech Student Associate Professor
Department of Electronics and Communication Department of Electronics and Communication
BNM Institute of Technology, Bangalore, India BNM Institute of Technology, Bangalore, India
e-mail: manubn88@gmail.com e-mail: prabha@bnmit.org
Design and Implementation of AMBAASB APB Bridge
234
978-1-4799-0386-3/13/$31.00 ©2013 IEEE

III. AMBA APB
The Advanced Peripheral Bus (APB) is part of the
AMBA hierarchy of buses and is optimized for minimal
power consumption and reduced interface complexity.
The AMBA APB appears as a local secondary bus that is
encapsulated as a single ASB slave device. APB pro-
vides a low-power extension to the system bus which
builds on ASB signals directly.
The APB Bridge appears as a slave module which han-
dles the bus handshake and control signal retiming on
behalf of the local peripheral bus. By defining the APB
interface from the starting point of the system bus, the
benefits of the system diagnostics and test methodology
can be exploited. The AMBA APB should be used to
interface to any peripherals which are low bandwidth and
do not require the high performance of a pipelined bus
interface.
IV. IMPLEMENTATION
The implementation technique involves modeling the
following:
1) Arbiter
2) Decoder
3) APB Bridge
4) Reset controller
5) Remap and pause controller
A. Arbiter
The AMBA bus specification is a multi-master bus
standard. As a result, a bus arbiter is needed to ensure
that only one bus master has access to the bus at any
particular point in time. Each bus master can request the
bus, the arbiter decides which has the highest priority and
issues a grant signal accordingly. Every system must
have a default bus master which is granted use of the bus
during reset, or when no other bus master requires the
bus. The ASB arbitration is controlled by the AREQ,
AGNT, BLOK and BWAIT signals. When an ASB
master requires use of the bus, it sets its AREQ output
line HIGH. This is sampled by the arbiter, on the falling
edge of BCLK, and the AGNT outputs change according
to the arbitration priority scheme used by the system. The
BLOK and BWAIT signals are used to extend the
granted period to allow masters to finish transfers before
bus master handover begins. If BLOK is set HIGH by the
current master, and a higher priority master requests the
bus, handover will not start until BLOK is set LOW,
showing that the locked transfer has finished.
The following arbitration priorities (from highest to
lowest) are implemented in the default system: Test in-
terface controller (highest), Bus master 1, Bus master 2,
ARM processor (lowest).
B. Decoder
The decoder performs three functions, it generates the
slave select signals (DSELx) for each of the bus slaves,
indicating that a read or write access to that slave is
Fig. 2. Decoder memory map [2]
Fig. 3. FSM Without decode cycle [2]
Fig. 4. FSM With decode cycle [2]
required, it generates the slave response signals
(BWAIT, BLAST and BERROR) during address-only
transfers, when no slave is selected, it can act as a simple
protection unit which prevents attempts to access a pro-
tected area of the memory map shown in Figure 2.
The decoder can be implemented in two ways with and
without decode cycles as shown in Figure 3 and Figure 4
respectively.
C. APB Bridge
The APB Bridge provides an interface between the
ASB and the APB. It continues the pipelining of the ASB
by inserting wait cycles on the ASB only when they are
needed. It inserts them for burst transfers or read trans-
fers when the ASB must wait for the APB. The imple-
mentation of this block contains: a state machine, which
is independent of the device memory map, ASB address,
and data bus latching, combinatorial address decoding
logic to produce the peripheral select PSELx
235

Fig. 5. Peripheral memory map [2]
Fig. 6. APB Bridge state machine [2]
signals. Figure 5 shows the peripheral memory map of
the peripheral bridge. The finite state machine which
controls the working of the peripheral bridge is shown in
Figure 6.
D. Reset Controller
The AMBA specification defines a single reset signal
BnRES which indicates the current reset status of the
system. Assertion (the falling edge) of BnRES is asyn-
chronous to BCLK. De-assertion (the rising edge) of
BnRES is synchronous to the falling edge of BCLK.
BnRES is only asserted during a Power-On Reset con-
dition, caused by the assertion of the POReset signal. The
POReset input is an asynchronous input, and hence a
synchronizing d-type is required to eliminate propaga-
tion of metastable values.
The state machine which controls the working of reset
controller is shown in Figure 7.
E. Remap and Pause Controller
The remap and pause controller has three modes.
Reset status: This enables software to determine whether
the last reset was a Power On Reset (POR) or a soft reset.
Fig. 7. Reset controller state machine [2]
Remap memory: On reset the internal RAM is mapped
out and the external memory is mapped into location
0x00000000 which is the boot location for the ARM
processor. The reset memory map is cancelled by writing
to a register in this peripheral.
Pause mode: The microcontroller only supports one
simple power saving mode called Pause. This halts all
bus activity (but not the system clock) and waits for an
interrupt signal from the interrupt controller before res-
tarting the system.
The Remap output register is used to hold the value of
Remap, which is used to determine the memory map that
is used by the system. It is set LOW on reset, and is set
HIGH when the Remap address is written to with any
value. Once set HIGH, it can only be set LOW by a
system reset.
V. SPECIFICATIONS
The design of AMBA ASB APB Bridge has the fol-
lowing specifications:
• 50 MHz bus clock
• 32 bit address bus
• 32 bit data bus
• Decode and without decode cycles
• Arbitration: Fixed priority [1:4]
• No. of bus masters: 4
• Peripheral: Remap and Pause
VI. SIMULATION RESULTS
Xilinx 13.2 is used for modeling of AMBA ASB APB
Bridge and ISim is used for simulation results. Figure 8
shows the results with the priority based arbitration
scheme and the read and then the write operations
performed on the remap and pause peripheral. PSELx
indicates that the peripheral is chosen and when
BWRITE is low it indicates read operation and when
high a write operation is performed on the peripheral.
The status of remap and pause peripheral is shown in
Figure 9 when no interrupts are coming from the ARM
processor the system stays in a low power mode, when
interrupted the pause signal goes high. The remap signal
indicates if the system has recovered from a power on
reset state and when high divides the internal memory
into internal and external memory.
The RTL schematic is shown in Figure 12. The design
is implemented on SPARTAN 3E and the results are
analyzed using ChipScope Pro which is shown in Figure
10 and 11. The synthesis reports are shown in Table 1
and Table 2.
236

Fig. 8. Simulation results of ASB APB Bridge
Fig. 9. Remap and Pause outputs
Fig. 10. Chipscope Pro analysis
Fig. 11. ChipScope Pro analysis with active high power on reset
Table 1. Area report
Instance Cells Cell Area Net Area Wireload
Bridge 234 2141 0 <none> (D)
Table 2. Power report
Instance Cells Leakage
Power (nW)
Dynamic
Power
(nW)
Total
Power
(nW)
Bridge 234 5591.030 32217.318 37808.
347
Fig. 12. RTL Schematic
ARBITER
GRANTS
ARBITER
REQUESTS
READ
WRITE
PERIPHERAL
SELECTED
REMAP
ADDRES
PAUSE
ADDRESS
GRANT
REQUESTS
DEFAULT
MASTER
POWER
ON RESET
RESET
CONTROLLER
APB BRIDGE
DECODER
REMAP AND
PAUSE
ARBITER
237

VII. CONCLUSIONS
This paper presents modeling and simulation results of
AMBA ASB APB Bridge. From the simulation results
we can conclude that upto four masters can be connected
over the ASB with a priority based arbitration scheme
and these masters can communicate with the peripherals
over the APB Bridge. In a way the APB Bridge provides
a direct translation of the ASB masters to the peripherals
connected to the APB.
VIII. REFERENCES
[1] AMBA Specifications 2.0, Copyright ARM Limited 1999.
[2] ASB Example AMBA System, Technical Reference Manual.
[3] Kanchan Warathe, Dinesh Padole, Preeti Bajaj “A Design Ap-
proach To AMBA (Advanced Microcontroller Bus Architecture)
Bus Architecture with Dynamic Lottery Arbiter”
978-1-4244-4859-3/09/$25.00 ©IEEE 2009.
[4] Anurag Shrivastavastava, G.S. Tomar, Kamal K Kalra “Efficient
Design and Performance analysis for AMBA bus Architecture
Based System-on-Chip”, 2010 International Conference
on Computational Intelligence and Communication Networks.
[5] ZHANG Qing-li, W Ming-yan, WANG Jin-xiang, YE Yi-zheng,
LAI Feng-chang, “The Design of AMBA AHBNCI Wrapper”
0-7803-7889-X/03/$17.00@2003 IEEE.
[6] USHA A. JADHAV, M.M Jadhav A HIGH THROUGHPUT
AMBA AHB PROTOCOL, “International Journal of
Engineering Science and Technology”,
Vol. 2(5), 2010,1233-1241.
[7] Verilog HDL by Samir Palnitkar, Prentice Hall 1996.
[8] SPARTAN 3E and ChipScope Pro reference manuals by
Xilinx Corp.
[9] An Overview of SoC Bus Architectures, J. Ganesan Vojin
Oklobdzija/Digital Systems and Applications.
238

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