APB protocol v1.0

APB Protocol v 1.0
● Introduction
● Transfers
● Operating States
● Signal Descriptions

Conventions
This section describes the conventions that this
specification uses:
•Typographical
●
Timing diagrams
•Signals

Typographical
● This specification uses the following typographical conventions:
● Italic - Highlights important notes, introduces special
terminology,denotes internal cross-references, and citations.
● Bold - Highlights interface elements, such as menu names. Denotes
ARM processor signal names. Also used for terms in descriptive lists,
where appropriate.
● Monospace - Denotes text that you can enter at the keyboard, such as
commands, file and program names, and source code.
● Monospace - Denotes a permitted abbreviation for a command or
option. You can enter the underlined text instead of the full command
or option name.

●monospace italic - Denotes arguments to monospace text where the
argument is to be replaced by a specific value.
●monospace bold - denotes language keywords when used outside
example code.
●<and> - Angle brackets enclose replaceable terms for assembler syntax
where they appear in code or code fragments. They appear in normal font
in running text. For example:
● MRC p15, 0 <Rd>, <CRn>, <CRm>, <Opcode_2>
● The Opcode_2 value selects which register is accessed.

Timing diagrams
● The figure named Key to timing diagram conventions
explains the components used in timing diagrams.
Variations, when they occur, have clear labels. You
must not assume any timing information that is not
explicit in the diagrams. Shaded bus and signal areas
are undefined, so the bus or signal can assume any
value within the shaded area at that time. The actual
level is unimportant and does not affect normal
operation.

Signals
● The signal conventions are:
● Signal level - The level of an asserted signal depends on
whether the signal is active-HIGH or active-LOW. Asserted
means HIGH for active-HIGH signals and LOW for active-
LOW signals.
● Prefix P - Denotes AMBA 3 APB signals.
● Suffix n - Denotes AXI, AHB, and AMBA 3 APB reset
signals.

About the AMBA 3 APB
● The APB is part of the AMBA 3 protocol family. It provides a low
cost interface that is optimized for minimal power consumption and
reduced interface complexity.
● The APB interfaces to any peripherals that are low-bandwidth and do
not require the high performance of a pipelined bus interface. The
APB has unpipelined protocol.
● All signal transitions are only related to the rising edge of the clock to
enable the integration of APB peripherals easily into any design flow.
Every transfer takes at least two cycles.
● The APB can interface with the AMBA Advanced High-performance
Bus Lite (AHB-Lite) and AMBA Advanced Extensible Interface
(AXI). You can use it to provide access to the programmable control
registers of peripheral devices.

Changes for AMBA 3 APB Protocol
Specification v1.0
● This version includes:
● A ready signal, PREADY, to extend an APB
transfer.
● An error signal, PSLVERR, to indicate the failure of
a transfer.

Transfers
● Write transfers
● Two types of write transfer are described in this
section:
● With no wait states
● With wait states.

The write transfer starts with the address, write data, write signal
and select signal all changing after the rising edge of the clock.
The first clock cycle of the transfer is called the Setup phase. After
the following clock edge the enable signal is asserted, PENABLE,
and this indicates that the Access phase is taking place. The
address, data and control signals all remain valid throughout the
Access phase. The transfer completes at the end of this cycle.
The enable signal, PENABLE, is deasserted at the end of the
transfer. The select signal, PSELx, also goes LOW unless the
transfer is to be followed immediately by another
transfer to the same peripheral.

With wait states
● Figure shows how the PREADY signal from the slave can
extend the transfer. During an Access phase, when PENABLE is
HIGH, the transfer can be extended by driving PREADY LOW.
The following signals remain unchanged for the additional
cycles:
● address, PADDR
● write signal, PWRITE
● select signal, PSEL
● enable signal, PENABLE
● write data, PWDATA.

Write transfer with wait states

● PREADY can take any value when PENABLE is LOW.
This ensures that peripherals that have a fixed two cycle
access can tie PREADY HIGH.
● NOTE - It is recommended that the address and write
signals are not changed immediately after a transfer but
remain stable until another access occurs. This reduces
power consumption.

Read transfers
● Two types of read transfer are described in this
section:
● With no wait states
● With wait states.

With no wait states
Figure shows a read transfer. The timing of the address, write, select,
and enable signals are as described in Write transfers on page 2-2. The
slave must provide the data before the end of the read transfer

With wait states
● Figure shows how the PREADY signal can extend the transfer. The
transfer is extended if PREADY is driven LOW during an Access
phase. The protocol ensures that the following remain unchanged for
the additional cycles:
● address, PADDR
● write signal, PWRITE
● select signal, PSEL
● enable signal, PENABLE.
● Figure shows that two cycles are added using the PREADY signal.
However, you can add any number of additional cycles, from zero
upwards.

Error response
● You can use PSLVERR to indicate an error condition on an APB
transfer. Error conditions can occur on both read and write
transactions.
● PSLVERR is only considered valid during the last cycle of an
APB transfer, when PSEL, PENABLE, and PREADY are all
HIGH
● It is recommended, but not mandatory, that you drive PSLVERR
LOW when it is not being sampled. That is, when any of PSEL,
PENABLE, or PREADY are LOW.

Error response contd..
● Transactions that receive an error, might or might not have changed
the state of the peripheral. This is peripheral-specific and either is
acceptable. When a write transaction receives an error this does not
mean that the register within the peripheral has not been updated.
Read transactions that receive an error can return invalid data. There
is no requirement for the peripheral to drive the data bus to all 0s for a
read error.
● APB peripherals are not required to support the PSLVERR pin. This
is true for both existing and new APB peripheral designs. Where a
peripheral does not include this pin then the appropriate input to the
APB bridge is tied LOW.

Write transfer
● Figure shows an example of a failing write transfer that
completes with an error.

Read transfer
● A read transfer can also complete with an error response, indicating
that there is no valid read data available. Figure 2-6 on page 2-7
shows a read transfer completing with an error response.

Mapping of PSLVERR
● When bridging:
● From AXI to APB An APB error is mapped back to
RRESP/BRESP = SLVERR. This is achieved by mapping
PSLVERR to the AXI signals RRESP[1] for reads and
BRESP[1] for writes.
● From AHB to APB PSLVERR is mapped back to HRESP =
ERROR for both reads and writes. This is ac

Operating States
Figure shows the operational activity of the APB.

The state machine operates through the following
states:
● IDLE - This is the default state of the APB.
● SETUP - When a transfer is required the bus moves
into the SETUP state, where the appropriate select
signal, PSELx, is asserted. The bus only remains in
the SETUP state for one clock cycle and always
moves to the ACCESS state on the next rising edge
of the clock.

Contd....
● ACCESS - The enable signal, PENABLE, is asserted in the
ACCESS state. The address, write, select, and write data
signals must remain stable during the transition from the
SETUP to ACCESS state. Exit from the ACCESS state is
controlled by the PREADY signal from the slave:
1 - If PREADY is held LOW by the slave then the peripheral bus remains in
the ACCESS state.
2 - If PREADY is driven HIGH by the slave then the ACCESS state is exited
and the bus returns to the IDLE state if no more transfers are required.
Alternatively, the bus moves directly to the SETUP state if another transfer
follows.

Signal Descriptions
AMBA 3 APB signals

APB protocol v1.0

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