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AMBA 2 AHB - General - Frequently Asked Questions (Downloaded date: Friday 4th July, 2008)
Contents AMBA ....General : What system support is required if a slave can be powered down or have its clock stopped? ....General : When can Early Burst Termination occur ....General: Can HTRANS change whilst HREADY is low? ....General: Can a BUSY transfer occur at the end of a burst? ....General: Can a master change the address/control signals during a waited transfer? ....General: Can an AHB master be connected directly to an AHB slave? ....General: Do all slaves have to support the BUSY transfer type? ....General: Does the address have to be aligned, even for IDLE transfers? ....General: How many masters can there be in an AHB system? ....General: How should AHB to APB bridges handle accesses that are not 32-bits? ....General: Is HREADY an input or an output from slaves? ....General: Is a default slave really necessary? ....General: Is a dummy master really necessary? ....General: Is it legal for a master to change HADDR when a transfer is extended? ....General: Is it specified that HPROT, HSIZE and HWRITE remain constant throughout a burst? ....General: The specification recommends that only 16 wait states are used. What should you do if more than 16 cycles are needed? ....General: What are the different bursts used for? ....General: What default state should be used for the HREADY and HRESP outputs from a slave? ....General: What is a default slave? ....General: What is the difference between a dummy bus master and a default bus master? ....General: What is the recommended default value for HPROT? ....General: What is the state of the AHB signals during reset? ....General: What sequences of transfers types (HTRANS) can occur on the bus? ....General: When a master rebuilds a burst which has been terminated early are there any limitations on how it rebuilds the burst? ....General: Why is a burst not allowed to cross a 1 kilobyte boundary? ....How do you connect an AHB Master to an AHB-lite system? ....How do you connect an AHB slave to an AHB-lite system? ....How do you connect an AHB-lite Master to a full AHB system? ....How do you connect an AHB-lite Slave to a full AHB system? ....How does AHB differ from AHB-lite? ....How many clock cycles should the reset signal in an AMBA system be asserted for?
AMBA 1) General : What system support is required if a slave can be powered down or have its clock stopped? Applies to: AMBA AHB If a slave access is attempted while that slave is in a power down state or has had its clock stopped, you must ensure that an access will cause the power/clock to be restored, or else configure the AHB decoder up to redirect any such accesses to the dummy slave so that the system does not hang forever when an access to the device is made when it is disabled. Redirecting the access in this way will ensure that random "IDLE" addresses are treated with the HREADY high and HRESP=OKAY default response, but real accesses (NONSEQ or SEQ) will be detected with an ERROR response.
2) General : When can Early Burst Termination occur
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Applies to: AMBA AHB Bursts can be early terminated either as a result of the Arbiter removing the HGRANT to a master part way through a burst, or after a slave returns a non-OKAY response to any beat of a burst. Note however that a master cannot decide to terminate a defined length burst unless prompted to do so by the Arbiter or Slave responses. All AHB Masters, Slaves and Arbiters must be designed to support Early Burst Termination.
3) General: Can HTRANS change whilst HREADY is low? Applies to:AMBA AHB In general, an AHB master should not change control signals whilst HREADY is low. However it is allowable to change HTRANS in the following conditions: • HTRANS = IDLE The AHB master is performing internal operations and has not yet committed to a bus transfer. However during the AHB wait states (HREADY low) the master may determine that a bus transfer is required and change HTRANS on the next cycle to NONSEQ. • HTRANS = BUSY HTRANS is being used to give the master time to complete internal operations, which may be entirely independent of HREADY (i.e. wait states on the AHB). Therefore HTRANS can change on the next cycle to any legal value, i.e. SEQ if the burst is to continue, IDLE if the burst has completed, NONSEQ if a separate burst is to begin. • HRESP = SPLIT/RETRY As stated in the AHB specification, a master must assert IDLE on HTRANS during the second cycle of the twocycle SPLIT or RETRY slave response so HTRANS will change value from the first cycle to the second cycle of the response. • HRESP = ERROR The master is permitted to change HTRANS in reaction to an ERROR response in the same way as in reaction to a SPLIT/RETRY response and cancel any further beats in the current burst (even if HBURST is indicating a defined-length burst). In this case HTRANS changes to IDLE on the second cycle of the response. Alternatively, the master is permitted to continue with the current transfers. See also: • AMBA 2.0 Specification • •
4) General: Can a BUSY transfer occur at the end of a burst? Applies to:AMBA AHB A BUSY transfer can only occur at the end of an undefined length burst (INCR). A BUSY transfer cannot occur at the end of a fixed length burst (SINGLE, INCR4, WRAP4, INCR8, WRAP8, INCR16, WRAP16). See also: • • • • • •
AMBA 2.0 Specification Do all slaves have to support the BUSY transfer type? Can a master deassert HLOCK during a burst? Why is a burst not allowed to cross a 1 kilobyte boundary? Is it specified that HPROT, HSIZE and HWRITE remain constant throughout a burst? What are the different bursts used for?
5) General: Can a master change the address/control signals during a waited transfer? Applies to:AMBA AHB
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Yes. If the address/control signals are indicating an IDLE transfer then the master can change to a real transfer (NONSEQ) when HREADY is low. However, if a master is indicating a real transfer (NONSEQ or SEQ) then it cannot cancel this during a waited transfer unless it receives a SPLIT, RETRY or ERROR response. See also: • AMBA 2.0 Specification • Is it legal for a master to change HADDR when a transfer is extended? • Can HTRANS change whilst HREADY is low?
6) General: Can an AHB master be connected directly to an AHB slave? Applies to:AMBA AHB Any slave which does not use SPLIT responses can be connected directly to an AHB master. If the slave does use SPLIT responses then a simplified version of the arbiter is also required. If an AHB master is connected directly to an AHB slave it is important to ensure that the slave drives HREADY high during reset and that the select signal HSEL for the slave is tied permanently high. See also: • • • • •
AMBA 2.0 Specification What is a default slave? Is a default slave really necessary? Is a dummy master really necessary? What is the difference between a dummy bus master and a default bus master?
7) General: Do all slaves have to support the BUSY transfer type? Applies to:AMBA AHB Yes. All slaves must support the BUSY transfer type to ensure they are compatible with any bus master. See also: • AMBA 2.0 Specification • Can a BUSY transfer occur at the end of a burst? • Do all slaves have to support the Split and Retry responses?
8) General: Does the address have to be aligned, even for IDLE transfers? Applies to:AMBA AHB Yes. The address should be aligned according to the transfer size (HSIZE) even for IDLE transfers. This will prevent spurious warnings from bus monitors used during simulation. See also: • AMBA 2.0 Specification • Can a master change the address/control signals during a waited transfer? • What address should be on the bus during the IDLE cycle after a SPLIT or RETRY?
9) General: How many masters can there be in an AHB system?
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Applies to:AMBA AHB The AHB specification caters for up to 16 masters. However, allowing for a dummy bus master means the maximum number of real bus masters is actually 15. By convention bus master number 0 is allocated to the dummy bus master. See also: • • • •
AMBA 2.0 Specification Is a dummy master really necessary? What is the difference between a dummy bus master and a default bus master? Can an AHB master be connected directly to an AHB slave?
10) General: How should AHB to APB bridges handle accesses that are not 32-bits? Applies to:AMBA APB, AMBA AHB The bridge should simply pass the entire 32-bit data bus through the bridge. Please note that when transfers less than 32-bits are performed to an APB slave it is important to ensure that the peripheral is located on the appropriate bits of the APB data bus. See also: • AMBA 2.0 Specification
11) General: Is HREADY an input or an output from slaves? Applies to:AMBA AHB An AHB slave must have the HREADY signal as both an input and an output. HREADY is required as an output from a slave so that the slave can extend the data phase of a transfer. HREADY is also required as an input so that the slave can determine when the previously selected slave has completed its final transfer and the first data phase transfer for this slave is about to commence. Each AHB Slave should have an HREADY output signal (conventionally named HREADYOUT) which is connected to the Slave-to-Master Multiplexer. The output of this multiplexer is the global HREADY signal which is routed to all masters on the AHB and is also fed back to all slaves as the HREADY input. See also: • AMBA 2.0 Specification • What default state should be used for the HREADY and HRESP outputs from a slave?
12) General: Is a default slave really necessary? Applies to:AMBA AHB If the entire 4 gigabyte address space is defined then a default slave is not required. If, however, there are undefined areas in the memory map then it is important to ensure that a spurious access to a non-existent address location will not lock up the system. The functionality of the default slave is extremely simple and it will often make sense to implement this within the decoder. See also: • AMBA 2.0 Specification • • •
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13) General: Is a dummy master really necessary? Applies to:AMBA AHB A dummy master is necessary in any system which has a slave that can give SPLIT transfer responses. The dummy master is required so that something can be granted the bus if all the other masters have received a SPLIT response. No logic is required for the dummy master and it can be implemented by simply tying off the inputs to the master address/control multiplexer for the dummy master position. The requirements for a dummy master are that HTRANS is driven to IDLE, HLOCK is driven low, and all other master outputs are driven to legal values. See also: • • • •
AMBA 2.0 Specification What is the difference between a dummy bus master and a default bus master? What is a default slave? Is a default slave really necessary?
14) General: Is it legal for a master to change HADDR when a transfer is extended? Applies to:AMBA AHB If a master is indicating that it wants to do a NONSEQ, SEQ or BUSY transfer then it cannot change the address during an extended transfer (when HREADY is low) unless it receives an ERROR, RETRY or SPLIT response. If the master is indicating that it wants to do an IDLE transfer then it may change the address. See also: • • • •
AMBA 2.0 Specification Can a master change the address/control signals during a waited transfer? Can HTRANS change whilst HREADY is low? What default state should be used for the HREADY and HRESP outputs from a slave?
15) General: Is it specified that HPROT, HSIZE and HWRITE remain constant throughout a burst? Applies to:AMBA AHB Yes, the control signals must remain constant throughout the duration of a burst. See also: • • • • •
AMBA 2.0 Specification Can a master deassert HLOCK during a burst? Why is a burst not allowed to cross a 1 kilobyte boundary? Can a BUSY transfer occur at the end of a burst? What are the different bursts used for?
16) General: The specification recommends that only 16 wait states are used. What should you do if more than 16 cycles are needed? Applies to:AMBA AHB For some slaves it is acceptable to insert more than 16 wait states. For example, a serial boot ROM which is only ever accessed at initial power up could insert a larger number of wait states and it would not affect the calculation of the system performance and latency once system power up has been completed. For other slaves a number of options exist. A SPLIT or RETRY response could be used to indicate that the slave is not yet able to perform the requested data transfer, or the slave could be accessed either in response to interrupts or after
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polling a status register, in either case indicating that the slave is now able to respond in an acceptable number of cycles. See also: • • • • •
AMBA 2.0 Specification What default state should be used for the HREADY and HRESP outputs from a slave? Is HREADY an input or an output from slaves? Can a master change the address/control signals during a waited transfer? Can HTRANS change whilst HREADY is low?
17) General: What are the different bursts used for? Applies to:AMBA AHB Typically a master would use wrapping bursts for cache line fills where the master wants to access the data it requires first and then it completes the burst to fetch the remaining data it requires for the cache line fill. Incrementing bursts are used by masters, such as DMA controllers, that are filling a buffer in memory which may not be aligned to a particular address boundary. See also: AMBA 2.0 Specification Can a master deassert HLOCK during a burst? Why is a burst not allowed to cross a 1 kilobyte boundary? Can a BUSY transfer occur at the end of a burst? Is it specified that HPROT, HSIZE and HWRITE remain constant throughout a burst? When a master rebuilds a burst which has been early terminated are there any limitations on how it rebuilds the burst? • What value should be used for HTRANS when an AHB master gets a RETRY response from a slave in the middle of burst? • Can a SPLIT or RETRY response be given at any point during a burst? • • • • • •
18) General: What default state should be used for the HREADY and HRESP outputs from a slave? Applies to:AMBA AHB It is recommended that the default value for HREADY is high and the default value for HRESP is OKAY. This combination ensures that the slave will respond correctly to IDLE transfers to the slave, even if the slave is in some form of power saving mode. See also: • AMBA 2.0 Specification • What is the state of the AHB signals during reset? • What is the recommended default value for HPROT?
19) General: What is a default slave? Applies to:AMBA AHB If the memory map of a system does not define the full 4 gigabyte address space then a default slave is required, which is selected when an access is attempted to the empty areas of the memory map. The default slave should use an OKAY response for IDLE/BUSY transfers and an ERROR response sequence for NONSEQ/SEQ transfers. See also: • AMBA 2.0 Specification • Is a default slave really necessary? • What is the difference between a dummy bus master and a default bus master?
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• Is a dummy master really necessary?
20) General: What is the difference between a dummy bus master and a default bus master? Applies to:AMBA AHB The term default bus master is used to describe the master that is granted when none of the masters in the system are requesting access to the bus. Usually the bus master which is most likely to request the bus is made the default master. The dummy bus master is a master which only performs IDLE transfers. It is required in a system so the arbiter can grant a master which is guaranteed not to perform any real transfers. The two cases when the arbiter would need to do this are when a SPLIT response is given to a locked transfer and when a SPLIT response is given and all other masters have already been SPLIT. See also: • AMBA 2.0 Specification • If a master is currently granted the bus by default, how many cycles before starting an non-IDLE transfer does it have to assert HBUSREQ? • What is a default slave? • Is a default slave really necessary? • Is a dummy master really necessary?
21) General: What is the recommended default value for HPROT? Applies to:AMBA AHB Many bus masters will not be able to generate accurate protection information and for these bus masters it is recommended that the HPROT encoding shows, Non-cacheable, Non-bufferable, Privileged, Data Accesses which corresponds to HPROT[3:0] = 4'b0011. See also: • AMBA 2.0 Specification • What default state should be used for the HREADY and HRESP outputs from a slave?
22) General: What is the state of the AHB signals during reset? Applies to:AMBA AHB The specification states that during reset the bus signals should be at valid levels. This simply means that the signals should be logic '0' or '1', but not Hi-Z. The actual logic levels driven are left up to the designer. HTRANS is the only signal specified during reset, with a mandatory value of IDLE. It is important that HREADY is high during reset. If all slaves in the system drive HREADY high during reset then this will ensure that this is the case. However, if slaves are used which do not drive HREADY high during reset it should be ensured that a slave which does drive HREADY high is selected at reset. See also: • AMBA 2.0 Specification • What is the recommended default value for HPROT?
23) General: What sequences of transfers types (HTRANS) can occur on the bus? Applies to:AMBA AHB The following examples show some of the sequences of HTRANS that can occur on the bus:
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A normal burst of four transfers followed by an IDLE. N-S-S-S-I A normal burst of four transfers which includes BUSY transfers. N-S-B-S-B-S-I A burst of four transfers followed by another burst. N-S-S-S-N-S-S-S-I A single transfer followed by a burst of four transfers. N-N-S-S-S-I A single transfer followed by an IDLE N-I An undefined length burst which concludes with a BUSY transfer. N-B-S-B-S-B-I An undefined length burst which concludes with a BUSY transfer and is followed immediately by another burst. N-B-S-B-S-B-N-S See also: • • • • • •
AMBA 2.0 Specification Can a BUSY transfer occur at the end of a burst? Can a master change the address/control signals during a waited transfer? Do all slaves have to support the BUSY transfer type? Can HTRANS change whilst HREADY is low? What value should be used for HTRANS when an AHB master gets a RETRY response from a slave in the middle of burst?
24) General: When a master rebuilds a burst which has been terminated early are there any limitations on how it rebuilds the burst? Applies to:AMBA AHB The only limitation is that the master uses legal burst combinations to rebuild the burst. For example, if a master was performing an 8 beat burst, but had only completed 3 transfers before losing control of the bus, then the remaining 5 transfers could be performed either by using a 1 beat SINGLE burst followed by a 4 beat INCR4 burst, or it could be performed using a 5 beat undefined length INCR burst. For simplicity it is recommended that masters use INCR bursts to rebuild the remaining transfers. See also: AMBA 2.0 Specification Why is a burst not allowed to cross a 1 kilobyte boundary? What are the different bursts used for? What value should be used for HTRANS when an AHB master gets a RETRY response from a slave in the middle of burst? • Can a SPLIT or RETRY response be given at any point during a burst? • • • •
25) General: Why is a burst not allowed to cross a 1 kilobyte boundary? Applies to:AMBA AHB If an AHB slave samples HSELx at the start of a burst transaction, it knows it will be selected for the duration of the burst. Also, a slave which is not selected at the start of a burst will know that it will not become selected until a new burst is started. 1 kilobyte is the smallest area an AHB slave may occupy in the memory map. Therefore, if a burst did cross a 1 kilobyte boundary, the access could start accessing one slave at the beginning of the burst and then switch to another on the boundary, which must not happen for the above reason.
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The 1 kilobyte boundary has been chosen as it is large enough to allow reasonable length bursts, but small enough that peripherals can be aligned to the 1 kilobyte boundary without using up too much of the available memory map. See also: AMBA 2.0 Specification Can a master deassert HLOCK during a burst? Can a BUSY transfer occur at the end of a burst? Is it specified that HPROT, HSIZE and HWRITE remain constant throughout a burst? When a master rebuilds a burst which has been early terminated are there any limitations on how it rebuilds the burst? • What are the different bursts used for? • What value should be used for HTRANS when an AHB master gets a RETRY response from a slave in the middle of burst? • Can a Split or Retry response be given at any point during a burst? • • • • •
26) How do you connect an AHB Master to an AHB-lite system? An AHB-lite system does not have any arbitration logic, so the full AHB master will be permanently granted. The full AHB master HBUSREQ output will be left unconnected, and the HGRANT input tied to logic '1' (1'b1). As AHB-lite does not support SPLIT or RETRY responses, the AHB-lite HRESP signal is a single bit, so the full AHB master HRESP[1:0] input should have HRESP[1] tied to logic '0' (1'b0). The full AHB master drives HLOCK in advance of the LOCKed transfer address phase, and this would normally be retimed in full AHB by an Arbiter module to produce HMASTLOCK, which is address phase aligned. The following Verilog code would implement the required AHB HLOCK -> AHB-lite HMASTLOCK retiming function. always @( negedge (HRESETn) or posedge (HCLK) ) begin if ((!HRESETn)) HMASTLOCK <= 1'b0; else begin if (HREADY) HMASTLOCK <= HLOCK; end end
27) How do you connect an AHB slave to an AHB-lite system? AHB slaves are fully AHB-lite compatible, so can be connected directly, unless they generate SPLIT or RETRY responses. If the AHB slave generates RETRY responses, you will need an Ahb2Ahb bridge (there are several examples in ARM's AMBA Design Kit (ADK)) between the AHB slave and the AHB-lite system. This Ahb2Ahb bridge will locally service the RETRY response, while holding HREADY low (1'b0) to the AHB-lite system. If the AHB slave generates SPLIT responses then in addition to the Ahb2Ahb bridge required for RETRY responses, you would also need a local Dummy master and an Arbiter. SPLIT responses require the arbiter to grant a different master, so we need the dummy master on this local full-AHB bus to drive IDLE cycles until the slave is able to complete the SPLIT transfer.
28) How do you connect an AHB-lite Master to a full AHB system? You will need logic to add the HBUSREQ and HGRANT signals, to advance the timing of the HLOCK output to be ahead of the LOCKed transfer address phase, and to support SPLIT or RETRY responses on HRESP.
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None of this logic is trivial, so you would need an AHB-lite master wrapper, such as the Lite2Ahb component found in ARM's AMBA Design Kit (ADK).
29) How do you connect an AHB-lite Slave to a full AHB system? An AHB-lite slave is already AHB compliant, with only the HRESP output (indicating OKAY or ERROR) needing to be extended to 2 bits to be compatible with the full AHB HRESP[1:0] bus. HRESP[1] would be driven to logic '0' (1'b0).
30) How does AHB differ from AHB-lite? AHB-lite is a simplified version of the full AMBA 2 AHB specification, supporting only a single Master. This removes the need for arbitration signals, HBUSREQ and HGRANT, and HRESP only needs to be 1 bit as SPLIT and RETRY slave responses are only used for multi-master support. The removal of any arbitration logic also means the AHB Master HLOCK output (driven ahead of the LOCKed transfer address phase) needs to be retimed to be the address phase aligned HMASTLOCK signal (a function usually performed by the AHB arbiter).
31) How many clock cycles should the reset signal in an AMBA system be asserted for? It is recommended that master and slave components should clearly state if they have a reset requirement greater than 1 or 2 cycles. It is also recommended that the system design should hold reset asserted for at least 16 cycles, unless it is known that a master or slave component has a longer reset requirement.
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