Patent Publication Number: US-7917669-B2

Title: Multiphase addressing method for transferring address information

Description:
FIELD OF THE INVENTION 
     Embodiments of this invention relate generally to an interface between two integrated circuits e.g. a memory and microprocessor interface or a memory and DMA controller interface. 
     BACKGROUND TO THE INVENTION 
     One conventional microprocessor memory interface includes an address bus, a data bus, and a control bus. The microprocessor uses the address bus to send an memory address to the memory. The number of address signal lines in the address bus depends upon the number of addressable memory locations in the memory. The data bus is used to transfer data from the microprocessor to the memory during a write transaction and to transfer data from the memory to the microprocessor during a read transaction. The data bus typically has 8*n data signal lines, where n is a natural number. The control bus typically includes control signal lines such as write_enable (WE) that are used by the microprocessor to control the type of transaction at the memory (read or write) and such as a clock signal (clk) that is used to synchronise the transaction. 
     The current flash memory interface  10  used in NOKIA mobile telephones is illustrated in  FIG. 1 . The interface  10  has a dedicated 9-bit upper address bus  2  and a 16-bit multiplexed data/address bus  4 . It also has a control bus  6  comprising: a system clock signal line (external clock)  3 , a chip select signal line (CSX)  5 , a read signal line (OEX)  7 , an address valid signal line (ADVX)  9  and a Ready signal line  11 . 
     During an initial phase of a memory access the dedicated address bus  2  conveys the 9 most significant address bits while the multiplexed bus  4  simultaneously conveys the 16 least significant address bits. The address bits are conveyed at the rising edge of a single external clock cycle. The address valid signal line (ADVX)  9  is asserted by the microprocessor when the multiplexed bus  4  is carrying address information. This signal is typically used to latch the address information from the address bus  2  and the multiplexed bus  4  into a latching device at the memory. 
     In a later part of the memory access, the address bus  2  is unused, while the multiplexed bus  4  is used to convey a data word per clock cycle. Four data words D 0 , D 1 , D 2  and D 3  are conveyed in series. 
     The interface also has a flow control mechanism that allows the memory to pace the data transfer by temporarily suspending it. The control bus includes a Ready signal line  11  that is controlled by the memory. The state of the Ready signal indicates whether or not a data word is to be transferred. The Ready signal is asserted high while continuing data transfer is possible and asserted low when continuing data transfer is suspended. 
     In  FIG. 1  a read memory access is illustrated. The transfer of the data word D 3  from the memory to the microprocessor is delayed while the Ready signal line  11  is asserted low by the memory. The Ready signal is asserted HIGH as the fourth data word D 3  is transferred from the memory. 
     The Ready signal line  11  enables paced burst mode access to the memory including crossing internal memory page boundaries. In burst mode access the data is read as a burst of words, if necessary from different memory segments used in the internal organization of the memory. The Ready signal can be used to suspend the data transfer while the next memory segment is fetched. 
     It would be desirable to improve the above-described interface. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one embodiment of the invention there is provided a method of transferring address information from a controller device to a target device via a multiplexed data/address bus comprising: transferring a first portion of address information in a first phase via the multiplexed data/address bus; transferring a second portion of address information in a second phase via a multiplexed data/address bus, wherein the first and second portions are distinct portions of the address information and the first and second phases are distinct and successive. 
     According to another, embodiment of the invention there is provided a controller device for transferring address information to a target device via a multiplexed data/address bus comprising: timing means for timing a first phase and a second phase following the first phase; addressing means for providing a first portion of address information on the multiplexed data/address bus during the first phase and for providing a second portion of address information on the multiplexed data/address bus during the second phase, wherein the first and second portions are distinct portions of the address information. 
     According to another embodiment of the invention there is provided a target device for receiving address information transferred from a controller device via a multiplexed data/address bus, the target device comprising: timing means for timing a first phase and a second phase following the first phase; addressing means for storing, in a first predetermined storage location, a first portion of address information received on the multiplexed data/address bus during the first phase and for storing, in a second predetermined storage location, a second portion of address information received on the multiplexed data/address bus during the second phase. 
     According to another embodiment of the invention there is provided a method of strobing and pacing data transfer between a controller device and a target device using a single control signal, comprising: strobing the transfer of a data word from the target device to the controller device by changing the state of the control signal as the data word is transferred; and pacing a transfer of data words from the target device to the controller device by disabling a state change of the control signal. 
     According to another embodiment of the invention there is provided a target device for transferring data between a controller device and the target device, the target device comprising: an interface comprising a bus for transferring data between the target device and the controller device and a control signal line for providing a control signal to the controller; and flow control means for strobing the transfer of data to the controller by changing the state of the control signal with each data word provided to the bus and for pacing the transfer of data to the controller by maintaining the state of the control signal. 
     According to another embodiment of the invention there is provided controller device for transferring data between a controller device and the target device, the controller device comprising: an interface comprising a bus for transferring data between the target device and the controller device and a control signal line for receiving a control signal from the target device; and flow control means for latching a data word provided on the bus by the target device only when the state of the control signal changes. 
     According to another embodiment of the invention there is provided a method of strobing and pacing data transfer between a controller device and a target device using a single control signal, comprising: strobing a data transfer from the target device to the controller by changing the state of the control signal with each data transfer; and pacing a data transfer from the controller to the target by changing the state of the control signal. 
     According to another embodiment of the invention there is provided a method of performing a burst read access at a memory device using a multiplexed data/address bus and a control signal comprising: transferring a first portion of address information in a first phase via the multiplexed data/address bus to the memory device; transferring second portion of address information in a second phase via a multiplexed data/address bus to the memory device; transferring a series of data words from the memory via the multiplexed data/address bus; toggling the state of the control signal at the memory device as each data word is transferred; and suspending the transfer of the series of data words from the memory via the multiplexed data/address bus and the toggling of the state of the control signal. 
     Embodiments of the invention therefore provide a higher performance controller device/target device interface that has either a reduced number of signal lines or has a wider data bus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention reference will now be made by way of example only to the accompanying drawings in which: 
         FIG. 1  illustrates a prior art read memory access; 
         FIG. 2  illustrates a controller device and a target device connected via interface; 
         FIG. 3  illustrates a 4 word burst read with pacing; and 
         FIG. 4  illustrates an 8 word burst write with pacing. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
       FIG. 2  illustrates a controller device  30  and a target device  40  connected via interface  50 . The controller  30  device is typically a microprocessor, ASIC or other type of integrated circuit controller than is able to write to and read from the target device which is typically a memory device. The interface  50  connecting the controller  30  and target  40  is new. Only as many signal lines in the interface  50  will be describe as is necessary to understand the novel aspects of its operation. The interface  50  may comprise additional unspecified signal lines that operate in a conventional fashion. 
     The interface  50  comprises: an N-bit multiplexed data/address bus  4  that operates a double rate, and a control bus  6  comprising a system clock signal line (external clock)  3 , a read signal line (OEX)  7 , a write enable signal line (WEX)  1 , an address valid signal line (ADVX)  9  and a Ready signal line  11 ′. The interface  50  does not comprise a dedicated address bus 
     The interface  50  uses a new multi-phase addressing mode. Instead of simultaneously providing the address information via a dedicated address bus and the multiplexed bus in a single transfer process at the rising edge of the clock cycle, the interface  50  provides the address information only via a multiplexed bus  4  in a sequence of successive transfer processes. 
     The address transfer may be at ‘double rate’ in which two phases of address information are driven onto the multiplexed bus  4  in one external clock cycle. A first address transfer phase occurs at the rising edge of a first clock cycle and a second address transfer phase occurs at the falling edge of the first clock cycle. The first address transfer phase transfers N bits and the second address transfer process transfers up to N bits. The least significant bits of the address information are transferred in the first phase. This allows a single phase short address mode to be used in which only the least significant bits of the address are transferred in the first phase and the most significant bits of the address are re-used from the previous access. 
     The interface  50  also has a flow control mechanism that allows the target  40  to pace the data transfer during a write access or a read access by temporarily suspending the transfer. This enables paced burst mode access to the target  40 , including crossing internal memory page boundaries. In burst mode memory access the data is read/written as a burst of words, if necessary from/to different memory segments used in the internal organization of the memory. The flow control can be used to suspend the data transfer while the next memory segment is fetched/accessed. 
     The interface  50  also has a strobe mechanism that decouples the response of the target  40 , in time, from the request from the controller  30  during a read access. This overcomes the problem of relative clock delays between the controller  30  and target  40  which become more critical as the data transfer rate is increased. This allows the new interface  50  to operate at higher data transfer rates. The strobe mechanism in the target  40  produces a strobe signal that is synchronous with the transferred data. It is consequently received by the controller  30  at the same time as the transferred data is provided on the multiplexed bus  4 . The controller  30  comprises a latch mechanism operated by the strobe signal that latches the transferred data. 
     Both the flow control mechanism and strobe mechanism during a read use the Ready signal  11 ′ and the flow control mechanism during a write uses the Ready signal  11 ′. 
     As in the prior art the Ready signal  11 ′ is controlled by the memory. However, during a read access, an absence of a state transition of the Ready signal  11 ′ is used for data pacing (as opposed to the state of the Ready signal in the prior art) and a state transition of the Ready signal  11 ′ is used as a strobe signal for clocking the transferred data into the controller  30 . During a write access, the state of the Ready signal  11 ′ is used for data pacing (as in the prior art). 
     The controller  30  comprises an address provider mechanism  32  and a flow control slave  34 . The address provider mechanism  32  receives the address information  33  as an input and is connected via the interface  50  to the external clock signal line  3 , the address valid signal line  9  and the multiplexed bus  4  as schematically illustrated in  FIG. 1 . The flow control slave receives a control signal  35  that indicates whether the access using the address information should be a read access or a write access. It is also connected via the interface  50  to the external clock signal line  3 , the write enable signal line WEX  1 , the read enable signal line OEX  7 , the Ready signal line  11 ′ and the multiplexed bus  4 . 
     The target  40  comprises an address interpreter mechanism  42 , an address latch  44 , memory circuitry  46  and a flow control master  48 . The address interpreter mechanism  42  is connected to the external clock signal line  3 , the address valid signal line ADVX  9  and the multiplexed bus  4  as schematically illustrated in  FIG. 1 . The flow control master  48  is connected to the external clock signal line  3 , the write enable signal line WEX  1 , the read enable signal line OEX  7 , the Ready signal line  11 ′ and the multiplexed bus  4 . It is also connected to receive an internal data pace signal  43  from the memory circuitry if the read or write operation is delayed and to transfer data to and from the memory  46  via a data bus  41 . 
     Burst Read Access 
       FIG. 3  illustrates a  4  word burst read with pacing. The Ready signal  11 ′ is used to strobe the data into the controller  30 . Each new data word D 0 , D 1 , D 2 , D 3  presented to the controller is accompanied by a respective change in state S 0 , S 1 , S 2 , S 3 , of the Ready signal  11 ′. The data can be paced by holding the state of the Ready signal  11  and preventing subsequent transitions. 
     The read burst access begins when the address provider mechanism  32 , at the rising edge of a first clock cycle, drives and holds the address valid signal line ADVX LOW and simultaneously drives the N least significant address bits onto the multiplexed bus  4 . The address provider mechanism  32 , at the falling edge of the same first clock cycle, then drives the remaining address bits onto the multiplexed bus  4  while the address valid signal line ADVX is held LOW. The address provider mechanism  32 , at the rising edge of the next clock cycle, then drives and holds the address valid signal line ADVX HIGH indicating the end of the multi-phase address transfer. 
     The address interpreter mechanism  42  responds to a LOW address valid signal ADVX  9  and a rising edge of the external clock signal  3 , by latching the N bits received on the multiplexed bus  4  into the address latch  44  as the N least significant bits. The address interpreter mechanism  42  responds to a LOW address valid signal and a falling edge of the external clock signal, by latching the bits received on the multiplexed bus, if any, into the address latch  44  as the most significant bits. 
     The flow control master  48  in the absence of a pace signal  43 , latches a different data word D 0 , D 1 , D 2  retrieved from the memory via the data bus  41  onto the multiplexed bus  4  at each edge E 0 , E 1 , E 2  of the external clock signal and simultaneously toggles the state of the Ready signal The state of the Ready signal at S 0  is switched LOW as the first data word D 0  is latched onto the multiplexed bus at the rising edge E 0  of the external clock signal  3 . The Ready signal  11 ′ is the inverse of the external clock signal  3  with a delay. 
     The flow control slave  34  uses each transition S 0 , S 1 , S 2  in the Ready signal  11 ′ to latch a different data word D 0 , D 1 , D 2  from the multiplexed bus  4 . 
     When the pace signal  43  is asserted by the memory circuitry  46 , the flow control master  48 , continues to latch the current data word D 2  on the multiplexed bus  4  and continues to hold the current state (LOW) of the Ready signal  11 ′. 
     As there are no transitions in the Ready signal  11 ′, the flow control slave  34  does not latch a data word from the multiplexed bus  4  and the read process is suspended. 
     When the pace signal  43  is de-asserted the flow control master latches the next data word D 3  retrieved from the memory  46  via the data bus  41  onto the multiplexed bus  4  at the next edge E 3  of the external clock signal  3  and simultaneously toggles the state of the Ready signal. 
     Flow control slave uses the transition in the Ready signal S 3  to latch the data word D 3  from the multiplexed bus  4 . 
     In the above described example, a non-return to zero Ready signal  11 ′ is used. The Ready signal can consequently be held in either a HIGH or LOW state. This allows data transfers of any length to be paced at any point. 
     An alternative, although less preferred implementation, uses a return to zero Ready signal  11 ′ is used. The Ready signal can consequently only be held in a LOW state. In this implementation the data must be transferred in word pairs as pacing can only occur when the Ready signal  11 ′ is LOW. 
     Burst Write Access. 
       FIG. 4  illustrates an 8 word burst write with pacing The state of the Ready signal  11 ′ is used to pace data into the target  40 . A LOW state will suspend future data transfers to the target  30  and a HIGH state will enable future data transfers to the target  30 . 
     The write burst access begins when the address provider mechanism  32 , at the rising edge of a first clock cycle, drives and holds the address valid signal line ADVX LOW and simultaneously drives the N least significant address bits onto the multiplexed bus of width N. The address provider mechanism  32 , at the falling edge of the same first clock cycle, then drives the remaining address bits onto the multiplexed bus of width N while the address valid signal line ADVX is held LOW. The address provider mechanism  32 , at the rising edge of the next clock cycle, then drives and holds the address valid signal line ADVX HIGH indicating the end of the address transfer. 
     The address interpreter mechanism  42  responds to a LOW address valid signal ADVX  9  and a rising edge of the external clock signal  3 , by latching the N bits received on the multiplexed bus  4  into the address latch  44  as the N least significant bits. The address interpreter mechanism  42  responds to a LOW address valid signal ADVX  9  and a falling edge of the external clock signal  3 , by latching the bits received on the multiplexed bus  4 , if any, into the address latch  44  as the most significant bits. 
     The flow control slave  32  in the controller  30  starts the transfer of data by asserting WEX LOW at a rising edge of the external clock signal  3  and then clocking data words D 0 , D 1 , D 2 , D 3 , D 4  onto the multiplexed bus one at a time at each successive edge of the external clock signal  3 . 
     When the pace signal  43  is asserted by the memory circuitry  46 , the flow control master  48 , stops the data transfer by de-asserting the Ready signal  11 ′ by making it LOW at point A in the Fig. 
     The flow control slave  34  responds to the LOW Ready signal  11 ′ by stopping the data transfer by de-asserting the write enable signal WEX at point B in the Fig. It should be appreciated that the halting of data transfer after a change in state of the Ready signal  11 ′ is not instantaneous. The halting of data transfer (point B) is delayed relative to the change of state of the Ready signal  11 ′ (point A) by one external clock cycle in this example. 
     When the pace signal is de-asserted at point C, some time later, the flow control master  48  restarts the data transfer by re-asserting the Ready signal  11 ′ by holding it HIGH. 
     The flow control slave  34  responds to the HIGH Ready signal  11 ′ by starting data transfer by asserting the write enable signal WEX  1  at point D. It should be appreciated that the re-starting of the data transfer after a change in state of the Ready signal is not instantaneous. The re-start of data transfer (point D) is delayed relative to the change of state of the Ready signal  11 ′ (point C) by one external clock cycle in this example. 
     Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. For example although the multi-phase transfer of the address information and the transfer of the data between the controller and target has been described using ‘double rate’ transfer, in which a transfer occurs twice with each external clock cycle e.g. at each edge of the clock cycle, other embodiments of the invention may use ‘single rate’ transfer, in which a transfer occurs only once with each external clock cycle e.g. only with the rising edge of the external clock signal or, alternatively, only with the falling edge of the rising clock. 
     In addition, although the multi-phase transfer of an address has been illustrated using a two-phase transfer, it should be appreciated that N phases can be used where N is a natural number greater than 1. 
     Although in the interface described above the first phase, for transferring a first portion of the address, is initiated at a first rising clock edge and the second phase, for transferring a second portion of the address, is initiated at a second rising clock edge, in other implementations the first phase is initiated at a first falling clock edge and the second phase is initiated at a second falling clock edge. The first falling clock edge and second falling clock edge are successive adjacent clock edges in a single clock cycle.