Abstract:
A microcontroller system has a first and a second block of non-volatile programmable memory and includes a program memory space allocation circuitry. In a first mode of operation, the first and second blocks of programmable memory are prevented from being written by commands external to the microcontroller system. In a second mode, however, the first and second blocks of programmable memory are prevented from being written by commands external to the microcontroller system but the second block of programmable memory can be written based upon execution of commands stored in the first block of programmable memory. By having circuitry to so allocate the programmable memories, the security of the programmable memories is enhanced.

Description:
TECHNICAL FIELD 
     The present invention relates to microcontroller systems having flash memory in a program memory address space and, in particular, to a microcontroller system that selectively allocates portions of the program memory space to either internal or external memories, or a combination thereof, for reasons of security of the internal memory contents. 
     BACKGROUND 
     Microcontroller systems including memory are known in the art. With a conventional microcontroller system, blocks of the memory may be locked such that a locked block cannot be interrogated. This locking thus secures program code contained in the locked blocks from unauthorized access by firmware piraters and from accidental erasing and programming. 
     However, conventionally, neither can the locked blocks be conveniently reprogrammed or accessed by an authorized user. It is desirable for a microcontroller system to be configurable such that blocks of memory ca be locked from unauthorized interrogation or code corruption, but can also be reprogrammed and/or accessed under a predetermined secure environment. 
     SUMMARY 
     In accordance with the present invention, a microcontroller system includes program memory space allocation means. In a first mode of operation, a first block of program memory is hidden for code fetching such that a processor is prohibited from retrieving program instructions from the first block of program memory. In a second mode, however, the first block of program memory is accessible for code fetching. In a third mode of operation, the program memory space allocation means allocates the program memory space such that the processor may retrieve program instructions only from an external memory, that is not unitarily formed in the integrated circuit. In some embodiments, the mode of operation of the program memory space allocation is controlled in response to the contents of a configuration register. 
     By having means to so allocate the program memory, the security of the program memory is enhanced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block functional illustration of an embodiment of a microcontroller system. 
     FIG. 2 is a block functional illustration of an embodiment of a flash operation controller of the FIG. 1 microcontroller system. 
     FIG. 3A is a table that summarizes security functions of the FIG. 1 microcontroller system. 
     FIG. 3B is a table that further details the activated mode of the security lock circuit. 
     FIGS. 4A to  4 C illustrate various states of program memory organization of the FIG. 1 microcontroller system. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates, in block functional form, a microcontroller system  100  in accordance with an embodiment of the present invention, unitarily formed in an integrated circuit. The system  100  includes a microcontroller unit (MCU) core  103 , designated by part number T8052 in FIG.  1 . In the described (preferred) embodiment, the microcontroller system  100  is compatible with standard 8×C58 microcontroller devices. 
     Embedded program memory  102  includes two independent program memory blocks  102   a  and  102   b.  The program memory blocks  102   a  and  102   b  are erasable and reprogrammable, and are preferably flash EEPROM of the type disclosed, for example, in U.S. Pat. No. 5,572,054, issued Nov. 5, 1996 and assigned to Silicon Storage Technology, Inc. of Sunnyvale, Calif., the assignee of the present patent application. Program memory block  102   a  (designated, for convenience of reference, as the “lower program block”) is addressable at the standard 87C58 32 Kbytes of internal ROM area, and program memory block  102   b  (designated for convenience of reference as the “upper program memory block”) is addressable at the high address end of the 8×C5× architecture. In addition, the microcontroller system  100  is configurable to address up to 64 Kbytes of program memory (not shown) external to the system  100 . The system  100  also includes on-chip RAM (not shown) and is configurable to address external data RAM (not shown). 
     A flash operation controller  104  (shown in more detail in FIG. 2, discussed later) includes circuitry for controlling access to and from the program flash memory  102 . A command/address decoder  106  interfaces the MCU core  103  to the addressable memory (whether program memory  102  or other addressable memory, internal or external), I/O ports, and configuration registers. Some of the configuration registers, particularly those within the flash operation controller, are discussed in greater detail later. When the system  100  is operating with the MCU core  103  (as opposed to an external processor, discussed below), this is known as “normal mode”. 
     I/O pads interface  108  includes circuitry for interfacing the various components of the system  100  to external devices (not shown), for example, external memory or an external host. Finally, test mode controller  110  assists in controlling the system  100  when it is operating in “external host mode”, wherein an external “host” processor (not shown) is operating in place of the MCU core  103 . An example of an external host is a PCB tester automated test equipment, a PC-controlled development board, or an OTP (one-time programmable) MCU programmer. During external host mode, in response to hardware commands decoded from control signal input pins, the system  100  performs test, erase and/or programming functions. 
     Security functions of the microcontroller system  100  are now discussed with reference to FIG. 2, which is a more detailed functional diagram of the flash operation controller  104 . In the context of the present invention, a primary function of the flash operation controller  104  is to control read and/or write access to the blocks  102   a  and  102   b  of the program memory  102 , in accordance with the value stored in a “security byte”. Specifically, the flash control unit  201  controls such access responsive to the value of the security byte, as indicated by a field of the SuperFlash® Configuration register (SFCF)  202  which contains a decoding of the security byte. The security byte is located at the highest address location, FFFFh, of the microcontroller system  100  memory space. That is, the security byte is located at the highest location in upper program memory block  102   b.    
     Depending on the value of the security byte, either the entire program memory  102  may be locked; just the upper program memory block  102   b  may be locked; or both the lower program memory block  102   a  and the upper program memory block  102   b  may be unlocked. (Again, the term “value of the security byte” is a shorthand notation herein for the value of the field in the SFCF register  202  that is a decoding of the security byte.) Precisely what it means for a block of memory to be “locked” or “unlocked” is discussed later. The table shown in FIG. 3A provides a summary of the effect that a particular value of the security byte has on the security functions of the system  100 . 
     When the security byte has a value FFh, both the program memory block  102   a  and program memory block  102   b  are unlocked. When the system  100  is delivered from the factory, or after a CHIP ERASE function whereby the contents of the program memory  102  are erased and there is no need for security, the default value of the security byte is FFh. 
     Referring also to FIG. 3B, if the security byte value is other than FFh (i.e., has an “activated” value), then security is activated to lock at least one of the program memory blocks  102   a  and  102   b.  In this case, the “move constant” (MOVC) instruction, which in some instances could be used for read access of data from the program memory  102 , is denied read access when the location of the MOVC instruction itself could be in either an unlocked flash memory location (i.e., a portion of the flash memory  102  is unlocked, as indicated by the activated value of the security byte) or may be in an external memory (as indicated by the signal at an External Enable, or EA#, input pin). Therefore, the contents of the internal flash program memory  102  are prevented from being read undesirably when any level of security is activated. 
     The effect of the security byte on program memory  102  programming operations (i.e., write access) is now discussed. If the system  100  is in external host mode and the security byte value is other than FFh, the following external host mode commands are not allowed on the locked program memory  102 : 
     sector erase, to erase the addressed sector; 
     block erase, to erase the addressed block; 
     write byte, to write a byte to the addressed location; 
     burst write, to write an entire addressed row by burst writing sequentially within the row if the byte location has been successfully erased and not yet programmed; and 
     verify byte, to read a code byte from the addressed location. 
     The read ID function, to read the chip “signature” and identification registers, is allowed even when the security byte is not FFh. 
     Put simply, when the system  100  is in external host mode, only the CHIP ERASE operation, to completely erase the program memory  102 , can be used to access the lockable flash memory locations. That is, the CHIP ERASE function operates regardless of the value of the security byte. As discussed above, in response to the CHIP ERASE function being executed, the security byte is set back to the default value of FFh. Then, because security is disabled, the external host may freely program (or otherwise access) the program memory  102 , via the SFDT “mailbox register”  204 . (The WDTD and WDTC registers shown in FIG. 2 are for controlling a watchdog timer and are not relevant to the present disclosure.) 
     If the security byte at location FFFFh has the value F5h, only the upper program memory block  102   b  is locked, and the lower program memory block  102   a  is not locked. 
     During concurrent programming mode, with the security byte at location FFFFh set to 05h, the lower program memory block  102   a  may be programmed (i.e., sector or block erase, write and verify operations) by program code executing from the upper program memory block  102   b,  but not by any program code executing from the external program code storage even though both program memory blocks  102   a  and  102   b  are security locked for any other access, such as External Mode Host Access. This security mechanism is known as “soft lock”. Contrary to the “hard lock”, when the security byte at location FFFFh is set to 55h (where both the upper  102   b  and lower  102   a  memory blocks are not accessible for any programming operation), the “soft lock” security mechanism allows the system to use a secure program code to perform in-system and in-field flash memory concurrent programming of the lower program memory block  102   a  under a predetermined secure environment. The table in FIG. 3A further illustrates this security lock feature. 
     More details of the program memory addressable space are now discussed with reference to FIGS. 4A to  4 C, particularly relative to how the program memory organization is adaptable to address safety and security concerns. More specifically, FIGS. 4A to  4 C show three program memory organization and assignment options of the microcontroller system  100 . 
     As discussed above, the program (flash) memory  102  of the microcontroller system  100  includes two blocks, lower program memory block  102   a  and upper program memory block  102   b.    
     In one embodiment, the lower program memory block  102   a  has 32 Kbytes, and occupies the address space 0000h to 7FFFh. The 32 Kbytes of the lower program memory block  102   a  are organized as 512 rows (256 of 128-byte sectors per page, 9 X-address lines) and 512 columns (64 bytes per row, 6 Y-address lines). The upper program memory block  102   b  has 4 Kbytes, and occupies the address space F000h to FFFFh. The 4 Kbytes of the upper program memory block  102   b  are organized as 128 rows (64 of 64-byte sectors per page, 7 X-address lines) and 256 columns (32 bytes per row, 5 Y-address lines). 
     As can be seen from FIGS. 4A to  4 C, the address space 8000h to EFFFh is always reserved for external memory (not shown), which may be, for example, memory of a development system or hardware programmer. 
     Referring now specifically to FIG. 4B, in “normal mode”, when instructions are being executed by the MCU processor core  103 , the upper program memory block  102   b  is “hidden for code fetching”. This is so even when internal code execution is enabled (#EA=1; see FIG. 3) such that code may be executed from the internal program memory  102 . By “hidden for code fetching”, it is meant that program instructions in the block “hidden for code fetching” cannot be fetched and executed from locations of the hidden block. 
     As shown in FIG. 4A, to make the upper program memory  102   b  visible for code fetching, so that the concurrent programming operation can be carried out by executing program instructions from the upper program memory  102   b,  the system  100  must be in internal mode (i.e., #EA=1), and the user must set a concurrent programming field in the SuperFlash® configuration register SFCF  202 . In one embodiment, the concurrent programming field is the most significant bit of the SuperFlash® configuration register SFCF  202  (FIG.  2 ). 
     As for the visibility of the lower program memory block  102   a,  while internal code execution is enabled (i.e., #EA=1), the lower program memory block  102   a  is always visible for code fetching. This can be seen from FIGS. 4A and 4B. If the external enable is asserted at hardware reset of the system  100 , program execution starts from the lower program memory block  102   a.    
     Finally, as can be seen from FIG. 4C, while internal code execution is not enabled (i.e., #EA=0), both lower program memory block  102   a  and upper program memory block  102   b  are hidden for code fetching. 
     By having circuitry to so allocate the program memory, the security of the program memory is enhanced. Furthermore, in combination with the security lock circuitry, the security of the program is further enhanced. 
     Attached hereto as Appendix A is the “SST89C58 Data Sheet” which is to be considered an integral part of this specification.