int e l CHAPTER 10 System Configuration 10.1 MEMORY / HARDWARE CONFIGURATION MCS BASIC-52 always requires at least 1K bytes of external memory. After reset, MCS BASIC-52 sizes the external memory. If less than 1K bytes of external memory are available, MCS BASIC-52 will not "sign-on." in fact, it will internally loop forever. This obviously is not too exciting, so it is wise to hang some external memory on the MCS BASIC-52 device. MCS BASIC-52 sizes consecutative external memory locations from 0000H until a memory failure is detected. The sizing operation is performed simply by writing a 5AH to an external memory location, then testing the location.
If the particular memory location passes this test, BASIC then writes a 00H to the location, then again, checks the location. MCS BASIC-52 only sizes the external memory from locations 0 through 0DFFFH. Memory locations 0E000H through 0FFFFH are reserved for user I/O and/or assembly language programs. The MCS BASIC-52 program resides in the 8K of ROM available in INTEL's 8052AH device and as a result requires that external memory be "partitioned" in a specific manner. The architecture of the 8052AH is NOT Von Neumann. This means that Data and Program Memory do not reside in the same physical address space on the 8052AH.
Specifically, the /RD (pin 17) and /WR (pin 16) pins on the 8052AH are used to enable DATA memory and /PSEN (pin 29) pin is used to enable PROGRAM memory. Depending on the hardware configuration, MCS BASIC-52 operates in two distinct "memory" modes. RAM ONLY MODE In this mode of operation, Read/write memory is connected to the MCS BASIC-52 device starting at memory address 0000H. Memory can be placed up to location 0FFFFH. In this mode of operation the decoded addresses are used to generate the CHIP SELECT (/CS) signal for the RAM devices. The RD pin on the 8052AH is used to generate the OUTPUT ENABLE (/OE) strobe and the /WR pin generates the WRITE ENABLE (/WE or /WR) strobe.
/PSEN is not used in the RAM only mode of operation. The RAM only mode of operation offers the simplest hardware configuration available for the MCS BASIC- 52 device. An example of this configuration is shown in Figure 1. Since /PSEN is not used in the RAM only mode, the user may not CALL assembly language routines. The RAM only also does not support EPROM programming. In general, the RAM only mode will be used only to "check out" the device during the initial system development stage. - 132 -