Question 11.28: In a certain application, we need 256K bytes of NV-RAM to st...
In a certain application, we need 256K bytes of NV-RAM to store data collected by an 8051 microcontroller. (a) Show the connection of an 8051 to a single 256K×8 NV-RAM chip. (b) Show how various blocks of this single chip are accessed
(a) The 256K×8 NV-RAM has 18 address pins (A0 – A17) and 8 data lines. As shown in Figure 14-18, A0 – A15 go directly to the memory chip while A16 and A17 are controlled by P1.0 and P1.1, respectively. Also notice that chip select of external RAM is connected to P1.2 of the 8051.
(b) The 256K bytes of memory are divided into four blocks, and each block is accessed as follows :
| Chip select | A17 | A16 | |
| P1.2 | P1.1 | P1.0 | Block address space |
| 0 | 0 | 0 | 00000H – 0FFFFH |
| 0 | 0 | 1 | 10000H – 1FFFFH |
| 0 | 1 | 0 | 20000H – 2FFFFH |
| 0 | 1 | 1 | 30000H – 3FFFFH |
| 1 | x | x | External RAM disabled |
. . . .
. . . .
For example, to access the 20000H – 2FFFFH address space we need
the following :
| CLR P1.2 ;enable external RAM MOV DPTR,#0 ;start of 64K memory block CLR P1.0 ;A16 = 0 SETB P1.1 ;A17 = 1 for 20000H block MOV A,SBUF ;get data from serial port MOVX @DPTR,A INC DPTR ;next location … |
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