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LM1247 Datasheet(PDF) 37 Page - National Semiconductor (TI) |
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LM1247 Datasheet(HTML) 37 Page - National Semiconductor (TI) |
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37 / 48 page  Control Register Definitions (Continued) Bits 6–4 This three bit field determines the ROM bank (0-7) selected when the upper two bits of the character address in Page RAM are 01 (Character Address = 01xxxxxxb) Bit 7 This bit is reserved and should be set to 0. ROM Bank Select Register B: BANKSEL_2-3 (0x8428) Res’d Bank Select 3 Res’d Bank Select 2 X B3AD[2:0] X B2AD[2:0] Bits 2–0 This three bit field determines the ROM bank (0-7) selected when the upper two bits of the Page RAM addresses are 10 (Character Address = 10xxxxxxb) Bit 3 This bit is reserved and should be set to 0. Bits 6–4 This three bit field determines the ROM bank (0-7) selected when the upper two bits of the Page RAM addresses are 11 (Character Address = 11xxxxxxb) Bit 7 This bit is reserved and should be set to 0. The actual address for any character in ROM is formed, in logic, from the address in the Page RAM, by this sequence (also see Figure 13): 1. The upper 2 bits of the character address in Page RAM are used to address one of the four 3 bit fields in Bank Select Register A or Bank Select Register B. As shown in Table 22, depending on which of the four values is present, the corresponding 3 bit bank address is obtained from the BANKSEL_0, BANKSEL_1, BANKSEL_2, or BANKSEL_3 field shown in the last column. TABLE 22. Address Lookup Character Address in Page RAM Upper Two Bits Three Bit Bank Address Source 00xxxxxxb 00b B0AD[2:0] 01xxxxxxb 01b B1AD[2:0] 10xxxxxxb 10b B2AD[2:0] 11xxxxxxb 11b B3AD[2:0] 2. Then, the 3 bit address obtained from B0AD[2:0], B1AD[2:0], B2AD[2:0] and B3AD[2:0] are used to select four of the eight 2 or 4 color ROM banks as shown in Table 23. The BxAD[2:0] column gives the range of three bit addresses and the next two columns give the corresponding ROM address range and the character type. TABLE 23. Resulting ROM Bank Address BxAD[2:0] Character ROM Address Range ROM Character Type 000b 0x000 - 0x03F 2 Color 001b 0x040 - 0x07F 2 Color 010b 0x080 - 0x0BF 2 Color 011b 0x0C0 - 0x0FF 4 Color 100b 0x100 - 0x13F 2 Color 101b 0x140 - 0x17F 2 Color 110b 0x180 - 0x1BF 2 Color 111b 0x1C0 - 0x1FF 4 Color 3. In summary, the final ROM character address is formed by concatenating (combining end to end) the three bits of the corresponding Bank Address Register with the lower six bits of the original character address in RAM. Since just the two highest bits of the Page RAM address are used, only 4 banks can be addressed at one time. www.national.com 37 |
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