LM1247 National Semiconductor, LM1247 Datasheet - Page 37

• Manufacturer Part Number: LM1247
• Description: 150 MHz I2C Compatible RGB Preamplifier with Internal 512 Character OSD ROM, 512 Character RAM and 4 DACs
• Manufacturer: National Semiconductor

Control Register Definitions
ROM Bank Select Register B:
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
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
3. In summary, the final ROM character address is formed by concatenating (combining end to end) the three bits of the
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.
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.
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.
Bits 6–4
Bit 7
Bits 2–0
Bit 3
Bits 6–4
Bit 7
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)
This bit is reserved and should be set to 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)
This bit is reserved and should be set to 0.
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)
This bit is reserved and should be set to 0.
BxAD[2:0]
000b
001b
010b
100b
101b
011b
110b
111b
Character Address in
Res’d
X
Page RAM
00xxxxxxb
01xxxxxxb
10xxxxxxb
11xxxxxxb
Character ROM Address Range
TABLE 23. Resulting ROM Bank Address
Bank Select 3
B3AD[2:0]
TABLE 22. Address Lookup
0x0C0 - 0x0FF
0x1C0 - 0x1FF
0x080 - 0x0BF
0x180 - 0x1BF
0x000 - 0x03F
0x040 - 0x07F
0x100 - 0x13F
0x140 - 0x17F
(Continued)
BANKSEL_2-3 (0x8428)
Upper Two Bits
00b
01b
10b
11b
37
Res’d
X
Three Bit Bank Address
Bank Select 2
B2AD[2:0]
B0AD[2:0]
B1AD[2:0]
B2AD[2:0]
B3AD[2:0]
ROM Character Type
Source
2 Color
2 Color
2 Color
4 Color
2 Color
2 Color
2 Color
4 Color
www.national.com