0x400-0x41f= Access Protection Register Memory
0x400=Block0 (0-0x7f) Protction
0x401=Block1 (0x80-0xFF) Protction
0x402=Block2 (0x100-0x17F) Protction
0x403=Block3 (0x180-0x1FF) Protction
0x404=Block4 (0x200-0x27F) Protction
0x405=Block5 (0x280-0x2FF) Protction
0x406=Block6 (0x300-0x37F) Protction
0x407=Block7 (0x380-0x3FF) Protction
0x408=PBAP
0x409=WPN
Byte in buffer on address 40FH is device revision information & is fixed.
Block Protection bytes (0x400-0x407) definition:
bit1,bit0
00 no access on Block
01 no access on Block
10 Read only
11 Read & write
bit7 must be 1 to allow bit 0 & bit 1 writing.
To remove read protection: first read chip, fill 0x400-0x407 with 0xff , Write chip ,then read again.