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CRC CODE EXAMPLE Figure 8–3
This routine tests the CRC–16 circuit in the DS5001FP
crcmsb equ 0C3h
crclsb equ 0C2h
org 00h ;after reset, CRC regs = 0000
begin:
mov p2,crcmsb ;p2=00 read crcmsb register
mov p3,crclsb ;p3=00 read crclsb register
mov crclsb, #075h ;check crc register operation
;data in = 75 result = E7C1
mov crclsb, #08Ah ;data in = 8A result = 37A7
mov crclsb, #00Bh ;data in = 0B result = 7D37
mov crclsb, #075h ;data in = 75 result = 31FD
mov crclsb, #0C7h ;data in = C7 result = 13B1
mov crclsb, #0AAh ;data in = AA result = 0B53
mov crclsb, #075h ;data in = 75 result = DA8A
mov crclsb, #0C7h ;data in = C7 result = 351A
mov crclsb, #055h ;data in = 55 result = F474
mov crclsb, #043h ;data in = 43 result = D6B5
nop ;delay after last write and before first read
;let CRC finish
mov p0 ,crcmsb ;p0=D6 read CRCMSB register
mov p1 ,crclsb ;p1=B5 read CRCLSB register
mov crclsb ,crclsb ;clear CRC, data in = B5 result = 00D6
nop ;need delay
mov crclsb ,crclsb ;cleared, data in = D6 result = 0000
nop
mov p2 ,crcmsb ;p1=00 read crcmsb register
mov p3 ,crclsb ;p1=00 read crclsb register
end_loop:
sjmp $
end
As mentioned, the CRC–16 function is optionally avail-
able to the application software. This is available
regardless of whether the automatic power–on CRC is
used. Although a CRC could be computed completely in
software, it would take much longer than using the
DS5001 facility. Using the CRC–16 hardware, the
DS5001 series can perform a CRC–16 on 64K bytes of
memory in approximately 500 ms. The CRC–16 logic
resides behind the two SFRs mentioned above. These
display the current CRC result and also serve as the
input locations. The software must sequentially write the
memory values into the CRC LSB at location 0C2h.
After a delay of one instruction cycle, the 16–bit result
will be available at 0C3h and 0C2h. The CRC–16 is a
superior method of checking the file validity compared
to a checksum. Using the DS5001 hardware, it can be
computed quickly. When using the CRC–16 hardware
as part of an application, the existing CRC should first
be cleared. This is done by writing the CRC back on
itself. This process makes the CRC–16 result equal to
0000h. The LSB is written back twice with a delay in
between for computation. The code example shown in
Figure 8–3 displays the CRC–16 result on ports 0 and 1.