HT80C51 User Manual
Peripheral Modules – Triple-DES Converter
Handshake Solutions
© Philips Electronics N.V. 2005
Page 83 of 132
DKEY
DES key register (write only)
addr = C1H
reset value = XX
bits
7
6
5
4
3
2
1
0
DKEY
bit
symbol
Function
DKEY.7
DKEY.6
DKEY.5
DKEY.4
DKEY.3
DKEY.2
DKEY.1
7bit slice of the key register. Used to shift in a 56bit key, which can be
stored in either key register KEY0 or KEY1.
DKEY.0
Ignored
DTXT
DES data register
addr = C2H
reset value = XX
bits
7
6
5
4
3
2
1
0
DTXT
bit
symbol
Function
DTXT.7
..
DTXT.0
8bit slice of a complete 64bit data block for encryption or decryption. A
write access shifts in 8bits, a read access shifts out 8bits.
5.8.3.
Interrupts
No interrupts generated.
5.8.4.
Operation
Write accesses to all three registers are supported. However only read accesses from DTXT are sup-
ported (read accesses from the other registers have no effect). Each supported access on the 8-bit
slice of either the key or the text register is followed by a permutation of the corresponding register.
Writing the 64-bit text register takes 8 write accesses to DTXT. Reading the text register takes 8 read
accesses from DTXT after which the contents of the text register are back in the original state. Writing
the 56-bit key register takes also 8 write accesses to DKEY. In these accesses the least significant bit
of each byte is discarded.
To support triple DES, the converter has two internal key registers: KEY0 and KEY1. After a key has
been shifted in, its value has to be stored in KEY0 or KEY1 before it can be used. A single-DES conver-
sion uses KEY0 and a triple-DES conversion uses first KEY0, then KEY1 and finally KEY0 again.
After a command has been written into DCON, it will be executed. Only the three least significant bits in
register DCON have a meaning.
The conversions take so little time that no additional synchronization mechanism (interrupt or ready bit
in status information) is needed. Instead the handshaking mechanism is used to obtain the required
synchronization between the micro-controller and the DES converter. As long as the unit is busy in a
conversion it does not accept any accesses to one of its SFRs. Therefore after having given a conver-
sion command, the micro-controller can immediately start reading the resulting text. If in the case of a
triple-DES conversion the result is not yet available, the first read access is held up in a handshake
until the conversion is completed (no busy waiting).
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