Sfr Access Snoop Function - Renesas M16C Series User Manual

M30245 Group

2.11.3 SFR Access Snoop Function

The CRC calculation circuit includes the ability to snoop write/read to/from the SFR addresses and to
execute CRC automatic calculation (SFR access snoop function). In order to execute CRC calculation for
data which have been written/read to/from the SFR, setting data to CRC input register again is not re-
quired. The target SFRs include USB-related registers, UART-related registers, and Serial Sound Inter-
face-related registers.
Operation (1) The bit 1 of CRC mode register selects either CRC-CCITT or CRC-16, and the bit 7 selects
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either LSB first or MSB first.
(2) The target SFR addresses are set to CRC snoop address register (bit 0 to 9 of CRCSAR).
Snooping of writing to the target SFR is enabled with CRC snoop on write enable bit (bit 15 of
CRCSAR), and snooping of reading from the target SFR is enabled with CRC snoop on read
enable bit (bit 14 of CRCSAR).
(3) The initial value 0000
16
(4) If writing into the target SFR is executed by either the CPU or DMA while "1" is set to CRC
snoop on write enable bit, the CRC calculation circuit will store the data written to the target
SFR in CRC input register, and executes CRC calculation. Similarly, if reading from the target
SFR by the CPU or DMA while "1" is set to CRC snoop on read enable bit, calculation circuit
will store the data read from the target SFR in CRC input register, and executes CRC calcu-
lation. The CRC calculation circuit can only calculate CRC codes on data 1-byte at a time.
Therefore, if a target SFR is accessed in a word (16-bit) bus cycle, only 1-byte data are stored
into CRC input register.
(5) When 1-byte data is stored in CRC input register, CRC codes are generated in CRC data
register based on the stored data and the content of CRC data register. Generation of CRC
codes for 1-byte of data is completed in two machine cycles.
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is set to CRC data register.
2. CRC Calculation Circuit