PIC24HJ64 Datasheet

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PIC24HJ32GP302/304, <strong>PIC24HJ64</strong>GPX02/X04, AND PIC24HJ128GPX02/X04 FIGURE 22-2: CRC GENERATOR RECONFIGURED FOR x 16 + x 12 + x 5 + 1 XOR SDOx D BIT 0 Q D BIT 4 Q D BIT 5 Q D Q BIT 12 D BIT 15 Q p_clk p_clk p_clk p_clk p_clk CRC Read Bus CRC Write Bus 22.2 User Interface 22.2.1 DATA INTERFACE To start serial shifting, a ‘1’ must be written to the CRCGO bit. The module incorporates a FIFO that is 8 deep when PLEN (PLEN) > 7, and 16 deep, otherwise. The data for which the CRC is to be calculated must first be written into the FIFO. The smallest data element that can be written into the FIFO is one byte. For example, if PLEN = 5, then the size of the data is PLEN + 1 = 6. The data must be written as follows: data[5:0] = crc_input[5:0] data[7:6] = ‘bxx Once data is written into the CRCWDAT MSb (as defined by PLEN), the value of VWORD (VWORD) increments by one. The serial shifter starts shifting data into the CRC engine when CRCGO = 1 and VWORD > 0. When the MSb is shifted out, VWORD decrements by one. The serial shifter continues shifting until the VWORD reaches 0. Therefore, for a given value of PLEN, it will take (PLEN + 1) * VWORD number of clock cycles to complete the CRC calculations. When VWORD reaches 8 (or 16), the CRCFUL bit will be set. When VWORD reaches 0, the CRCMPT bit will be set. To continually feed data into the CRC engine, the recommended mode of operation is to initially “prime” the FIFO with a sufficient number of words so no interrupt is generated before the next word can be written. Once that is done, start the CRC by setting the CRCGO bit to ‘1’. From that point onward, the VWORD bits should be polled. If they read less than 8 or 16, another word can be written into the FIFO. To empty words already written into a FIFO, the CRCGO bit must be set to ‘1’ and the CRC shifter allowed to run until the CRCMPT bit is set. Also, to get the correct CRC reading, it will be necessary to wait for the CRCMPT bit to go high before reading the CRCWDAT register. If a word is written when the CRCFUL bit is set, the VWORD Pointer will roll over to 0. The hardware will then behave as if the FIFO is empty. However, the condition to generate an interrupt will not be met; therefore, no interrupt will be generated (See Section 22.2.2 “Interrupt Operation”). At least one instruction cycle must pass after a write to CRCWDAT before a read of the VWORD bits is done. 22.2.2 INTERRUPT OPERATION When the VWORD4:VWORD0 bits make a transition from a value of ‘1’ to ‘0’, an interrupt will be generated. 22.3 Operation in Power Save Modes 22.3.1 SLEEP MODE If Sleep mode is entered while the module is operating, the module will be suspended in its current state until clock execution resumes. 22.3.2 IDLE MODE To continue full module operation in Idle mode, the CSIDL bit must be cleared prior to entry into the mode. If CSIDL = 1, the module will behave the same way as it does in Sleep mode; pending interrupt events will be passed on, even though the module clocks are not available. DS70293B-page 242 Preliminary © 2008 Microchip Technology Inc.
PIC24HJ32GP302/304, <strong>PIC24HJ64</strong>GPX02/X04, AND PIC24HJ128GPX02/X04 22.4 Registers The CRC module provides the following registers: • CRC Control Register • CRC XOR Polynomial Register REGISTER 22-1: CRCCON: CRC CONTROL REGISTER U-0 U-0 R/W-0 R-0 R-0 R-0 R-0 R-0 — — CSIDL VWORD bit 15 bit 8 R-0 R-1 U-0 R/W-0 R/W-0 R/W-0 R/W-0 R/W-0 CRCFUL CRCMPT — CRCGO PLEN bit 7 bit 0 Legend: R = Readable bit W = Writable bit U = Unimplemented bit, read as ‘0’ -n = Value at POR ‘1’ = Bit is set ‘0’ = Bit is cleared x = Bit is unknown bit 15-14 Unimplemented: Read as ‘0’ bit 13 CSIDL: CRC Stop in Idle Mode bit 1 = Discontinue module operation when device enters Idle mode 0 = Continue module operation in Idle mode bit 12-8 VWORD: Pointer Value bits Indicates the number of valid words in the FIFO. Has a maximum value of 8 when PLEN is greater than 7, or 16 when PLEN is less than or equal to 7. bit 7 CRCFUL: FIFO Full bit 1 = FIFO is full 0 = FIFO is not full bit 6 CRCMPT: FIFO Empty Bit 1 = FIFO is empty 0 = FIFO is not empty bit 5 Unimplemented: Read as ‘0’ bit 4 CRCGO: Start CRC bit 1 = Start CRC serial shifter 0 = CRC serial shifter turned off bit 3-0 PLEN: Polynomial Length bits Denotes the length of the polynomial to be generated minus 1. © 2008 Microchip Technology Inc. Preliminary DS70293B-page 243
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