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HCS370IST Datasheet(PDF) 11 Page - Microchip Technology |
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HCS370IST Datasheet(HTML) 11 Page - Microchip Technology |
11 / 36 page 2002 Microchip Technology Inc. Preliminary DS41111D-page 11 HCS370 3.2 Code Word Format A KEELOQ code word consists of 32 bits of hopping code data, 32 bits of fixed code data, and between 3 to 5 bits of status information. Various code word formats are shown in Figure 3-1 and Figure 3-2. 3.2.1 HOPPING CODE PORTION The hopping code portion is calculated by encrypting the counter, discrimination value, and function code with the Encoder Key (KEY). The hopping code is cal- culated when a button press is debounced and remains unchanged until the next button press. The synchronization counter can be either a 16- or 20- bit value. The Configuration Option Counter Select (CNTSEL) will determine this. The counter select option must be the same for both Encoder 1 and Encoder 2. If the 16-bit counter is selected, the discrimination value is 10 bits long and there are 2 counter overflow bits (OVR0, OVR1). Set both bits in production and OVR0 will be cleared on the first counter overflow and OVR1 on the second. Clearing OVR0 with OVR1 set will only detect the first overflow. Clearing both OVR bits will effectively give 12 constant bits for discrimination. If the counter is 20 bits, the discrimination value is 8 bits long and there are no overflow bits. The rest of the 32 bits are made up of the function code also known as the button inputs. The discrimination value can be programmed with any value to serve as a post decryption check on the decoder end. In a typical system, this will be pro- grammed with the 8 or 10 Least Significant bits of the serial number. This will be stored by the receiver sys- tem after a transmitter has been learned. The discrimi- nation bits are part of the information that is to form the encrypted portion of the transmission. 3.2.2 FIXED CODE PORTION The 32 bits of fixed code consist of 28 bits of the serial number (SER) and a copy of the 4-bit function code. This can be changed to contain the whole 32-bit serial number by setting the Extended Serial Number (XSER) configuration option to a 1. If more than one button is pressed, the function codes are logically OR’ed together. The function code is repeated in the encrypted and unencrypted data of a transmission. TABLE 3-4: FUNCTION CODES 3.2.3 STATUS INFORMATION The status bits will always contain the output of the Low Voltage (VLOW) detector and Cyclic Redundancy Check (CRC). If Queue (QUEN) is enabled, button queue information will be included in the code words. FIGURE 3-1: CODE WORD DATA FORMAT (16-BIT COUNTER) Button Function Code 2 S0 xx1x 2 S1 x1xx 2 S2 1xxx 2 S3 xxx1 2 S4 111x 2 S5 11x1 2 Fixed Code Portion (32 Bits) CRC 2 Bits VLOW 1-Bit SERIAL NUMBER (28 Bits) C1 C0 S2 S1 S0 S3 BUT 4 Bits Counter Overflow 2 Bits DISC 10 Bits Synchronization 16 Bits Counter 15 0 S1 S2 S0 S3 OVR1 OVR0 Transmission Direction LSB First Hopping Code Portion (32 Bits) With XSER=0, 16-bit Counter, QUEN=0 Status Information (3 Bits) BUT 4 Bits Fixed Code Portion (32 Bits) QUE 2 Bits CRC 2 Bits VLOW 1-Bit SERIAL NUMBER (32 Bits) Q1 Q0 C1 C0 BUT 4 Bits Counter Overflow 2 Bits DISC 10 Bits Synchronization 16 Bits Counter 15 0 S2 S1 S0 S3 OVR1 OVR0 Hopping Code Portion (32 Bits) With XSER=1, 16-bit Counter, QUEN=1 Status Information (5 Bits) |
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