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HCS412SN Datasheet(PDF) 6 Page - Microchip Technology |
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HCS412SN Datasheet(HTML) 6 Page - Microchip Technology |
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6 / 44 page  HCS412 DS41099C-page 6 Preliminary © 2002 Microchip Technology Inc. 2.2 Architecture Overview 2.2.1 WAKE-UP LOGIC AND POWER DISTRIBUTION The HCS412 automatically goes into a low-power Standby mode once connected to the supply voltage. Power is supplied to the minimum circuitry required to detect a wake-up condition; button activation or LC sig- nal detection. The HCS412 will wake from Low-power mode when a button input is pulled high or a signal is detected on the LC0 LF antenna input pin. Waking involves powering the main logic circuitry that controls device operation. The button and transponder inputs are then sampled to determine which input activated the device. A button input activation places the device into Encoder mode. A signal detected on the transponder input places the device into Transponder mode. Encoder mode has priority over Transponder mode so a signal on the transponder input would be ignored if it occurred simultaneously to a button activation; ignored until the button input is released. 2.2.2 CONTROL LOGIC A dedicated state machine, timer and a 32-bit shift reg- ister perform the control, timing and data manipulation in the HCS412. This includes the data encryption, data output modulation and reading of and writing to the onboard EEPROM. 2.2.3 EEPROM The HCS412 contains nonvolatile EEPROM to store configuration options, user data and the synchroniza- tion counter. The configuration options are programmed during pro- duction and include the read protected security-related information such as crypt keys, serial number and dis- crimination value (Table 7-2). The 64 bits (4x16-bit words) of user EEPROM are read/ write accessible through the low frequency communi- cation path as well as in-circuit, wire programmable during production. The initial synchronization counter value is pro- grammed during production. The counter is imple- mented in Grey code and updated using bit writes to minimize EEPROM writing over the life of the product. The user need not worry about counter format conver- sion as the transmitted counter value is in binary for- mat. Counter corruption is protected for by the use of a semaphore word as well as by the internal circuitry ensuring the EEPROM write voltage is at an accept- able level prior to each write. The EEPROM is programmed during production by clocking (S2 pin) the data into the DATA pin (Section 7.0). Certain EEPROM locations can also be remotely read/written through the LF communication path (Section 4.3). 2.2.4 CONFIGURATION REGISTER The first activation after connecting power to the HCS412, the device retrieves the configuration from EEPROM storage and buffers the information in a con- figuration register. The configuration register then dic- tates various device operation options including the RC oscillator tuning, the S2/RFEN/LC1 pin configuration, low voltage trip point, modulation format,... 2.2.5 ONBOARD RC OSCILLATOR AND OSCILLATOR TUNE VALUE (OSCT) The HCS412 has an onboard RC oscillator. As the RC oscillator is susceptible to variations in process param- eters, temperature and operating voltage, oscillator tuning is provided for more accurate timing character- istics. The 4-bit Oscillator Tune Value (OSCT) (Table 2-2) allows tuning within ±4% of the optimal oscillator speed at the voltage and temperature used when tuning the device. A properly tuned oscillator is then accurate over temperature and voltage variations to within ±10% of the tuned value. Oscillator speed is significantly affected by changes in the device supply voltage. It is therefore best to tune the HCS412 such that the variance in oscillator speed be symmetrical about an operating mid-point (Figure 2-7). ie... • If the design is to run on a single lithium battery, tune the oscillator while supplying the HCS412 with ~2.5V (middle of the 3V to 2V usable battery life). • If the design is to run on two lithium batteries, tune the oscillator while supplying the HCS412 with ~4V (middle of 6V to 2V battery life). • If the design is to run on 5V, tune the oscillator while supplying the HCS412 with 5V. Say the HCS412’s oscillator is tuned to be optimal at a 6V supply voltage but the device will operate on a sin- gle lithium battery. The resulting oscillator variance over temperature and voltage will not be ±4% but will be more like -7% to -15%. Programming using a supply voltage other than 5V may not be practical. In these cases, adjust the oscilla- tor tune value such that the device will run optimally at the target voltage. (i.e., If programming using 5V a device that will run at 3V, program the device to run slow at 5V such that it will run optimally at 3V). |
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