HCS412 Datasheet, PDF (6/44 Pages) Microchip Technology – KEELOQ Code Hopping Encoder and Transponder

HCS412
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).
DS41099C-page 6
Preliminary
© 2002 Microchip Technology Inc.