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Supply, reset and clock management
ST7LITEUS2, ST7LITEUS5
6
Supply, reset and clock management
The device includes a range of utility features for securing the application in critical
situations (for example in case of a power brown-out), and reducing the number of external
components.
6.1 Main
features
●
Clock management
–
8 MHz internal RC oscillator (enabled by option byte)
–
External clock Input (enabled by option byte)
●
Reset sequence manager (RSM)
●
System integrity management (SI)
–
Main supply low voltage detection (LVD) with reset generation (enabled by option
byte)
–
Auxiliary voltage detector (AVD) with interrupt capability for monitoring the main
supply
6.2
Internal RC oscillator adjustment
The ST7 contains an internal RC oscillator with a specific accuracy for a given device,
temperature and voltage. It can be selected as the start up clock through the CKSEL[1:0]
option bits (see
). It must be calibrated to obtain the frequency
required in the application. This is done by software writing a 10-bit calibration value in the
RCCR (RC Control register) and in the bits [6:5] in the SICSR (SI Control Status register).
Whenever the microcontroller is reset, the RCCR returns to its default value (FFh), i.e. each
time the device is reset, the calibration value must be loaded in the RCCR. Predefined
calibration values are stored in Flash memory for 3.3 and 5 V V
DD
supply voltages at 25°C,
as shown in the following table.
Table 5.
Predefined RC oscillator calibration values
RCCR
Conditions
ST7LITEUS2/ST7LITEUS5 address
RCCRH0
V
DD
=5 V
T
A
=25 °C
f
RC
=8 MHz
DEE0h
(1)
(CR[9:2] bits)
1.
DEE0h, DEE1h, DEE2h and DEE3h are located in a reserved area butare special bytes containing also
the RC calibration values which are read-accessible only in user mode. If all the Flash space (including the
RC calibration value locations) has been erased (after the readout protection removal), then the RC
calibration values can still be obtained through these two addresses.
RCCRL0
DEE1h
1)
(CR[1:0] bits)
RCCRH1
V
DD
=3.3 V
T
A
=25 °C
f
RC
=8 MHz
DEE2h
1)
(CR[9:2] bits)
RCCRL1
DEE3h
1)
(CR[1:0] bits)
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