M41T00AUDD1E STMICROELECTRONICS [STMicroelectronics], M41T00AUDD1E Datasheet - Page 23

• Manufacturer Part Number: M41T00AUDD1E
• Description: Serial real-time clock with audio
• Manufacturer: STMICROELECTRONICS [STMicroelectronics]

5.5
Trickle charge circuit
The M41T00AUD includes a trickle charge circuit to be used with a backup capacitor. It is
illustrated in
supply input. (The input nature is not depicted in the figure.) The trickle charge output
function is a secondary capability, and reduces the need for external components.
To enable trickle charging, two switches must be closed. A diode is present to prevent
current from flowing backwards from V
path.
An additional switch allows the diode to be bypassed through a 20k resistor. This should
charge the capacitor to a higher level thus extending backup life. This switch automatically
opens when the device switches to backup thus preventing capacitor discharge to V
Furthermore, at switchover to backup, the other switches open as well. The application
must close them after power up to re-enable the trickle charge function.
The use of two switches in the chain is to protect against accidental, unwanted charging as
might be the case when using battery backup. Additionally, one of the two switches
requires four bits to be changed from the default value before it will close. This prevents
single bit errors from closing the switch. The four bits, TCHE3:TCHE0, reside in register
09h at bits D3 to D0.
The control bit for the second switch, TCH2, resides in register 08h at bit D5. With this bit in
a separate register, two bytes must be written before charging will occur, again protecting
against inadvertent charging due to errors.
The control bit for the bypass switch, TCFE, resides in register 09h at bit D6.
To enable trickle charging, the user must set TCHE3:TCHE0 to 5h, and TCH2 to 1. To
bypass the diode, TCFE must be set to 1. All three fields must be enabled after each power
up.
Figure 14. Trickle charge circuit
V CC
Table
TCHE
14. V
TCHE/ = 5h
TCHE = 5h
CLOSED
BACK
OPEN
is a bi-directional pin. Its primary function is as the backup
TCH2
BACK
TCH2 = 0
TCH2 = 1
CLOSED
OPEN
to V
CC
. A current limiting resistor is also in the
TCFE
TCFE = 0
TCFE=1
CLOSED
OPEN
940 Ω
20 Ω
180 Ω
V BACK
ai13327
CC
.
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