Functional description ST7540
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6.4 ST7540 Mains access
ST7540 can access the Mains in two different ways:
● Synchronous access
● Asynchronous access
The choice between the two types of access can be performed by means of Control
Register bit 14(see Ta b l e 1 2) and affects the ST7540 data flow in Transmission Mode as in
Reception Mode (for how to set the communication Mode, see Section 6.5).
In data transmission mode:
● Synchronous Mains access: on clock signal provided by ST7540 (CLR/T line) rising
edge, data transmission line (TxD line) value is read and sent to the FSK Modulator.
ST7540 manages the Transmission timing according to the BaudRate Selected.
● Asynchronous Mains access: data transmission line (TxD line) value enters directly to
the FSK Modulator. The Host Controller manages the Transmission timing (CLR/T line
should be neglected).
In data reception mode:
● Synchronous Mains access: on clock signal recovered by a PLL from ST7540 (CLR/T
line) rising edge, value on FSK Demodulator is read and put to the data reception line
(RxD line). ST7540 recovers the bit timing timing according to the BaudRate Selected.
● Asynchronous Mains access: Value on FSK Demodulator is sent directly to the data
reception line (RxD line). The Host Controller recovers the communication timing
(CLR/T line should be neglected).
76
600
--
132.5
600
--
1 75684 76335 1 132161 132813
1200
0.5 75684 76335
1200
0.5 132161 132813
1 75358 76660 1 131836 133138
2400
0.5 75358 76660
2400
0.5 131836 133138
1 74870 77148 1 131348 133626
4800
0.5 74870 77148
4800
0.5 131348 133626
1 73568 78451 1 130046 134928
Table 8. ST7540 synthesized frequencies
