How and when should I switch from synchronous mode to asynchronous mode when generating Caller ID on the CMX605 and CMX615?
A. View Diagram Begin a synchronous transmission of ‘channel seize’ pattern by loading the Data Buffer with 0x55h. Initially read the Status Register to clear it then enable the FSK transmission using bit 6 of the Mode Register. The IRQ will go low almost immediately the FSK is enabled because the data buffer becomes available. Ignore this IRQ and IRQN will stay low. After approx 7.5ms (9 bit periods) a Tx Underflow will occur. This will be flagged but no interrupt will occur because the previous one was not serviced. Provided no load of the Data Buffer takes place the 0x55h will re-circulate indefinitely giving the required channel seize pattern. After the required period, the Data Buffer should be loaded with $FF. The FSK transmission will change to continuous marks. Wait a few bit periods and then select asynchronous mode. The underflow condition will still exist and so the UART will continue to produce continuous marks. When the continuous marks period ends; 1. Load the data buffer
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