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Application Note
AN_289 FT51A Programming Guide
Version 1.0
Document Reference No.: FT_000962 Clearance No.: FTDI# 483
15
Copyright © 2015 Future Technology Devices International Limited
PMM is entered and exited by setting the PMM bit (PCON.0). In addition, use of the switchback
feature is possible to affect a return from PMM to the full speed mode. This allows both hardware
and software to cause an exit from PMM. It is the responsibility of the software to test for UART
activity before attempting to change speed, as a modification of the clock divider bits during a
UART operation will corrupt the data.
The switchback feature allows a system to burst to a faster mode when required by an external
event. Enable this feature by setting the PCON bit 2, a qualified interrupt (interrupt which has
occurred and been acknowledged) or serial port reception or transmission cause the
microcontroller to return to full speed mode. An interrupt must be enabled and not blocked by a
higher priority interrupt. Software should manually return the microcontroller to PMM after the
event is completed. The following sources can trigger the Switchback:
external interrupt 0/1,
serial start bit detected, UART,
transmit buffer loaded, UART,
reset,
external reset.