
EM358x
56
Rev. 0.4
Figure 7-2. nBOOTMODE and nRESET Timing
Timing for a power-on-reset is similar except that OSCHF does not begin oscillating until up to 70 µsec after both
core and HV supplies are valid. Combined with the maximum 250 µsec allowed for HV to ramp from 0.5 V to
1.7 V, an additional 320 µsec may be added to the 512 OSCHF clocks until nBOOTMODE is sampled.
If the mode is selected (nBOOTMODE is low after 512 clocks), the FIB monitor software begins execution. In
order to filter out inadvertent jumps into themonitor, the FIB monitor re-samples the nBOOTMODE signal after a
3 ms delay. If the signal is still low, then the device stays in monitor mode. If the signal is high, then monitor mode
is exited and the normal program begins execution. In summary, the nBOOTMODE signal must be held low for
4 ms in order to properly invoke the monitormode.
After nBOOTMODE has been sampled, PA5 is configured as a floating input like the other GPIO configurations.
The GPIO_BOOTMODE bit in the GPIO_DBGSTAT register captures the state of nBOOTMODE so that software
may act on this signal if required.
Note:
To avoid inadvertently asserting nBOOTMODE, PA5’s capacitive load may not exceed 250 pF.
7.6 GPIO Modes
7.6.1
Analog Mode
Analog mode enables analog functions, and disconnects a pin from the digital input and output logic. Only the
following GPIO pins have analog functions:
PA0 and PA1 can be the differential IO pins for the USB device.
PA4, PA5, PB5, PB6, PB7, and PC1 can be analog inputs to the ADC.
PB0 can be an external analog voltage reference input to the ADC, or it can output the internal analog voltage
reference from the ADC. The Ember software selects an internal or external voltage reference.
PC6 and PC7 can connect to an optional 32.768 kHz crystal.
Note:
When an external timing source is required, a 32.768 kHz crystal is commonly connected to PC6 and PC7.
Alternatively, when PC7 is configured as a digital input, PC7 can accept a digital external clock input.
When configured in analog mode:
The output drivers are disabled.
The internal pull-up and pull-down resistors are disabled.
The Schmitt trigger input is connected to a high logic level.
Reading GPIO_PxIN returns a constant 1.
7.6.2
Input Mode
Input mode is used both for general purpose input and for on-chip peripheral inputs. Input floating mode disables
the internal pull-up and pull-down resistors, leaving the pin in a high-impedance state. Input pull-up or pull-down
mode enables either an internal pull-up or pull-down resistor based on the GPIO_PxOUT register. Setting a bit to
0 in GPIO_PxOUT enables the pull-down and setting a bit to 1 enables the pull up.
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