Voltage Input And Ground; Communication - IDTECH SecureHead User Manual

The NCS is pulled low when the host is communicating with the device.

3.7. Voltage Input and Ground

The VIN signal is the power input for the device and has an operating range of 3.0 to 3.6 volts DC.
The GND signal is logic ground. The head case GND signal is chassis ground which is connected to
the head case. For optimum ESD protection, this signal should be connected to earth ground.

3.8. Communication

When the host has a frame to send, it simply clocks it out. When the device has a frame to send, it
raises its data available (DAV) signal and waits for the host to clock in the frame. The host normally
clocks out IDLE characters to clock in a frame from the device. Since the device typically loads its one
transmit buffer with IDLE byte when it has nothing to transmit, the first 1 byte clocked out from the
device after the DAV signal is asserted could be IDLE bytes instead of a valid byte. If this is the case,
simply discard this byte. To detect whether the device has a frame to send, the host can either monitor
the DAV signal or, optionally, periodically clock in up to two bytes from the device to see if the device
has sent a valid data. Up to two bytes should be clocked in instead of just one because the first byte
could be IDLE byte that was loaded into the device's transmit buffers before the device had anything
to send. The host should look at each byte it clocks in to see if it is a valid byte. If a valid byte is found,
then the subsequent bytes will contain the frame.
Copyright © 2010-2013, International Technologies & Systems Corporation. All rights reserved.
SecureHead SPI Interface with FPC User Manual
Page 13 of 67