LEA-M8F - Hardware Integration Manual
UBX-14000034 - R03
Early Production
Information
Hardware
description
Page 10 of 30
enabled by connecting pin 5 (
D_SEL
) to ground - see 1.6.2 below. For speed and clock frequency specifications,
see the
LEA-M8F Data Sheet
1.5.5
DDC interface for External DAC Control
A dedicated DDC (I2C) interface (pins
SDA_DAC, SCL_DAC
) is provided for implementations in which the LEA-
M8F controls an external voltage controlled frequency reference via a DAC. This is set up via FTS specific
configuration messages. See the
u-blox M8 Receiver Description Protocol Specification
[2] and the
u-blox M8F
Applications Guide
[3] for more information. The DDC pins
SDA_DAC
and
SCL_DAC
have internal 10 k
Ω
pull-
up resistors.
When the LEA-M8F is configured to discipline a voltage controlled oscillator via the dedicated DDC interface
(
SDA_DAC, SCL_DAC
) a choice must be made with respect to the DAC component. The recommended types
(TI or Microchip) offer 16 or 12 bit resolution respectively and should be chosen for the desired performance/cost
requirements. This section shows a suggested circuitry for implementing a 16 bit TI DAC analog filter
combination connected to a VCTCXO/VCOCXO. Note that 12 bit DAC may not provide sufficient resolution if
used over the full circuit voltage range and hence may compromise the controlled frequency performance.
Implementing a circuit using a smaller DAC voltage range and adding the output to a fixed low noise offset
voltage would be beneficial.
Figure 3: 16 bit DAC connection for VCOCXO control
1.6
I/O and Control Pins
1.6.1
RESET_N
RESET_N
is an input-only pin with an internal pull-up resistor. The LEA-M8F performs an automatic reset
function on application of the power supply but this input may also be used to re-start the device during
operation if necessary. The pin must be held low for at least 10 ms to ensure
RESET_N
is detected. Leave
RESET_N
open for normal operation. The
RESET_N
input complies with the power supply VCC voltage level and
can be actively driven high. Use this pin only to reset the module. Do not use
RESET_N
to turn the module on
and off since the reset state increases power consumption.
1.6.2
D_SEL
The
D_SEL
pin selects the available communication interfaces available at the module pins. This allows a choice
between UART+DDC or SPI control of the module, see Table 3 below. If
D_SEL
is left open both UART and DDC
are available. If pulled low, a single SPI interface is available. See the
LEA-M8F Data Sheet
