MAX-8 / MAX-M8 - Hardware Integration Manual
UBX-15030059 - R05
Hardware description
Page 6 of 31
Production Information
1.3.1
VCC_IO: IO Supply Voltage
VCC_IO
from the host system supplies the digital I/Os. The wide range of
VCC_IO
allows seamless
interfacing to standard logic voltage levels independent of the VCC voltage level. In many applications,
VCC_IO
is simply connected to the main supply voltage.
☞
Without a
VCC_IO
supply, the system will remain in reset state.
1.3.2
V_BCKP: Backup supply voltage
If there is a power failure on the module supply (VCC_IO), the real-time clock (RTC) and battery backed
RAM (BBR) are supplied through the
V_BCKP
pin. Thus orbit information and time can be maintained
and will allow a hot or warm start. If no backup battery is connected, the module performs a cold start
at every power up if not aiding data are sent to the receiver
☞
Avoid high resistance on the
V_BCKP
line: During the switch from main supply to backup supply,
a short current adjustment peak can cause high voltage drop on the pin with possible
malfunctions.
☞
If no backup supply voltage is available, connect the
V_BCKP
pin to
VCC_IO
.
☞
As long as power is supplied to the u-blox 8 / M8 modules through the
VCC_IO
pin, the backup
battery is disconnected from the RTC and the BBR to avoid unnecessary battery drain (see Figure
1). In this case,
VCC_IO
supplies power to the RTC and BBR.
Figure 1: Backup battery and voltage (for exact pin orientation, see the
MAX-8 Data Sheet
[1]
and
MAX-M8 Data Sheet
[2])
Single Crystal feature on MAX-8C / MAX-M8C
On MAX-8C / MAX-M8C, the reference frequency for the RTC clock will be internally derived from the
main clock frequency (26 MHz) when in Backup Mode (does not have a 32 kHz oscillator). This feature
is called “single crystal” operation. In the event of a power failure, the backup battery at
V_BCKP
will
supply the 26 MHz crystal oscillator, as needed to maintain the time. This makes MAX-8C / MAX-M8C
a more cost efficient solution at the expense of a higher backup current, as compared to other MAX-
8 / MAX-M8 variants that use an ordinary RTC crystal. Therefore, the capacity of the backup battery
at
V_BCKP
must be increased if Hardware Backup Mode is needed (See the
MAX-8 Data Sheet [1] and
MAX-M8 Data Sheet [2] for specification).
If the battery used cannot provide the increased current consumption for the needed time on MAX-
8C / MAX-M8C, the “single crystal” feature can be permanently disabled. Then the backup current
will be same as on MAX-8 / MAX-M8 modules without “single crystal” feature. But the time
information is not maintained during off time. So the customer either aides the time at every startup
to the receiver or the hot and warm start performance will be degraded because of missing time
information.
Send this string to disable the “single crystal” feature: “B5 62 41 09 00 01 01 92 81 E6 39 93 2B EE
30 31”.
