Gertboard overview
A lot of people want to know more details about the Gertboard. So I decided to release this text which
will be part of the Gertboard manual.
Principle diagram Gertboard.
Raspi
strapping area
D
in
1k
A
MCP4802
Motor
controller
L6203
3x
Micro controller
12x
ATmega
74xx244
A
D
out
MCP3002
SPI/dbg
ULN2803a
Above is a principle diagram
1
of the Gertboard.
Each ‘circle’ in the diagram above is a header pin. That gives you access to a wide range of control
combinations. To start off you would probably use the strapping area to connect various components
to the Raspberry-Pi. However there is nothing which prevents you from connecting the motor
controller PWM pins to the ATmega device. The ATmega device has a separate 6-pin header
compatible with the JTAGIce pins. Again you can connect these to the Raspberry-Pi SPI interface to
program the device straight from the Linux environment. (Requires SPI drivers & software which are
need to be provided by Linux volunteers).
The major building blocks are:
12x buffered I/O
3x push button
6x Open collector driver (50V, 0.5A)
48V, 4A motor controller
28-pin dual in line ATmega device
2-channel 8/10/12 bits Digital to Analogue converter.
2-channel 10 bits Analogue to Digital converter.
Buffered I/O.
12 pins which can be used as input or output.
Each buffer can be set to input or output mode using a jumper.
1
A ‘principle diagram’ is a coarse overview of the most important parts of the system. It is not correct in all details. For that
you must look at the board schematics.
open collector (6x)
SPI
PWM
I/O
UART
I/O
1k
I/O ports 4-12 look like this:
Raspi
input
1k-10k
74xx244
output
I/O
1k
In order to make the port function as input you have to place the ‘input’ jumper.
In order to make the port function as output you have to place the ‘output’ jumper.
You can place both jumpers but the port will not function well in that case.
In both the input and output mode the LED will tell you what the logic level is on the I/O pin. The
LED will be
on
when the level is
high
and it will be
off
when the level is
low.
Depending on the type of 74xx244 buffer chosen the LED might show random behaviour if the port is
not driven. In that case it may also go on or off on the smallest of electronic changes.
There is a third option: If neither the input nor output jumper is placed the I/O pin can be used as a
simple ‘logic’ detector. You can connect the I/O pin to some other logic (3V3) point and use the LED
to check if the connect point is seen as high or low.
There is a series resistor between input buffer and the GPIO port. This is to protect the BCM2835 if
the user programs the GPIO as output but also leaves the ‘input’ jumper in place. The BCM2835 input
is a high impedance input and thus even a 10K series resistor will not produce a noticeable change in
behaviour when it is used as input.
Push buttons.
The Gertboard has three push buttons. They are connected to the BUF1, BUF2 and BUF3 jumpers on
the BCM2835 side.
Thus the first three I/O port looks like this:
Raspi
input
1k
1k-10k
74xx244
output
I/O
1k
In order to use a push button the user must do the following:
1/ Do not place the ‘input’ jumper.
2/ Program the GPIO pin to be set to input mode.
3/ Program the GPIO pin to use the internal pull-up resistor.
Optionally you can place the output jumper to show the button state on the LED.
Beware: the push buttons can
not
be used on GPIO0 or GPIO1 as those two pins have a 1K8 pull-up
resistor on the board. When the button is pressed the voltage on the input will be
3.3*1000/(1000+1800) = 1.1V
That is not an I/O voltage which can reliable seen as low.
Open collector driver.
The Gertboard uses six ports of a ULN2803a for open collector driver. The ULN2803a can withstand
up to 50V and drive 500mA on each of its ports. The device has integrated protection diodes.
Each output port looks like this:
common
OUT
Raspi
The ‘common’ pin is, as the name states, common for all open collector drivers. It is not connected to
any other point on the Gertboard. For more details see the datasheet of the ULN2803a.
As with all I/O devices the open collector drivers can also be connected to the ATmega controller e.g.
to drive a number of relays or motors.
Motor controller.
The Gertboard has place for a L6203 (Miniwatt package) motor controller. This is a rather expensive
part so not everybody might want to mount that. The motor controller is for brushed DC motors.
The controller has two input pins A and B. The pins can be driven high or low according to the
following truth table:
A
B
Motor action
0
0 No movement
0
1 Rotate one way
1
0 Rotate opposite way from above
1
1 No movement
The speed of the motor can be controlled by applying a Pulse-Width-Modulated (PWM) signal to
either the A or B pin.
The motor controller has temperature protection but no current protection. The Gertboard holds a fuse
to provide that function. Again for more details see the datasheet of the L6203.
ATmega device.
The Gertboard can hold a 28-pin ATmega device. This can be any of
ATmega48A/PA, 88A/PA, 168A/PA or 328/P in a 28PIN DIP package. The device has a 12MHz
ceramic resonator. So the pins 9 and 10 cannot be used. All other pins are brought out to headers. The
boards has a separate 6-pin header which is compatible with the JtagIce interface. The reset pin (1)
has a 10K pull-up resistor. I even slavishly copied the diode in parallel with that pull-up resistor
although nobody has been able to tell me what its function is.
The PD0/PD1 pins (ATmega UART TX and RX) are also brought out to pins placed adjacent to the
Raspberry-Pi UART pins so you only need to place two jumpers to connect the two devices.
Beware the ATmega devices on the Gertboard operate at 3.3Volts. That is in contrast to the ‘Arduino’
system which runs at 5V. It is also the reason why the device does not have a 16MHz. Clock. In fact
at 3V3 the maximum operating frequency according to the specification is just
under
12MHz.
Digital to Analogue converter.
The Gertboard uses a MCP48xx digital to analogue converter from Microchip. The device comes in
three different types: 8,10 or 12 bits. Each supports 2 channels. The device is controlled by an SPI
interface. It is connected to the SPI chip select 1.
For details see the datasheet of the MCP4802
Analogue to Digital converter.
The Gertboard uses a MCP3002 10-bit analogue to digital converter from Microchip. It supports 2
channels with a sampling rate of ~72ksps. The device is controlled by an SPI interface. It is connected
to the SPI chip select 0.
For details see the datasheet of the MCP3002.
To make it easier to interface the A/D and D/A converter to the Raspberry-Pi all SPI signals from the
two devices are placed adjacent to the Raspberry-Pi SPI interface signal. To connect both devices you
have to place 5 jumpers: Clock, Mosi, Miso and the two chip selects. Both A/D and D/A devices
chip-select signals have a 10K pull-up so the device will not be accessed if the chips select pin is not
connected.
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