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Step 9USB Interface, Power Supply
The connection to the PC is very straightforward. The DPScope uses a USB-to-serial converter cable from FTDI. This cable has a FT232R chip built into the USB connector and translates the USB data stream into a standard RS-232 asynchronous serial data stream (output levels on the serial side are TTL, not true RS-232 - this is what the microcontroller needs anyway). That way the scope does not have to deal with a complex USB interface but sends and receives data as if the connection were a simple RS-232 link.
On the PC side there is a driver that emulates a RS-232 connection as well, so the scope application only has to deal with a standard serial connection, too. Data transfer rate is still a respectable 500 kBaud (the converter cable could go up to 1 Mbaud but the transmitted data volume is too small to really need full bandwidth - each record is approx. 0.5 KByte, so even at 40 frames/sec this amounts to just 20 KByte/sec or around 200 kBaud average serial data rate (note that each data byte needs a start bit and a stop bit, so for each byte actually 10 bits are sent).
The interface uses software handshaking (the scope responds to each transmission from the PC with an acknowledge packet), so while CTS and RTS are physically connected - just in case a later revision wants to use them - they are not used in the current design.
The converter cable also makes the USB port's 5V supply available. The DPScope needs about 250mA of supply current, a USB port can provide up to 500 mA, so the scope can get its power from the USB line and does not need an external power supply. One word of caution though, some USB ports have rather large variation in supply voltage (sometimes down to little more than 4V). In my experience this is mostly true when connecting to an unpowered USB hub or when many other power-hungry devices are connected to the same USB hub, so try to avoid this. The DPScope assumes a 5V supply and uses that as its voltage reference, so while it will function even at lower voltage, any deviation directly impacts its voltage accuracy. Some USB ports (laptops are notorious) are also quite noisy, this will show up as noise on the measured signals.
If you are absolutely unable to provide close to 5V from USB, or the USB supply turns out to be too noisy, then the DPScope allows to use an external power supply (7.5V/500mA). All you need to do is add the regulator (REG, a simple 7805 linear regulator) and a buffer capacitor (C5), hook up the power supply, and set the jumper (PWR_SEL) to "external supply". REG and C5 are very generic types and should be available in any hobby electronics store (and in any case are most likely already in your drawer).
On the PC side there is a driver that emulates a RS-232 connection as well, so the scope application only has to deal with a standard serial connection, too. Data transfer rate is still a respectable 500 kBaud (the converter cable could go up to 1 Mbaud but the transmitted data volume is too small to really need full bandwidth - each record is approx. 0.5 KByte, so even at 40 frames/sec this amounts to just 20 KByte/sec or around 200 kBaud average serial data rate (note that each data byte needs a start bit and a stop bit, so for each byte actually 10 bits are sent).
The interface uses software handshaking (the scope responds to each transmission from the PC with an acknowledge packet), so while CTS and RTS are physically connected - just in case a later revision wants to use them - they are not used in the current design.
The converter cable also makes the USB port's 5V supply available. The DPScope needs about 250mA of supply current, a USB port can provide up to 500 mA, so the scope can get its power from the USB line and does not need an external power supply. One word of caution though, some USB ports have rather large variation in supply voltage (sometimes down to little more than 4V). In my experience this is mostly true when connecting to an unpowered USB hub or when many other power-hungry devices are connected to the same USB hub, so try to avoid this. The DPScope assumes a 5V supply and uses that as its voltage reference, so while it will function even at lower voltage, any deviation directly impacts its voltage accuracy. Some USB ports (laptops are notorious) are also quite noisy, this will show up as noise on the measured signals.
If you are absolutely unable to provide close to 5V from USB, or the USB supply turns out to be too noisy, then the DPScope allows to use an external power supply (7.5V/500mA). All you need to do is add the regulator (REG, a simple 7805 linear regulator) and a buffer capacitor (C5), hook up the power supply, and set the jumper (PWR_SEL) to "external supply". REG and C5 are very generic types and should be available in any hobby electronics store (and in any case are most likely already in your drawer).
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