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  USB DTMFTransceiver  SOFTMARK PO Box 1609Hornsby NSW 2077 The USB DTMF Transceiver is able to generate and receive DTMF tones from varioussources. The card is controlled by the USB port. Windows automatically finds the card and the project is ready to be used immediately. Software for this card works on Windows 98 and  XP   USB DTMF Transceiver Kit description The DTMF signal consists of a mixture of two sine tones: Figure 1.  The DTMF tone is generated when you press a button which consists of a mixtureof row and column frequencies. DTMF tones can represent one of 16 different states or symbols. Applications Paging systemsRepeater systemsMobile radioPABX systemsComputer systemsFax machinesRobotics Input configurations for MT8880 MT8880, which is used in this kit, is manufactured using state of the art CMOS technology.There are two input configurations, single ended and differential input configurations. Figure 2.  Single ended input configuration  Figure 3.  Differential input configurationTo find out more about settings, refer to the technical data sheet for MT8880. How it works Communication with MT8880 takes place over a 4-bit bus consisting of D0 to D3 and additionalbits which select modes of operation. These additional bits are chip select (CS), read/write(RW) and register select (RS). MT8880 is active only when CS is zero. RW determines the datadirections: 1 = Read (data from MT8880 by USB controller) and 0 = write (data from USBcontroller to MT8880). RS determines whether the transactions involve data (DTMF tones) or internal MT 8880 functions (instructions or status): 1 = instructions/status and 0 = data. Once theMT8880 is set up, the USB controller writes 000 to CS, RW and RS. This is done in order to sendDTMF tones. The USB controller can also write 010 to read the DTMF tones.Before you use MT8880, you have to set it up. You have to initialize both modes separately. For receiver mode, initializing takes place after the “DTMF Receiver” button is pressed (Figure 4). After that, the receiver is ready to read DTMF signals supplied to the audio input pin. Audio inputis configured to be a single ended input amplifier built inside MT8880. Amplifier gain can bechanged as per the description for the single input amplifier (Figure 2). How to receive and send DTMF codes  Figure 4.  Virtual panel of the DTMF Transceiver Pressing the “DTMF Receiver” button will set registers CRA and CRB for receiving mode and youwill be able to receive DTMF signals from inputs. Each DTMF signal will be appended to thevalue currently displayed. The displayed value can be reset at any time by pressing the “ResetDisplay” button.If you would like to transmit DTMF tones, you must firstly press the “DTMF Transmitter” button(Figure 4). After that, any numerical button pressed will generate DTMF tones. DTMF output willbe amplified by IC2. The audio level can be changed by setting the potentiometer built directly onthe PC board (VR1). LM386’s amplifier can generate around 1W of audio power and it can beconnected to a small speaker (speaker is not included in the kit). Each received tone is displayedon the DTMF Receiver display. The next received DTMF tone is appended to the value on thedisplay. This display can be cleared by pressing the “Reset Display” button. How to connect the DTMF Receiver directly to the phone line This will require differential input configuration for MT8880 (Figure 3). Please find moreinformation regarding this in the technical data sheet for MT8880. Building the card The PCB top view (element side) is shown in Figure 5 below. Initialize DTMF ReceiverInitialize DTMF Transmitter 
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