The AS2591 is a CMOS IC used for driving LCDs with four backplanes and 24 (or 31) segments. This arrangement provides 12 (or 16) 7-segment digits " and 12 symbols. It is meant to provide an easy interface for the miniature alpha-numeric displays used in cellular handsets. Combined with Austriamicrosystems' AS2525 single chip telephone, the display driver helps build a "handsfree" cell phone with a low component count.*
The display device is controlled through a simple two-wire interface and a chip select (CS) signal. Digits are strobed in serially, with a "write-digit" or "shift&write" command for the controller.
The device includes a lapsed time counter, which is started five seconds after power up and is incremented every second. The timer is actually re-started every time the device gets a "write-digit" or "shift&write" command (except for commands to display "-", "o,", or "o").
Table 1: Hex codes provide digits telling last number dialed, time on line, even caller ID information.
While a "write-digit," "shift&write" or "blinking" command will immediately force the display to change, the content of the timer is displayed five seconds after receiving a command. This is useful in cellular telephone applications, where the last "write-digit" or "shift&write" command is tied to the telephone number dialed. The timer automatically gives a display indication of the time online.
The AS2591 incorporates a buffer for 32 digits. Digits are entered from the most right position and shifted left by new entries (see Table). Symbols will be displayed independently from selecting the upper bank of the 32 digit buffer to be displayed.
The symbol data - reflecting the status of the cell phone or PDA - is not effected by a "blinking" or by a "write&shift" command. Depending on to layout of the display, the symbols can be displayed independently from the digit segments (displaying phone numbers or elapsed time). The symbols are in the upper bank of the AS2591's 32 digit buffer. With typical 12/16 digit LCD displays, the symbol mapping does not need to be stored in EEPROM.
Figure 1: An integrated LCD digit driver provides the driving and contrast control voltage for alpha-numeric letters, numbers and symbols.
The serial interface uses three lines: chip select (CS), the serial clock line (SCLK), and the data input (DI). A single data frame consists of a 3-bit command and 5-bit data word. In sequence, the CS line goes high, and then each bit of the command and data word is strobed in with the transitions of the serial clock. The command and data word is strobed in serially from most significant bit (MSB) to least significant bit (LSB), though it appears to come in backwards. The sequence is C3, C2, C1, D4, D3, D2, D1, and D0. The actual execution of the command - the actual display of the digit - commences at the falling edge of the CS signal.
Certain symbols such as those indicating "key lock activated," "mute activated," "memory redial key has been pressed," "shift key has been pressed," and those indicating "program mode" are always enabled. Other symbols, like those showing "loudspeaker on," "DTMF dialing mode selected," "loop disconnect (pulse) dialing mode selected," "volume control keys activated" and "last number redial has been activated" can be enabled through the serial interface.
Figure 2: The interface between the LCD display and digit drivers is extremely simple (no pulldown resistors required).
With the LCD output voltage (VLCD) set to 3.27 V, each segment will get 2.97V. This voltage can actually be programmed through the serial bus to reflect a range from 2.83 V (low) to 3.09V (high) for optimizing the contrast of the LCD. Any 1/3, 2/3, and 3/3 voltage states are generated internally. In applications with varying VDD (as with voltage and frequency scaling applications), the use of a separate voltage regulator at VLCD is recommended in order to assure a constant contrast.
Build a feature phone
The operation of the AS2591 display module can be demonstrated in conjunction with the AS2525 single-chip feature phone on the Austriamicrosystems AN525 demonstration kit. The demo-kit contains the circuit board for the two chips, along with a handset, microphone, data sheets and instruction manuals.
Figure 3: Wiring diagram for a reference design.
To make the demonstration board operational, the user connects the handset (included in demo kit); he connects the telephone line interface (taking care to distinguish between the outer and inner/middle pins), and the handsfree microphone (users are reminded to observe the polarity; the capsule case is negative). While the feature phone will function without the LCD display module, the user is offered options for connecting near the on-board EPROM or the phone's keypad. A loudspeaker is not included in the demo kit; an impedance of 25 to 50 ohms is recommended.
The user will need to set some switches on the demo board. If you're operating as a speakerphone with microphone and speaker, you press a button to dial or respond to an incoming call. Therefore, the hookswitch is set to respond one way. If you use the handset, and pick it up to make a call or to respond to a call, the hookswitch is set another way. For operation in handset mode, the hookswitch must be activated. For operation in the handsfree mode, the "h/free"-switch must be toggled. The same switch can also be used to activate a "loudhearing"
Mode (when the hook-switch is in the off-hook position), which enables a speaker output to supplement the handset volume.
Figure 4: Connector and Jumper locations.
There are two ways of switching between DTMF and Pulse mode: either by hardware or by software (not both), the applications note advises. The various parameters for Pulse and DTMF mode (dialing rate, m/b-ratio, tone duration, etc.) must be preset in the programming EPROM. Alternately, the board can be programmed for hardware switching.
Selecting the AC impedance requires some additional thought, since the user needs to place resistors and capacitors in parallel with the line interface. The internal AC impedance of the phone chip is 1000 ohms. By adding a capacitor, the synthesized impedance becomes more complex.
A parallel resistor (DC-decoupled by the capacitor VSS lowers the total AC impedance by making a parallel connection between the IC's internal impedance (1000 ohms), and the external load resistors (which are typically 10 k-ohms).
For example, to set an AC impedance of 600 ohms, the load resistors could be 1.8 k-ohms (in parallel with 10k, in parallel with the 1000 ohms.
RAC = 1000 ohm // 10k-ohm // 1k8 = 600 ohms
On the demo board, two jumpers are provided to quickly implement the AC impedance change. A table printed on the demo board PCB (next to the line connector) describes the settings for nominal and complex impedances.
For sidetone cancellation, a passive complex network (two resistors and one capacitor) must be connected. This should be 10 times the value of the (complex) line termination. In other words, the sidetone network resistor values are each 10 times of line termination resistor values, while the sidetone network capacitor value is 1/10 of line termination capacitor value.
Example: for a line termination of 270 ohms + 750 ohm // 150nF, the equivalent sidetone network would be 2.7 k-ohms + 7.5 k-ohms // 15nF
If you're using a 600-ohm impedance setting), the sidetone resistor (10 times the value of the load resistor) should be connected in parallel to the sidetone network to compensate for the additional load. On the demoboard, an 8-pin DIP socket is provided for the sidetone network to allow easy insertion of the components.
Although transmit and receive gains can be set by software, additional frequency shaping may be required (depending on handset's acoustic characteristics). This can be done by modifying the resistor-capacitor network of the handset.
*This article, which appeared in the January 19th issue of Planet Analog magazine, represents a distillation of Austriamicrosystems' Applications Note AN525. You can also visit the company's website at www.austriamicrosystems.com.