SAN3010 Ver la hoja de datos (PDF) - South African Micro Electronic Systems

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SAN3010

SINGLE CHIP TELEPHONE INTERFACE FOR KEYBOARD ENTRY VIA uC

South African Micro Electronic Systems 

SAN3010

4 Revision status

SAN3010 Application Note (this document):

AN3010 Schematic:

AN3010 Sample Software

Rev.: A02

Rev.: A01

5 General Description

The interface must be capable of forcing the keyboard rows (hereafter indicated as R1...R4) both low (Vss) and

high (VDD) and forcing the keyboard columns (hereafter indicated as C1...C4) to high (VDD). Only one row or column

is forced at the same time while the remaining rows/columns must be high ohmic (hereafter indicated as Hi Z).

Since only one row/column must be driven at the same time, a decoder (IC5 = 74138) was implemented to save

pin count of the µC. 3 output ports were used for row/column selection, 1 output port to disable all rows/columns

and 1 output port for the 3-state buffer inputs to select forcing high or low .

One µC pin must be configured as input for handshake.

If 9 output + 1 input pins are available from the µC (8 row/column selectors, 1 force hi/lo selector , 1 handshake

input) , IC5 = 74138 can be omitted.

IC3 & 4 (=74HC125) are separately addressable 3-state buffers.

A simple discrete NOR gate (Q1,D2,D3,R9..11) is used for synchronization.

6 Hardware configuration

The SA253x Single Chip Telephone can be connected to a µC for key entry when the following conditions are met:

Supply voltage: The SA253x works at 4V VDD, its high/low input levels are 30/70% VDD.

If the µC is supplied with >4V a current limiter (serial resistor) must be added to the row/column pins of the

SA253x.

If the µC supply is <4V it must provide adequate high level (> 70%VDD of SA253x = >2.8V)

Row driver : 4 Tristate output pins, each separately adjustable as either output (Hi/Lo) or tristate . Only one row is

driven at the same time, all other rows/columns are high ohmic.

Column driver: Tristate output pins, each separately adjustable as either output (Hi) or tristate. Only one column is

driven at the same time, all other rows/columns are high ohmic.

Synchronization: The SA253x is intentionally designed for low standby power, so no oscillator is running as long as

no key is pressed. All the external row/column driving is done asynchronously. The scanning sequence is

synchronized by triggering a rising edge on specific columns (see table 1).

Therefore a 1-bit input must be provided for the synchronization.

7 Application schematic:

See also: Appendix A:

One solution which only requires 5 output pins and 1 input pin is shown in the attached schematic. A common µC

(80C51-family) is used. This controller is widely known, so both hardware and software description in this

application note should be easily understood which simplifies adaption to any other controller.

The rows/columns are driven by 8 separately selectable tristate buffers (e.g. 74HC125). Power supply is 5V,

therefore resistors R2...R8 are necessary to provide adaption to the 4V-logic of the SA253x.

R1 is always connected by a 5kOhms resistor to prevent collision of two outputs when it initiates the key entry in an

unsynchronous state.

2/13

sames

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