Serial Communications Module

For Evaluation Kits

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9

is tristated and the input is read. The pin remains in a

high-impedance state until a write operation is per-

formed. As mentioned before, all I/O pins, except P3,

are inputs on power-up.

Note: If an I/O pin is being used for both reads and

writes, take care to avoid bus contention.

Since software can only change a single pin at a time,

the DS3900 is much slower when communicating using

the bit I/O commands. For some applications, this is

acceptable since the 57600 baud communications

along with the streamline 2-byte command system allow

pins to be changed hundreds of times a second. If the

design requires higher performance than 100s of pin

changes per second the DS3900 may not be a good

choice for that general-purpose bit I/O application.

Application Information

The DS3900 has decoupling on its PC board, but due to

the high periodic current draw of the RS232 transceiver.

It is recommended to place a decoupling capacitor

(47µF, Tantalum capacitor) next to the connector on the

board that the DS3900 will be mounted.

Using the Integral Pullups

Moving the switches to the on (labeled “ON” on the

switch) position enables the integral pullups. This con-

nects 4.7k

Ω pullup resistors to the SDA and SCL

buses, and it eliminates the need to place external

pullup resistors on the application board for 2-wire

communication. In the event that the application board

already has pullups present, disconnect the integral

pullups by placing the switches in the off position. This

eliminates the extra (unnecessary) pullup strength,

because having both the integral pullup and the appli-

cation board’s pullup on at the same time could poten-

tially cause problems for either the DS3900 or the

2-wire slaves, which are required to pull the buses low.

Additionally, when P10 and P11 are used as bit I/O

pins, it can be advantageous to disconnect the pullups

to reduce the through current when either signal is dri-

ven low. For bit I/O operations, writing the signal to a ‘1’

enables a PMOS driver that drives the signal high.

However, it is also possible to use P10 and P11 as

open-collector I/O pins by enabling the pullups and

performing only read and write ‘0’ operations. As long

as a ‘1’ is never written to P10 or P11 it appears as an

open-collector output. This is how the firmware manufac-

tures the open-collector output for the 2-wire interface.

Intended Use

The DS3900 module and the bundled software are

intended only to be used to evaluate Maxim and Dallas

Semiconductor’s integrated circuits. The software is pro-

vided “as is”, without warranty of any kind, express or

implied, including but not limited to the warranties of

merchantability, fitness for a particular purpose and non-

infringement. In no event shall Dallas Semiconductor be

liable for any claim, damages or other liability, whether in

an action of contract, tort or otherwise, arising from, out

of, or in connection with the software or the use or other

dealings in the software.