Maxim Integrated High-Speed Microcontroller Users Guide: Network Microcontroller Supplement - Manual (Page 168)

High-Speed Microcontroller User’s
Guide: Network Microcontroller
Supplement
168
SECTION 21: 1-WIRE BUS MASTER
The 1-Wire master contained within the DS80C400 was designed to offload the task of 1-Wire communications from the microcontroller.
Its main target is a network between the DS80C400 and a small number of local 1-Wire devices. This would include any 1-Wire chips
or permanently attached iButtons ® located on the same PC board as the DS80C400. A minimum of external devices must be con-
nected to the designated 1-Wire master pins in order to obtain proper function. This section discusses the hardware setup required,
as well as the minimum steps necessary, to bring the 1-Wire master up and running.
HARDWARE SETUP
The 1-Wire system typically requires the slave devices to obtain their power parasitically from the master or, more specifically, from the
1-Wire (OW) line coming from the master. This I/O line is historically rated to function between 2.8V and 5.25V. The DS80C400, howev-
er, is a low-voltage microcontroller with a nominal I/O supply voltage (V CC3 ) of 3.3V, leaving little operational headroom. This requires the
parasitic power supplied by 1-Wire line to come from an external source, namely a pullup resistor to an external supply. Some devices,
however, need a more direct connection to the parasite power source. This is necessary to obtain the large amounts of current they
require without dropping below the minimum input voltage. The direct connection is obtained through a strong pullup bypass transistor.
The two pads on the DS80C400 involving the 1-Wire master both require external pullup resistors. Each pad should be pulled up by
the same power-supply voltage that is used to power the slave devices. OWSTP is the control signal for the gate of the strong pullup
transistor. A 10k Ω resistor should pull up the OWSTP output to (a recommended) 5V. The drain of the transistor should be connected
to the OW line. In parallel with the strong pullup transistor is the OW weak pullup resistor. This resistor, typically being around 2.2k Ω,
provides the power supply to the slaves most of the time. The following figure shows the described circuit.
SETTING UP AND USING THE 1-WIRE MASTER
The default state of the 1-Wire master is to remain completely shut down until accessed. When an application calls for the master’s fea-
tures, certain steps must be taken to bring it out of this shutdown state. The following is a quick guide to show the minimum setup
requirements for the master to start communications followed by some basic communication code.
2.2kΩ
10kΩ
OWSTP
OW
5V
To slave
devices VSS
DS80C400
NOTE: If not using the strong pullup feature, the
10kΩ resistor and the strong pullup transistor
can be omitted.
iButton is a registered trademark of Maxim Integrated Products, Inc.
Figure 21-1. Typical 1-Wire External Hardware Configuration
Maxim Integrated