MCP8021 datasheet(30/66 Pages) MICROCHIP

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MCP8021/2

DS20006265D-page 30

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4.8

DE2 Communication Port

A half-duplex 9600 baud UART interface is available

to communicate with an external host. The port is used

to configure the MCP8021/2 and also for status and

Fault messages.

4.8.1

COMMUNICATIONS INTERFACE

A single wire, half-duplex, 9600 baud, 8-bit bidirectional

communications interface is implemented using the

open-drain DE2 pin. The interface consists of eight data

bits, one Stop bit and one Start bit. The implementation

of the interface is described in the following sections.

The DE2 interface is an open-drain interface. The

open-drain output is capable of sinking a minimum of

1 mA of current while maintaining less than 50 mV at

the output.

A 5K resistor should typically be used between the

host transmit pin and the MCP8021/2 DE2 pin to allow

the MCP8021/2 to drive the DE2 line low when the

host TX pin is at an active-high level.

The auto-baud frequency is temperature-dependent,

as illustrated in Figure 2-5. To establish proper DE2

communication, it is recommended to synchronize the

host frequency by proceeding the auto-baud function

alternatively, as described in Section 4.8.5 “Auto-

Baud Function”. The time from receiving the last bit

of a command message to sending the first bit of the

response message ranges from tDE2_RSP to tDE2_WAIT,

corresponding to 0 µs to 3.125 ms. The host should

refrain from sending additional messages until the

previously requested message has been received in

order to prevent overwriting the driver response

message.

4.8.2

PACKET FORMAT

Every internal driver status change will cause the

driver to send a message to the microcontroller. The

interface uses a standard UART baud rate of 9600 bits

per second.

In the DE2 protocol, the transmitter and the receiver

do not share a clock signal. A clock signal does not

emanate from one transmitter to the other receiver.

Due to this reason, the protocol is asynchronous. The

protocol uses only one line to communicate, so the

transmit/receive packet must be done in Half-Duplex

mode. A new transmit message is allowed only when

a complete packet has been transmitted and

responded to.

The host must listen to the DE2 line in order to check

for contentions. In case of contention, the host must

release the line and wait for at least three packet

length times before initiating a new transfer.

Figure 4-4 illustrates a basic DE2 data packet.

4.8.3

PACKET TIMING

While no data are being transmitted, a logic ‘1’ must

be placed on the open-drain DE2 line by an external

pull-up resistor. A data packet is composed of one

Start bit, which is always a logic ‘0’, followed by eight

data bits and a Stop bit. The Stop bit must always be a

logic ‘1’. It takes ten bits to transmit a byte of data.

The device detects the Start bit by detecting the transi-

tion from logic ‘1’ to logic ‘0’ (note that while the data

line is Idle, the logic level is high). Once the Start bit is

detected, the next data bit’s “center” can be assured to

be 24 ticks minus 2 (worst-case synchronizer uncer-

tainty) later. From then on, every next data bit center is

16 clock ticks later. Figure 4-5 illustrates this point.

4.8.4

MESSAGE HANDLING

The driver will not transition to Sleep mode while a

message is being received. If a message reception is

in progress before the OE = 0 to Sleep mode transition

delay (tSLEEP) times out, the message will be fully

received and the contents applied to the Configuration

registers if applicable. The SLEEP bit will then be

checked and the system enters Sleep mode if the

SLEEP bit is still active.

4.8.5

AUTO-BAUD FUNCTION

The MCP8021/2 devices provide an auto-baud feature

that allows a host, communicating on the DE2

communications link, to determine the actual baud rate

being used by the MCP8021/2 device. The feature

allows the host to request a 0x55 byte transmission

from the MCP8021/2. The host then determines the

MCP8021/2 baud rate and adjusts the host internal

Baud Rate Generator (BRG) to match the MCP8021/2

baud rate.

The DE2 pin is used to trigger the auto-baud feature.

The host sets the DE2 signal to a logic low for a period

of time (auto-baud Break window) that ranges

between 1.29 ms and 2.0 ms. The host then releases

the DE2 pin back to the host UART control. The host

UART then raises the DE2 pin to a logic high value.

The MCP8021/2 driver will respond with a standard

NACK (‘0b00nnnnnn’, where ‘nnnnnn’ are the six

Least Significant bits (LSbs) received) if the DE2 link

was held low for less than 1.29 ms and the byte was

not interpreted as a valid command. The MCP8021/2

driver will ignore the current message if the DE2 link is

held low for more than 2.0 ms.

If the driver receives a valid auto-baud request in the

allotted time frame, the driver will enter an Auto-Baud

state, indicating an auto-baud message has been

requested. When the auto-baud function is activated,

the DE2 subsystem will disable sending all unsolicited

messages to the host.

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