MCP8021 datasheet(31/66 Pages) MICROCHIP

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DS20006265D-page 31

MCP8021/2

If the internal Auto-Baud state is set, the driver will wait

for a minimum of 0.86 ms and a maximum of 1.19 ms.

After the wait time has expired, a 0x55 data byte will

be immediately sent on the DE2 link by the driver.

The driver will wait 2.00 ms after sending the 0x55

baud rate data over the DE2 link before transmitting

any other messages. The driver will then exit the Auto-

Baud state and resume normal DE2 operations. The

2.00 ms wait is needed to allow the host to complete

the auto-baud verification and update the host UART

Baud Rate Generator.

The MCP8021/2 device will always monitor the DE2

link for a logic low before attempting to transmit.

The MCP8021/2 device will preempt all DE2

communications upon receiving a logic low on the DE2

link which lasts longer than ten bit times at 9600 baud

(Break sequence).

The MCP8021/2 device will wait for a period up to

2 ms for the DE2 link to change to a logic high state

after the initial detection of a logic low on the DE2 link.

If the DE2 link fails to rise to a logic high level within

2 ms of the initial logic low level, the auto-baud

message will be canceled and no message will be

sent. The auto-baud function will then be complete.

The driver will send any pending unsolicited messages

after the auto-baud function has finished.

4.8.6

MESSAGING INTERFACE

A command byte will always have the Most Significant

bit (MSb) 7 set to ‘1’. Bits 6 and 5 are reserved for

future use and should be set to ‘0’. Bits[4:0] are used

for commands. That allows for 32 possible commands.

4.8.6.1

Host to MCP8021/2

Messages sent from the host to the MCP8021/2

devices consist of either one or two 8-bit bytes. The

first byte transmitted is the command byte. The

second byte transmitted, if required, is the data for the

command.

If a multibyte command is sent to the MCP8021/2

devices and no second byte is received by the

MCP8021/2 devices, then a “Command Not Acknowl-

edged” message will be sent back to the host after.

The host must start sending the 2nd byte of a two-byte

command within 1 ms of completion of the first byte to

prevent a NACK message. Once the second byte Start

bit is received, the MCP8021/2 internal receiver logic

will handle the reception of the data byte. If the data

byte Stop bit is not received within the expected recep-

tion time for the last received bit, the MCP8021/2 will

respond with a NACK message.

4.8.6.2

MCP8021/2 to Host

A solicited response byte from the MCP8021/2

devices will always echo the command byte with bit 7

set to ‘0’ (response) and with bit 6 set to ‘1’ for

Acknowledged (ACK) or ‘0’ for Not Acknowledged

(NACK). The second byte, if required, will be the data

for the host command. Any command that causes an

error or is not supported will receive a NACK

response.

The MCP8021/2 may send unsolicited command

messages to the host controller. All messages to the

host controller do not require a response from the host

controller.

4.8.7

MESSAGES

4.8.7.1

SET_CFG_0

There is a SET_CFG_0 message that is sent by the host

to the MCP8021/2 devices to configure the devices.

The SET_CFG_0 message may be sent to the devices

at any time. The host is responsible for making sure the

system is in a state that will not be compromised by

sending the SET_CFG_0 message. The SET_CFG_0

message format is indicated in Table 4-6. The response

is indicated in Table 4-7.

4.8.7.2

GET_CFG_0

There is a GET_CFG_0 message that is sent by the

host to the MCP8021/2 devices to retrieve the device

Configuration register. The GET_CFG_0 message format

is indicated in Table 4-6. The response is indicated in

Table 4-7.

4.8.7.3

STATUS_0 and STATUS_1

There is a STATUS_0 and STATUS_1 message that is

sent by the host to the MCP8021/2 devices to retrieve

the device STAT0 and STAT1 registers. Unsolicited

STATUS_0 and STATUS_1 messages may also be

sent to the host by the MCP8021/2 devices to inform

the host of status changes. The unsolicited STATUS_0

and STATUS_1 messages will only be sent when a

status bit changes to an active state. The STATUS_0

and STATUS_1 message format is indicated in

Table 4-6. The response is indicated in Table 4-7.

When a STATUS_0 or STATUS_1 message is sent to

the host in response to a new Fault becoming active,

the FAULT bit will be cleared, either by the host issuing

a STATUS_0 or STATUS_1 request message, or by

the host toggling the OE pin low then high. The FAULT

bit will stay active and not be cleared if the Fault condi-

tion still exists at the time the host attempted to clear

the Fault.

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