MCP39F511N datasheet(15/61 Pages) MICROCHIP

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DS20005473B-page 15

MCP39F511N

4.0

COMMUNICATION PROTOCOL

All communication to the device occurs in frames. Each

frame consists of a header byte, the number of bytes in

the frame, a command packet (or command packets)

and a checksum. It is important to note that the maxi-

mum number of bytes in either a Receive or Transmit

frame is 35.

FIGURE 4-1: MCP39F511N Communication Frame.

This approach allows for single, secure transmission

from the host processor to the MCP39F511N with

either a single command or multiple commands. No

command in a frame is processed until the entire frame

is complete and the checksum and number of bytes are

validated.

The number of bytes in an individual command packet

depends on the specific command. For example, to set

the instruction pointer, three bytes are needed in the

packet: the command byte and two bytes for the

address you want to set to the pointer. The first byte in

a command packet is always the command byte.

4.1

Device Responses

After the reception of a communication frame, the

MCP39F511N has three possible responses, which are

returned with or without data, depending on the frame

received. These responses are either:

• Acknowledge (ACK, 0x06): Frame received with

success; commands understood and commands

executed with success.

• Negative Acknowledge (NAK, 0x15): Frame

received with success; however, commands not

executed with success, commands not understood

or some other error in the command bytes.

• Checksum Fail (CSFAIL, 0x51): Frame received

with success; however, the checksum of the

frame did not match the bytes in the frame.

Note:

There is one unique device ID response

used to determine which MCP39FXXX

device is present: [NAK(0x15) + ID_BYTE].

If the device is interrogated with 0x5A, i.e.

it receives 0x5A as the first byte instead of

the standard 0xA5 first header byte, a

special NAK is returned followed by an

ID_BYTE. For the MCP39F511N the

ID_BYTE is 0x03.

4.2

Checksum

The checksum is generated using simple byte addition

and taking the modulus to find the remainder after

dividing the sum of the entire frame by 256. This oper-

ation is done to obtain an 8-bit checksum. All the bytes

of the frame are included in the checksum, including

the header byte and the number of bytes. If a frame

includes multiple command packets, none of the com-

mands will be issued if the frame checksum fails. In this

instance, the MCP39F511N will respond with a CSFAIL

response of 0x51.

On commands that are requesting data back from the

MCP39F511N, the frame and checksum are created in

the same way, with the header byte becoming an

Acknowledge (0x06). Communication examples are

given in Section 4.5 “Example Communication

Frames and MCP39F511N Responses”.

4.3

UART Settings

The default baud rate is 115.2 kbps and can be

changed using the UART bits in the System

Configuration Register. Note that the baud rate is

changed only at system power-up, so when changing

the baud rate, a Save To Flash command followed

by a power-on cycle is required.

The UART operates in 8-bit mode, plus one start bit

and one stop bit, for a total of 10 bits per byte, as

shown in Figure 4-1.

FIGURE 4-1:

UART Transmission, N-8-1.

Note:

If a custom communication protocol is

desired, please contact a Microchip sales

office.

Header Byte (0xA5) Number of Bytes

Command Packet1 Command Packet2

...Command Packet n

Checksum

Command

BYTE0

BYTE1 BYTE2 BYTE N

BYTE N

Frame

IDLE

START

IDLE

STOP

D0 D1

D3 D4

D5 D6 D7

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