H8SSLASH2224 datasheet(575/1071 Pages) RENESAS

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Section 14 IEBus™ Controller (IEB) [H8S/2258 Group]

Rev. 5.00 Aug 08, 2006 page 489 of 982

REJ09B0054-0500

(5) Message Length Field

The message length field is a field for specifying the number of transfer bytes. The message length

field is comprised of message length bits, a parity bit, and an acknowledge bit.

The message length has eight bits and is output MSB first. Table 14.3 shows the number of

transfer bytes.

Table 14.3 Contents of Message Length Bits

Message Length bits (Hexadecimal)

Number of Transfer Bytes

H'01

H'02

H'FF

H'00

1 byte

2 bytes

255 bytes

256 bytes

Note:

* If a number greater than the maximum number of transfer bytes in one frame is

specified, communications are performed in multiple frames depending on the

communications mode. In this case, the message length bits indicate the number of

remaining communications data after the first transfer. In this LSI, after the first transfer,

the message length bits must be specified to the number of remaining communications

data by a program, since these bits are not automatically specified by the hardware.

This field operation differs depending on the value of bit 3 in the control field: master transmission

(bit 3 in the control bits is 1) or master reception (bit 3 in the control bits is 0).

(a) Master Transmission

The master unit outputs the message length bits and parity bit. When the parity is correct, the

slave unit returns the acknowledgement and enters the following data field. Note that the slave

unit does not return the acknowledgement in broadcast communications.

In addition, when the parity is not correct, the slave unit decides that the message length field

is not correctly received, does not return the acknowledgement, and returns to the waiting

(monitor) state. In this case, the master unit also returns to the waiting state, and

communications end.

(b) Master Reception

The slave unit outputs the message length bits and parity bit. When the parity is correct, the

master unit returns the acknowledgement.

When the parity is not correct, the master unit decides that the message length bits are not

correctly received, does not return the acknowledgement, and returns to the waiting state. In

this case, the slave unit also returns to the waiting state, and communications end.

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